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Bob McDavitt's ideas for sailing weather around the South pacific

31 December 2011

BOBGRAM issued 1Jan 2012

WEATHERGRAM
YOTREPS
Issued 1 January 2012
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina is steady and strong in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 2.26 on 25 Dec and on 31 Dec. It is likely to persist for the next few months, encouraging the South Pacific Convergence Zone SPCZ and subtropical ridge slightly pole-wards.

That MJO of enhanced convection mentioned last week moving across Australia is now weakening away. That means we may be in for a few quiet weeks in the tropics.

TC GRANT has also weakened away – There is a branch of the South pacific Convergence Zone SPCZ over the northern Coral Sea, with a weak extension towards New Caledonia---- a (new) small Tropical Low may form in this zone during the week, hovering west of Vanuatu for a few days and then broadening over New Caledonia on Thu 5 Jan. It may visit Fiji on Fri 6 Jan and then wander south along the dateline early next week.

The main branch of the SPCZ is still stretching from Tuvalu to Tokelau to Northern and Southern Cooks. As mentioned last week it has formed a tropical low, around 1005 near Southern Cooks. This system is not well placed for development and should move south then away to the southeast.

SUBTROPICAL RIDGE STR
High has nosed across Tasmania and into SW Tasman Sea this weekend, and should move across North Island on Tuesday and Wednesday. Next High is expected to cross Tasmania on Friday 6Jan and spread around Southend of South Island on Sun 8 Jan, followed by a Northerly flow.

TASMAN SEA/NZ AREA
A transitional trough made for a wet end to 2011 in NZ, and thundery conditions for the interior North Island for New Year 2012. Some of the cold air left behind aloft from this trough may induce more inland showers for North Island on Monday afternoon.

After Wednesday's brief High, another transitional series of troughs are expected to bother mainly the South island on Thursday-Friday-Saturday 5-6-7 Jan with a few fronts.

Should be OK coming and going from Northland this week

Changing timezones:
Samoa and Tokelau have sacrificed Friday 30 Dec altogether so that they are now on the western side of the date line and can trade with Australia and NZ on the same days of the week. For those of you on the eastern side of the date line these old verses of Robbie Burns famous rhyme may still apply tonight:
We two have paddled in the stream,
from morning sun till dine ;
But seas between us broad have roared
since the days of auld lang syne.
For auld lang syne, my friend,
for auld lang syne,
we'll take a cup of kindness yet,
for the sake of auld lang syne.
And here's a hand my trusty friend
And give us a hand o' thine
And we'll take a right good-will draught,
for the sake of auld lang syne.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

25 December 2011

BOBGRAM issued 26 Dec 2011

WEATHERGRAM
YOTREPS
Issued 26 December 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina continues to increase in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 2.02 on 19 Dec and 2.26 on 25 Dec. It is likely to persist for the next few months, displacing South Pacific Convergence Zone SPCZ and subtropical ridge slightly pole-wards.

TC FINA briefly formed in the Coral Sea, but encountered strong winds aloft and lost its top… the low centre is still hovering mid way between Queensland and New Caledonia and drifting south or southeast… it is expected to wander and fade to west of or over North Island on Thu /Fri 29/30 Dec.

TC GRANT deepened east of Darwin on Christmas Day, and is expected to weaken as it wanders further east and reaches the Coral Sea around Thu 29 Dec and then may deepen again and move towards New Caledonia by Sat 31 Dec/Sun 1 Jan. Avoid.

Cyclone risk is winding up this week… an MJO cycle of enhanced convection is expected to arrive there this week also some equatorial westerly winds have reached the Solomons, so all is set for good cyclone formation especially in the Coral Sea.

The South Pacific Convergence zone, SPCZ, has been active over Coral Zone, New Caledonia, and Fiji to Niue and to the southeast--(where SE winds are converging into northerly winds) to Fiji, and Samoa to Niue. This part of the SPCZ may drift north and east across Southern Cooks and allow some tropical lows to develop there on Thu/Fri 29/30 Dec. Avoid.

There is a low that has come form the tropics near 34S 160W today and this should over to the southeast and fade by Wednesday.
Another branch of SPCZ has been lurking between Tahiti and Marquesas over past few days, but I think this will fade away soon.

SUBTROPICAL RIDGE STR
Big fat high stalled over New Zealand during the past week and should now wander to east along 40S reaching 150W by Sun 1 Jan. There is a squash zone of enhanced easterly wind between it and the SPCZ.

Next high is likely to nose across Tasmania on Thu 29 Dec and into South Tasman Sea from Friday 30 Dec, brining clearing weather.

TASMAN SEA/NZ AREA
A transitional trough is expected to take from Wed 28 Dec to maybe as long as wed 4 Jan to cross NZ. During Wednesday and Thursday an increasing NE flow is likely to spread a rain band southwards over the North islands. At about the same time a southerly change is expected over the South Island. These may converge together and stall the rain band for a while over central NZ or the northern parts of the South Island. Location, timing and intensity of peak rain are still changing.

As for Northland—winds should become light and variable there for a time on Sun 1 Jan, then another period of easterly winds may occur next week.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

19 December 2011

BOBGRAM 19 Dec 2011

WEATHERGRAM
YOTREPS
Issued 19 December 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina is increasing in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 1.32 on 3 Dec and 2.02 on 19 Dec. It is likely to continue through our cyclone season, displacing South Pacific Convergence Zone SPCZ and subtropical ridge slightly pole-wards.


TC Washi, in the Philippines, turned out to be more severe than its satellite imagery was indicating, and death toll in the landslides is now exceeding 650. As for last Wednesday/Thursday's rain in Nelson, MetService had issued warnings of 500mm+ for the hills from a stalled trough, and this amount was delivered to a wide region also producing landslides.

In the equatorial regions things are changing …. Equatorial westerlies have arrived in Timor Sea and Norherlies have appeared over Coral Sea, just as a MJO cyclone of increased tropical convection arrives in the Coral Sea. Already small tropical lows are appearing—tonight one is near 5S 155E.

GFS and EC models are both now picking formation of a tropical depression near 10S 130E on Tue 20 Dec, deepening into a more intense feature as it makes land fall near Darwin around Christmas Day --- this can still change so please check BOM for updates.

The South Pacific Convergence zone, SPCZ, is active from Coral Zone (where SE winds are converging into northerly winds) to Fiji, and Samoa to Niue. A tropical depression should develop to west of New Caledonia on Tues 20 Dec, deepen and be slow-moving for the Christmas weekend then move SE across /near New Caledonia and weaken away by 29 Dec. Worth avoiding.

A sub-tropical low is expected to form between Minerva and Niue on Fri 23 Dec, move SE deepen and peak near 35S 160E on Sun 25 Dec and then move further SE and weaken, leaving behind a tropical trough/convergence zone worth avoiding.

SUBTROPICAL RIDGE STR
Large High at 45S is slow-moving over NZ/180 area this week — there is a squash zone of enhanced easterly winds between this and the SPCZ, mainly near 20 to 30S this week. Next week the main part of this high is likely to get east of NZ, and the squash zone may turn into a strong NE flow onto northern NZ during the spring tides following the New moon. Be aware of this strengthening NE with the approach of New Year.

TASMAN SEA AREA
NE tail winds for start of Sydney-Hobart race on 26th, then, in typical fashion, a deep tropically enforced LOW is expected to cross Tasmania around Thu 29 Dec, followed by a intense cold high--- strong pressure gradient between high and low makes for a southerly buster around Friday 30 Dec … maybe too late to bother the race this year, maybe not - Depends on the timing.

Next Sunday is Christmas Day. I'll take time-out to feel the spirit. So, next edition of weathergram will be Monday 26 Dec.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

10 December 2011

BOBGRAM issued 11 Dec 2011

WEATHERGRAM
YOTREPS
Issued 11 December 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina is increasing in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 1.32 on 3 Dec and 1.61 on 10 Dec. This La Nina is likely to continue through our cyclone season, displacing the South Pacific Convergence Zone SPCZ somewhat southwest of its normal position and thus helping promote a quieter-than-normal cyclone season in our area.

There was a MJO – an enhanced pulse of tropical convection – over Western Australia last week but it seems to be fading away before it gets to the Coral Sea, so I now think the cyclone threat in the South Pacific remains low for the next few weeks.

SPCZ weakened and split into two last week, one zone from Solomons to Northern Vanuatu, and other zones near Tokelau, Southern Cooks and Southern French Polynesia, only weakly kinked together. The weather engine in the South Pacific is turning over more slowly than normal. A LOW may form out of the zone near Southern Cooks; its passage to the south may be blocked by a HIGH for a few days but should be able to get going from Thu/Fri 15/16 Dec. leaving behind a zone of rising pressure and light variable winds over Southern Cooks /Niue. OK for diving, no good for sailing.

SUBTROPICAL RIDGE STR
The long wave pattern seems to still be dominating the weather in our part of the South pacific, with a long wave ridge near the date line and a long wave trough to south of Australia.

The surface HIGH that is east of NZ has been acting like a road block to troughs upstream over recent days, and should continue like this until Tue 13 Dec, then weaken and move east along 45S

Next high should push across Tasmania into the South Tasman Sea on Sat 17 Dec. This is expected to take a pat along 50S, crossing southern NZ around Mon 19 Dec and then blossom when it encounters upper support from then long wave ridge and expand to 1030+ to Southeast of South Island on Tue 20 Dec.


TASMAN SEA/NZ AREA
NZ may be next to a long wave ridge, but will spend the next week under attack from some short wave transitory troughs.
The first produced a small area of intense rain between Norfolk and Raoul yesterday, and its low is fading as it moves south along 160E tonight. Remaining rain band is visiting northern NZ next few days: may be useful over Northland Sunday night / Monday and briefly heavy over Taranaki/Nelson high ground on Monday.

This trough may act as the entree and help condition NZ's atmosphere to enhance the impact of the main course: a new LOW should form in the Tasman Sea on Tues 13 Dec and take its time, finally crossing North Island on Sunday 18 – some models have this feature going faster than this. It is preceded by a frontal zone that is expected to cross NZ on Thu 15 Dec and followed by an upper trough good for scattered outbreaks of thunderstorms and finally some cooler south to southwest winds. If a southerly crosses Canterbury on Thursday afternoon it may be preceded by hot air and the cold shock may trigger hail.

For those still planning to sail into Northland:
Northeast winds over Northland this week, these are likely to be strong on Thursday ahead of the front (avoid Thu) then northwest and showery until Sunday. Next west to southwest/southerly winds for Northland are likely on Sun /Mon 18/19 Dec.

I will be travelling next Sunday, so next edition of Weathergrams is postponed to Mon 19 Dec.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

03 December 2011

BOBGRAM issued 4 Dec 2011

WEATHERGRAM
YOTREPS
Issued 4 December 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Welcome to the months of the southern summer. For practical purposes many climatologists take the months of December to February to be summer, but for logical reasons I wait until the Solstice for my summer to start.

Either way, the southern hemisphere cyclone season seems set soon to start, with what looks to be a developing tropical depression in the Indian Ocean neat 13S 90E today, possibly set to take a path to the southeast but to fade before making land fall between Shark Bay and Perth in WA around Sun 11 Dec. It is riding on the wave of a zone of enhanced tropical convection, called a Madden Julian Oscillation or MJO that seems set to bring the Monsoon from Indonesia to Northern Australia over the next few weeks.

Australia had a very wet November and is still a place favoring upper troughs, whilst the upper ridge partners have been favuored about and east of the date line.

La Nina is increasing in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 1.0 on 26th Nov and 1.32 on 3 Dec. It is likely to continue through our cyclone season, displacing the South Pacific Convergence Zone SPCZ somewhat southwest of its normal position and thus helping promote a quieter-than-normal cyclone season is our area. The MJO is likely to reach the Coral Sea around mid-December, so that is likely to be the first period of increased threat.

SPCZ weakened west of the date line last week, from Solomons to northern Vanuatu to north of Fiji, and strengthened east of the dateline from Samoa to Southern Cooks. That little low near 25S 165W today is attached to a branch of the SPCZ which is active over the Southern Cooks. The Low is expected to wander to SE and the activity should ease by wed 7 Dec.

SUBTROPICAL RIDGE STR
Large High at 35 to 40S east of NZ and south of Southern Cooks managed to reach over 1040 hPa last week … It is now slowly weakening and moving east mainly along 30/35S.

New High is expected to cross Tasmania on Mon 5 Dec and then build in the mid Tasman Sea and cross southern and central NZ on Fri 9 to Sat 10 Dec. This has the makings of a fine arrangement for pre-Christmas outdoor functions in NZ next weekend.

TASMAN SEA/NZ AREA
Between the Highs there are troughs. The one over Tasman /NZ today is in two parts… first frontal rain band is over much of the northern areas of both Island tonight and should clear off on Monday. Second part is expected to deepen into a Low in mid-Tasman during Monday and then cross northern and central NZ on Wednesday along with some heavy downpours. After that second rain band moves off, the incoming high is expected to bring dry and clearing weather to NZ, good for a few days of haymaking.

Avoid the Tasman from Mon 5 to Wed 7 Dec.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

27 November 2011

BOBGRAM issued 27 Nov 2011

WEATHERGRAM
YOTREPS
Issued 27 November 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

There was a solar eclipse with the New Moon on Friday and this is a Perigean moon. The perigee (= moon-closest-to-earth) occurred 30 hours before new moon. This means that the tides over weekend and on Monday are more extreme than normal - but not quite as extreme as this time last month (Oct perigee was within 7 hours of New moon).

We are having a minor La Nina. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 18 Nov it was 0.90 and on 26th 1.0. Computer modelling suggests it will continue for next few months, possibly making for a quieter than normal cyclone season. At this stage the first Madden Julian oscillation- a period of enhanced convection –may be starting to appear to west of Australia over next week or two, and so this is an early indication it may reach the South Pacific around mid December.

Last week was reasonably quiet on the South pacific. The South Pacific Convergence Zone SPCZ has two branches: one from Solomons to northeast Vanuatu to Fiji/Wallis/Futuna to Samoa, and the other from Tokelau to Northern Cooks. It is expected to stay is much the same position for the next week or so.

SUBTROPICAL RIDGE STR
High that is crossing to east of northern NZ today should move smartly along 35S over the next few days. Another HIGH is expected to develop at around 40S to 45S in the Tasman Sea on Monday and to spend this week travelling sedately eastwards, crossing the Chatham Islands on Tues/Wed 29/30 Nov. There should be a humid and muggy northerly flow over NZ (and an easterly flow for Northland) for several days.

Another High is expected to travel across Tasmania on Thu /Fri 1 and 2 Dec. This is expected to travel east along 45S, crossing South Island on Wed/Thu/Fri 7/8/9 Dec. More on this in the next weathergram.


TASMAN SEA/NZ AREA
Between the Highs there are troughs. One of these is moving across NZ Sunday night and Monday, and should bring some heavy rain to western South Island and some strong to gale NW winds to southern and eastern districts, but just a moderate wind change to Northland.

Next trough should cross NZ on Sat 3 to Mon 5 Dec, and bring a change in Northland from Sun 4 Dec to West then SW then SE winds.

SAILING TO/FROM NORTHERN NZ.
There are still some cruising yachts sailing to NZ, and indeed this looks to be a good week to approach from the north, with the high now at 35S pulling off to the east at last, so that there is a northerly flow over Northland on Fri/Sat 2/3 Dec. Even the trough crossing Northland may be OK for anyone arriving Sun 4 Nov, but it may deepen into a low on Mon/Tue/Wed 5/6/7 Dec and that may be worth avoiding.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

19 November 2011

BOBGRAM issued 20 Nov 2011

WEATHERGRAM
YOTREPS
Issued 20 November 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina continues in a weak to moderate fashion. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 11 Nov it was 1.02, AND ON 18TH, 0.90 Computer modelling suggests it will continue into the southern hemisphere cyclone season, but as a minor player, making a quieter than normal cyclone season.

The first tropical depression of the new cyclone season made its appearance during the past week, bringing several days of heavy rain to northern Fiji and moving across central Tonga. It has moved off to the southeast and faded away now.

The South Pacific Convergence Zone SPCZ has two branches: one from Solomons to north of Vanuatu to north of Fiji, and the other from Tuvalu to Samoa to Southern Cooks. The SPCZ that is now north of Fiji is likely to move onto Fiji and Tonga this weekend 26/27 Nov.

SUBTROPICAL RIDGE STR
The High that has been lingering in north Tasman Sea for past two weeks is finally on its way east, but slowly and will still be able to form a squash zone between it and the front over the South island on Monday—making gusty conditions for central NZ and wet conditions wit that front. Avoid.

This high then mainly lingers at 30S 180, waiting for a Low to go east of NZ, and finally the High is expected to move off to east itself on Wed/Thu 24/25 Nov.

New High is expected to move across Tasmania on Thu 24 Nov and weaken as it extends across northern NZ on Fri/Sat 25/26 Nov

TASMAN SEA/NZ AREA
On Monday there is an enhanced front + squash zone, a classic spring front, to avoid. A low is expected to form between Christchurch and Chathams on Monday and deepen as it goes east. Another front is expected to cross NZ on Wed 23 Nov, followed by strong SW flow on Thu 24/ Fri 25 Nov (with large ocean swells) and then winds should ease for the election weekend.

SAILING TO/FROM NORTHERN NZ.
If you have Internet access you may be able to check the weather patterns at bit.ly/7daywx or bit.ly/ecoz

For Northland there is, at this stage, expected to be a northerly flow for much of next week, so plan to come south as that high waddles off to the east on Thu 25 Nov and plan to get to Opua before the next SW change (somewhere around 3 Dec).

I plan to be in Nelson for Coastguard conference next weekend… this may delay my next weathergram, depends on how well I travel.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

13 November 2011

BOBGRAM issued 13 Nov

WEATHERGRAM
YOTREPS
Issued 13 November 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina continues in a weak to moderate fashion. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 5 Nov it was 0.88 and on the 11th, 1.02. Computer modelling suggests it will continue into the southern hemisphere cyclone season, but as a minor player. .

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu towards Wallis Futuna and Tonga. Activity on the SPCZ increased last week and a series of small LOW pressure systems are expected to form near Wallis/Futuna by Mon 14 Nov and then move over Tonga on Tuesday 15 Nov and linger near Niue on Wed /Thu 16/17 Nov. Maybe another may form over Tonga on Friday 18 Nov. Avoid… this area is likely to be full of squally showers all week.

SUBTROPICAL RIDGE STR
High is lingering in north Tasman Sea this week with a ridge occasionally nosing out along mainly 30S--- there is a moderate squash zone of enhanced trade winds in the Tropics between 20 and 25S, lasting all week, especially from Kermadecs/Minerva region to New Caledonia. Be aware of the squash zone and the zone of light variable winds within the STR.


TASMAN SEA/NZ AREA
NZ is continuing with a disturbed westerly flow, typical of spring. Fronts mainly cross the South island on Tue 15 Thursday 17th and Sat 19th Nov, followed by SW flows over the whole country, including Northland, on 16th, late 17th to 18th and on 20th Nov (plus or minus a day). None of these fronts are expected to be major.

SAILING TO/FROM NORTHERN NZ.
If you have Internet access you may be able to check the weather patterns at bit.ly/7daywx or bit.ly/ecoz

Those in Tonga /Niue/Southern Cooks should wait until the SPCZ settles down—maybe early next week. /Fiji/ New Caledonia to NZ is possible, so long as you are prepared for a strong SE for starters, a zone of light winds near 30S, and possible SW winds near Northland (timing of these keeps coming-and-going, so update).

I plan to be in Whangarei on Friday arvo addressing the ICA crowd about weather.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

06 November 2011

BOBGRAM issued 6 Nov 2011

WEATHERGRAM
YOTREPS
Issued 6 November 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

La Nina continues in a weak to moderate fashion. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 30th it was 0.96 and on 5 Nov, 0.88. Computer modelling suggests it will continue into the southern hemisphere cyclone season. This tends to reduce the tropical cyclone activity around and east of the dateline, but may increase the risk around the Coral Sea. But it is a minor player, and other factors are also likely to influence our weather, providing more variety and variability than last year.

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu towards Wallis Futuna. Also in the past week it has been active across north Tonga /Niue/Southern Cooks and then has stretched southwards into the middle latitudes.
The weak trough that moved to east of Tonga today is expected to continue east and cross Niue on Monday and hover around Southern Cooks from Wed 9 to Friday 11 Nov.
Then SPCZ is expected to move south and build over Fiji/Tonga on the Sat/Sun 12/13 Nov weekend and trigger the development of a LOW to south of Niue that will deepen and move southeast.

SUBTROPICAL RIDGE STR
Last weeks BFH (Big fat High) has moved on. Following that BFH there was a weakness in the STR around NZ, and that helped deflect a low from out of the Southern Ocean so that it could sideswipe southern NZ on Fri 4/Sat 5 with a dusting of snow. Gone now. A new high H1 has formed in the North Tasman Sea… this is a weak one and it should fade into a ridge by Wednesday 9 Nov as another high centre H2 crosses the South Island. H2 should then travel east along 40S to East of NZ. Then another high H3 is expected to form in central Tasman Sea on Fri 11 Nov and move over Northland on the weekend.

So it's a week of rather weak highs and not much in the way of squash zones in the trade winds.


TASMAN SEA/NZ AREA
NZ is getting a disturbed westerly flow inbetween and on the southern side of these weak passing highs. A front is likely to cross the South Island on Tuesday. Another may arrive with 'gusto' over Tasmania on Thursday, and be preceded by a strong NW flow over South Island by Friday morning, then weaken as it crosses South Island on Friday afternoon and fade over the North Island on Saturday. These fronts are not expected to bother Northland much.


SAILING TO/FROM NORTHERN NZ.
If you have Internet access you may be able to check the weather patterns at bit.ly/7daywx or bit.ly/ecoz

Those in Tonga/Fiji/ New Caledonia have reasonable trade winds for departure. There is a disturbed SW flow over Northland until Wed 9 Nov. Then the problem is the way the STR is hovering around Northland, making it a light wind zone, requiring some motor sailing. At this stage it seems that the next front to bring wind to Northland may be a NW on Tue 15 then a SW/S on Wed 16/ Thu 17 Nov.
Note that I will be UNAVAILABLE from 8 to 12 Nov—attending to MetService display at Christchurch show.


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

30 October 2011

BOBGRAM issued 30 Oct 2011

WEATHERGRAM
YOTREPS
Issued 30 October 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

This new La Nina episode continues in a weak to moderate fashion. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 22nd Oct it was 1.0, and on 30th it was 0.96. Computer modelling suggests this La Nina will continue into the southern hemisphere cyclone season. This tends to reduce the tropical cyclone activity around and east of the dateline, but may increase the risk around the Coral Sea.

Some Insurance policies take the start of the cyclone season to be 1 Nov- and indeed the risk rises rapidly during Nov and December.

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu to Wallis Futuna and to Samoa, then … slightly weaker … to Southern Cooks. This is its 'normal position' and it has shown a build up of activity during the last week.

It appears that a tropical Low is forming on the SPCZ around the Wallis/Futuna area during Monday and may move across northern/central Tonga on Tuesday, and then take a path ESE-wards to north of Niue on Wednesday and over Southern Cooks on Thursday, then southwards into the mid-latitudes. Avoid.

SUBTROPICAL RIDGE STR
A BFH (big fat high, central pressure over 1036 hPa) has built well to east of NZ and is hovering around 45S 140W. There is a squash zone of enhanced trade winds on the north side of this high near 25S 150W also enhancing this squash Zone a sit moves slowly south. Avoid.

The next high is expected to form slowly in the mid Tasman Sea around 30S but its formation is expected to be delayed until Sun 6 Nov, and this high may just stay put all next week. During this time there will be the characteristic subtropical ridge link connecting this high to the BFH. The STR link should move north next week to 30S, fading out the squash zone.

TASMAN SEA/NZ AREA
An upper low is forming tonight near 28S 175E and it contains thundery squalls. This feature is expected to be collected by the remains of an old frontal system that is currently draped over southern NZ and a trough that is in the Tasman Sea . The combination is expected to move across NZ on wed 2 Nov and lead in a disturbed W to SW flow that looks likely to cover NZ and the area north of NZ from Thu 3 Nov until at least Fri 11 Nov. Drat.

SAILING TO/FROM NORTHERN NZ.
If you have Internet access you may be able to check the weather patterns at http://bit.ly/7daywx or http://bit.ly/ecoz
Those in Tonga may as well wait for that tropical Low to go away on Tuesday before thinking of departure, just it case it changes track.
The STR will remain as a zone of light winds roughly along 30S from wed 3 Nov until at least Fri 11 Nov, with a disturbed westerly flow over Northland. This isn't the easiest pattern for getting to NZ but we may be able to sort out some zig-zags across the S/SW winds for a few boats with have enough fuel to motor across the STR light-wind-zone.

Note that I will be UNAVAILABLE from 8 to 12 Nov—attending to MetService display at Christchurch show.


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

22 October 2011

BOBGRAM issued 23 Oct 2011

WEATHERGRAM
YOTREPS
Issued 23 October 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

A La Nina episode is kicking in. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 16th Oct it was 1.0, and on 22nnd Oct it was 1.2. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season.
This helps intensify the subtropical ridges STR and knocks then pole-wards. The stronger highs in the STR helps make stronger trade winds. There is one of these squash zones of enhanced trade winds around New Caledonia and another to NW of Fiji at present, and there is likely to be more of these squash zones this week.

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu to Wallis Futuna… slightly weak over Tokelau/Samoa the strong again over Northern Cooks. It had a brief welcome visit to Tuvalu mid-last-week, and is expected to drift SOUTH this week.

SUBTROPICAL RIDGE STR
This is a week of Big Fat Highs (BFH)
The one building east of NZ today is expected to make steady progress to the east along around 40 to 45S and grow to over 1030hPa when well south of French Polynesia FP from Wed 26 Oct.

A new BFH is likely to cross Tasmania on Mon 24 Oct and then squeeze around South Island on Wed/Thu 26/27 Oct and then build to maybe over 1035 as moves off to east of NZ from Fri 28 Oct to Mon 31 Oct.

A squash zone of enhanced trade winds is likely to form between this new BFH and the SPCZ=== most likely around 20S to 25S form Tonga to New Caledonia from Friday 28 Oct to Tuesday 1 Nov

TASMAN SEA/NZ AREA
A transitional trough between the two Highs is expected to be windy in the east and west in the west as it crosses South Island on Monday, and then weaken as it stalls over the North Island on Tuesday/Wednesday, 25/26 Oct.

Next trough should reach South Island around Sat 29 Oct --- as a LOW is likely to form off Sydney. This front should stall over central NZ as that Low move SE along the frontal zone. Fronts should then cross Northland around Tue /Wed 1 or 2 Nov, followed by a moderate westerly wind, and Thu 3 Nov followed by a moderate SW wind.

SAILING TO/FROM NORTHERN NZ.
If you have Internet access check the weather patterns at http://bit.ly/7daywx or http://bit.ly/ecoz

The BFHs are in charge this week. One leaves light winds over Northland until Wed 26 Oct then E or NE/NW winds over Northland from Thu 27 Oct to Tue 01 Nov. This provides a good window for arriving in NZ, especially for those leaving Tonga /Fiji/New Cal on Mon/Tue 24/25 Oct. AT this stage the winds over Northland on 2-3 Nov are expected to be mod W /SW, so this may be OK as well.

Because the BFH may be making a squash zone from Tonga to New Cal from Fri 28 Oct to Tue 1 Nov, we may as well treat next weekend as a stay ashore weekend, maybe a good time to complete the voyage planning rather than a target date for departure.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

16 October 2011

BOBGRAM issued 16 Oct 2011

WEATHERGRAM
YOTREPS
Issued 16 October 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Northern Hemisphere cyclone season is easing off this week.

A La Nina episode is kicking in. This helps intensify the subtropical ridges STR and knocks then pole-wards. The stronger highs in the STR helps make stronger trade winds. There is one of these squash zones of enhanced trade winds over the seas of French Polynesia at present as a high moves east along 30S and this should last until Tuesday 18 Oct UTC. Another squash zone is expected to form in the Coral Sea on Tuesday 18 Oct UTC and this should spread east reaching New Caledonia /Vanuatu on Thu 20 Oct, Fiji on Fri 21 Oct and Tonga on Sat 22 Oct.

From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 9th it was 0.91, and on 16th Oct it was 1. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season. At this stage is expected to be a weak La Nina, and NOT as strong as the extreme La Nina we had last summer.

The South Pacific Convergence Zone SPCZ is expected to hover near its normal position this week between 10 and 15S—this means that Tuvalu and Tokelau may see some desperately needed showers for a change.

SUBTROPICAL RIDGE STR
The week starts with a BFH (big fat high) near 40S 150W moving east – this is the cause of the squash zone over French Polynesia. It is expected to be followed by another High of weaker intensity near 35S 160W from Wednesday 19th October. And another High should form in the Tasman Sea on Tue 18 to Thu 19 Oct, crossing northern NZ on Sat/Sun/Mon 22/23/24 Oct (Labour weekend in NZ)


TASMAN SEA/NZ AREA
Of major concern to anyone sailing to NZ at present is a trough now in the Tasman Sea. One low in this trough is forecast to cross the South Island on Monday and another should cross northern NZ on Wednesday--- this brings vigorous SW winds and heavy SW swells to the zone between northern NZ and as far north as 30S, maybe 25S, on Wed 19 and Thu 20 Oct. AVOID.

For this year's COASTAL CLASSIC departing Auckland for Bay of Islands on Friday,
The forecast at this stage is for a ho-hum SW between 10 and 20 knots.


SAILING TO/FROM NORTHERN NZ.
The main thing is to avoid 25S to NZ on Wed 19 and Thu 20 Oct, please

There is a squash zone slowly spreading east from Coral Sea, expected to reach New Caledonia around Thu 20 Oct and onto Tonga by Sat 22 Oct. It may be a good idea to depart before this zone arrives.


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

09 October 2011

BOBGRAM issued 9 Oct 2011

WEATHERGRAM
YOTREPS
Issued 9 October 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Cruising sailors in South Pacific may be in for an interesting delight this weekend /next week with the waning moon, especially around the third quarter- This usually triggers a spawning of a coral worm called Palolo (or Balolo in Fijian). Sometimes the overnight spawn may be large enough to colour the sea – stuff can be netted and eaten raw or cooked. Ask the locals.

Northern hemisphere cyclone season is still busy: PHILIPPE still in North Atlantic, staying offshore, and now IRWIN and JOVA in NE Pacific.

Equatorial ocean temperatures in west pacific are near normal, and in east pacific are below normal. A La Nina episode is kicking in. This helps intensify the subtropical ridges STR and knocks them pole-wards (in both hemispheres). The stronger highs in the STR help make stronger trade winds.

From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 2nd Oct its 30day running mean was 1.2, and on 9th it was 0.91. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season. At this stage is expected to be a weak La Nina, and NOT as strong as the extreme La Nina we had last summer.

The South Pacific Convergence Zone SPCZ gets knocked south by a La Nina. This was certainly the case early this year, and indeed SPCZ has not been visiting Tokelau or Tuvalu much this year… this explains their drought which is now getting desperate. The SPCZ over past week is now hovering between Solomons and inbetween NE of Vanuatu and SW of Fiji. Rather weak.

SUBTROPICAL RIDGE STR
The STR is usually around 30S at this time of the year, but at present seems to be more near 35/40S. And some BFH (big fat high) specimens are on show this week, getting over 1035hPa near 35S 100W and near 37S 140W. These BFHs are also blocking highs so that troughs get directed around them rather taking a normal journey southeastwards. They also strengthen the easterly winds in the tropics on their northern side - be aware of the prospects of a squash zone of enhanced easterly winds between French Polynesia and Niue from Tue 11 Oct, with huge surf/swell over the Cooks. Avoid.

For Tasman Sea, next high should move in from west on Thu 13 Oct, but will be a slow-mover…should fade over Northland on Sun/Mon 16/17 Oct. The following High is likely to catch up with this and cross the North Island more sedately next week from 18 to 20 Oct. If this turns out to be the case, it may offer a few reasonable voyages from Tropics to NZ / Australia.


TASMAN SEA/NZ AREA
Broad multi-edged trough is expected to cross New Zealand this week. First trough already has a tight and intense low centre moving south along eastern Tasman Sea, and this should cross southern NZ on Monday 10 Oct, preceded by strong NE flow over NZ. Second trough has a low that should move east over southern NZ on Wed and then the trough line should cross remainder of NZ on Thu 13 Oct, followed by cold SW flow that may be thundery.

SAILING TO/FROM NORTHERN NZ.
If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then you may be interested in following the ICA All Points Rally, see http://www.islandcruising.co.nz/ for this voyage.

If you time your voyage to depart Tonga/Fiji/New Caledonia before the squash zone of an incoming high arrives (New Caledonia already sat 15 Oct, and Fiji /Tonga by Tue 18 Oct. Also time it to arrive in NZ after the SW winds following Thursday trough have faded away (around 16 Oct) – then that has the offerings of a reasonable voyage.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

02 October 2011

BOBGRAM 2 Oct 2011

WEATHERGRAM
YOTREPS
Issued 2 October 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

This edition comes from Ohope beach… "Is that a dark fin I see before me?"
I shall be discussing weather with local community groups here on Monday.

I hope you managed to avoid the stronger than normal tidal currents of the past week, all due to a Perigean new moon - currents should be quieter this week, allowing for better diving :)

Northern hemisphere cyclone season is still going strong. NALGAE is second destructive typhoon to visit Philippines in a week, and OPHELIA and PHILIPPE are still in North Atlantic, staying offshore.

The tropical ocean temperatures in west pacific are near normal, however there is a slow intensification of a cool anomaly around Galapagos --- so there is a tendency towards a La Nina, but it is more in the east than in the west.

From the atmosphere, the Southern Oscillation Index SOI is positive and increasing: its 30day running mean was 0.75 on 21 Sep and 1.2 on 2nd October. First time is has exceeded 1 since April, so yes there a change occurring in the air. Over next few weeks I shall explain what this may do to the coming South Pacific cyclone season.

The South Pacific Convergence Zone SPCZ has been in its normal zone from Solomons to Fiji then southeast across Tonga and to south of Southern Cooks. The eastern side to this zone is weakening at present as a new zone has formed – rather weak - over Coral Sea. This trough is at present linked via upper jetstreams with a mid-latitude feature and should cross New Caledonia on Monday and Fiji on Tue/wed 4/5 Oct and fade over Tonga on wed/Thu 5/6 Oct. Another tropical trough may form between Southern Cooks and French Polynesia this week, speaking up for the 'proper' SPCZ.

SUBTROPICAL RIDGE STR
The STR seems to be south of its norm position of around 25S. That High around 1030hPa near 35 to 40S to east of NZ should continue to migrate along that latitude this week, out to way southeast of French Polynesia--- with a slight enhancement of the trade winds on its northern side.

The next High is likely to take a path along slightly more southern latitudes---around 45S to 47S—crossing Tasmania on Tue 4 Oct and skirting southern South Island on Thu 6 Oct, then spreading north be cover North Island by Sun 9 Oct.


TASMAN SEA/NZ AREA
The low that is currently in the Tasman Sea is under a slow-moving long wave trough and so is likely to take its time crossing the NZ area this week. Its main centre should move onto central NZ by Tuesday 4 Oct, and then form a secondary 'back' in the Tasman Sea on Wednesday 5 Oct. This complex should finally get to east of the North Island by Sat 8 Oct—mostly being pushed off its perch by the rising pressures of the High, rather than moving via its own pressure falling techniques. Some spats of heavy rain with the low's frontal bands are likely here and there Mon and Tuesday, and the cold southerlies arriving on Tuesday may bring a dash of snow to the Kaikoura ranges.

SAILING TO/FROM NORTHERN NZ.
If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then you may be interested in following the ICA All Points Rally, see http://www.islandcruising.co.nz/ for this voyage.

The slow-moving Low crossing NZ is actually GOOD NEWS, as it provides a reasonably steady westerly flow over seas north of NZ from Tues 4 to Friday 7 Oct. OK there may be a mediocre front and southerly change near 30-35S on Saturday 8 Oct, but then the forecast is for light winds over Northland on Sun 9 Oct and steady northerly flow there on Mon/Tue 10/11 Oct. So if you can arrange to depart New cal/Fiji/Tonga at the right time (considering the incoming trough) then a relatively straight forward voyage is in the offering this week. Not 100% but not bad either.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

24 September 2011

BOBGRAM issued 25 Sep 2011

WEATHERGRAM

YOTREPS

Issued 25 September 2011

Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

 

Northern Hemisphere is still at peak cyclone season, with HILARY west of Mexico, OPHELIA and PHILIPPE in Atlantic and NESAT about to cross northern Philippines.

 

The tropical ocean temperatures in mid pacific continue to be near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos---so much that it has exceeded USA's Climate prediction centre CPC's threshold and they have called it a new La Nina.  It is sort of neutral with bursts of La Nina at times.

 

From the atmosphere, the Southern Oscillation Index SOI is positive and increasing: its 30day running mean was 0.44 in 17 Sep and 0.75 on 21 Sep.  Some lingering La Nina weather patterns persist in the atmosphere, and the Subtropical ridge STR in the Australia/Tasman Sea area is now being taken south of its normal position.

 

The South Pacific Convergence Zone SPCZ is persisting from Solomons to Fiji then southeast across Tonga and to south of Southern Cooks.  The section across northern Vanuatu/Fiji/Tonga is weak at present but is likely to activate from Thursday 29 Sep to Sunday 2 Oct.

 

SUBTROPICAL RIDGE STR

The STR seems to be south of its norm position of around 25S.  That High near 40S in the South Tasman Sea tonight is expected to travel northeastwards across NZ this week and may be located near 30S 170W by wed 28 Sep and then wander southeast to 38S 130W in early October.  Its intensity should stay below 1030hpa so that the trade winds on its northern side are likely to be enhanced a little but stay less than strong.

A large area of heavy SW swell associated with a low from the southern ocean is expected to reach NZ south coast by Wed 28 Sep and then move into the eastern flank of the High in early Oct – so avoid the centre of this high as it will have the combination of light winds and heavy swells – no good even for a power boat.

 

TASMAN SEA/NZ AREA

There is a developing Low between Sydney and Lord Howe Island tonight feeding off the jetstream aloft and the east Australian current from beneath. It should  move east along 30S until wed 28 Sep and then when it gets knocked a little to north of Norfolk Island,  but may redevelop near and to east of Kermadecs on Fri/Sat 30 Sep/1 Oct.   This low has strong to gale easterly winds on its south side, worth avoiding.

After that high crosses central an northern New Zealand on Tuesday 27 Sep, a North to NW flow is expected over the country for the remainder of the week : down-slope winds may be a problem for eastern coastlines, with temperatures foehning up into the 20s.

 

Looking further ahead a Low from the southern ocean is likely to reach Southland by Sunday 2 Oct, turning the flow over NZ to a squally westerly, then from Mon 3 to Wed 5 Oct next week, this low may deepen east of NZ,  bringing a polar chilled southerly outbreak to NZ eastern coastlines. Avoid.

 

 

SAILING TO/FROM NORTHERN NZ.

If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then you may be interested in following the ICA All Points Rally, see http://www.islandcruising.co.nz/ for this voyage.

 

On Mon 3 Oct to Wed 5 Oct a polar chilled flow hits Northland with W then SW then S winds, so don't plan on departing this week unless you are quick enough to reach Northland by end of Sun 2 Oct – and if you come south from New Caledonia, then also watch the progress of that low wandering along 30S.   

 

The terms used are more fully explained in the METSERVICE Yacht Pack.

           More info at http://weathergram.blogspot.com

Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

18 September 2011

BOBGRAM issued 18 Sep 2011

WEATHERGRAM

YOTREPS

Issued 18 September 2011

Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

 

North Atlantic, still boisterous with cyclones ROKE and SONCA – they seem to be staying offshore.

 

The tropical ocean temperatures in mid-pacific are near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos--- so much that it has exceeded USA's Climate prediction centre CPC's threshold and they have called it a new La Nina.  Over the entire Pacific, it is sort of neutral with bursts of La Nina at times.

 

From the atmosphere, the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.49 on 10 Sep and 0.44 in 17 Sep.   There has been some lingering La Nina weather patterns in the atmosphere last month, but the sub tropical ridge STR is now near its normal position. 

 

In fact: the South Pacific this week seems to be a continuation of last week- with trade winds in the tropics, disturbed westerly winds to south of 30S and the subtropical ridge STR sitting mainly along 25S—something like the seasonal norm, and this week's disturbed westerlies are looking to be quieter than last week's.

 

SUBTROPICAL RIDGE STR

A steady STR staying along about 25S.   STR in this position should maintain fresh to strong SE winds in the Coral Sea. 

One weak high cell should cross the North Tasman Sea on Monday 19 Sep and North Island on Tuesday 20 Sep then weaken away.  Another high cell is likely to weaken away in north Tasman Sea on Thursday and Friday 22/23 Sep.  Next high should advance across Tasmania on Sat 24 Sep, Tasman Sea on Sun 25 Sep and North Island on Mon 26 Sep.  These systems are not penetrating past the dateline at this stage.

 

TASMAN SEA/NZ AREA

Disturbed westerly flow in Tasman Sea latitudes is a good sign--- it is caused by the warmth of the extra sunlight in the Southern Hemisphere getting further south.  The stronger temperature difference between tropics and polar regions is what feeds these westerlies.  The equinox is around Friday 23 Sep, so this is THE week of the year when we have the strongest temperature difference.

 

Last week the disturbances were severe at times… hail in Wellington, a tornado in Auckland and, today,  more hail in Auckland and Bay of Plenty.  It seems that the coming week should be slightly quieter. The main disturbances should cross NZ on Mon 19 Sep (followed by cold air over southern districts), Thu/Fri 22/23 Sep and Sat/Sun 24/25 Sep (with more cold air in the south).

 

 The timing of these sequences may still change and anyone following the Rugby games in NZ can catch an update on the weather forecasts for the games from

http://www.metservice.com/towns-cities/rugby-world-cup-2011

 

SAILING TO/FROM NORTHERN NZ.

If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then the next few months is the optimum time.  You can join the ICA All Points Rally, see http://www.islandcruising.co.nz/, for this voyage.  My suggestion is that you try and meet a front at 30S – that way you will avoid meeting a front upon arrival in Northland. -

 

I'm just back this evening from a great Boat Show in Auckland.

Great new site-- thanks to organizers, and to Auckland City for helping sorting the parking!

 

The terms used are as explained in the METSERVICE Yacht Pack.

           More info at http://weathergram.blogspot.com

             Feedback to home: bobmcd@xtra.co.nz

               work: bob.mcdavitt@metservice.com

10 September 2011

BOBGRAM issued 11 Sep 2011

WEATHERGRAM
YOTREPS
Issued 11 September 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Three cyclones (KATIA, MARIA and NATE) in the North Atlantic and some areas of interest to wets of Mexico…so Northern Hemisphere Cyclone season is still raging.

The South Pacific this week offers trade winds in the tropics, disturbed westerly winds to south of 30S and the subtropical ridge STR sitting mainly along 25S. The daily maps look reasonably close to seasonal norm, but just for this week.

The tropical ocean temperatures in mid pacific are near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos. This has prompted USA's Climate prediction centre to call this a new La Nina, but Australian and NZ climatologists are happy enough to call it neutral - let's compromise with neutral + bursts of La Nina at times.

From the atmosphere, the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.46 on 2 Sep and .49 on 10 Sep. There have been some lingering La Nina weather patterns in the atmosphere--- the STR was further south than normal earlier this month, but is near its normal latitude this week.


SUBTROPICAL RIDGE STR
No large highs this week in our area, just a weak STR along about 25S. Next high should be moving off Australia and along 30S into northern Tasman Sea from Thu 15 Sep to crossing NZ on 22/23 Sep. This provides a good window for sailing westwards across the north Tasman or eastwards across the mid Tasman Sea.

Rising pressures this week over eastern Australia are helping to enhance strong SE winds in the Coral Sea.

TASMAN SEA/NZ AREA
The Low that crossed southern NZ today in followed by a period of squally SW wind and heavy swells in the Tasman/NZ area until at least Wednesday 14 Sep, and these then spread eastwards of NZ as a disturbed westerly flow. Avoid.

Another trough is likely to deepen in the central Tasman Sea on Friday 16/ Sat 17 Sep and cross North island on Sat 17/ Sun 18 Sep then move off to east of NZ - but this feature is still somewhat uncertain, so if it concerns you then get updates on it .

SAILING TO/FROM NORTHERN NZ.
The Island Cruising Association ICA fleet are starting to think about their All Points Rally from Tonga/Fiji/Vanuatu/New Caledonia to Opua. Well, my idea about the best time to leave from any of these places is so that you encounter a front at 30S – one that is followed by a mediocre ridge. The reasoning is that this voyage takes around a week and fronts in this part of the world have a frequency of at least once a week, sometime more frequent. The disturbed westerlies over Northland this week do not offer much opportunity for this pattern. So let's just enjoy another week of tropical bliss this week, and check in again next Sunday/Monday.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to home: bobmcd@xtra.co.nz, work: bob.mcdavitt@metservice.com

04 September 2011

BOBGRAM issued 4 Sep 2011

WEATHERGRAM
YOTREPS
Issued 4 September 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

We are now moving into the month where daylight increases fastest in the Southern hemisphere ---It breaks out onto the Antarctic circle at around the equinox – that's the Antarctic dawn, so its their coldest time of the year. Since the westerly winds of the roaring 40s get their energy from the temperature difference between the tropics and the polar regions, these winds are normally at their strongest and furthest north at this time of the year--- some call these equinoctial gales—I like to think of them as the gales of the Antarctic dawn.

This year things are not quite normal. The tropical ocean temperatures are near normal so the ENSO is in neutral and not having much impact at this stage. However the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.38 on 28 Aug and 0.46 on 2 Sep. This has been producing some lingering La Nina weather patterns in the atmosphere--- and one which has been happening in the South Pacific has taken the current high of the subtropical ridge south of "normal", weakening the roaring 40s.
HOWEVER, it now looks as though the South West Pacific may be in for a period of roaring 40s weather next week and possibly the following week as well.

4 Cyclones are raging in the Northern hemisphere… In NW Pacific TALAS is weakening over Japan and a new one (currently called TC SIXTEEN) is waiting in the winds. Around USA, Southern Louisiana seems to be coping OK with LEE whilst KATIA awaits offshore (KATIA is the replacement name for the now retired name KATRINA).

In our tropics, the South Pacific Convergence Zone got weak over the past week. There are clusters of convection north of Solomons, just east of Samoa and around Northern cooks but northing organised. The main convection that went from southeast of New Caledonia to southeast of Fiji (where it is tonight) is fed by a jet stream rather than convergence--- and will bring squally rain to southern Tonga until late Monday UTC (avoid). This system should continue heading east southeast and bring variable winds of a trough over Southern Cooks on Tue 6 Sep UTC.

SUBTROPICAL RIDGE STR
The current HIGH making its way eastwards across NZ is somewhat south of the normal STR September position, so kiwis should enjoy its light winds whilst they can. Meanwhile it is likely to produce strong trade winds near 20S all the way from about 160W to 180 until Wed 7Sep. Fiji Met Service have a gale warning is place on this squash zone.

Next high is expected to move east off Australia next week , around Tue 13 Sep , and along 25 to 30S into the North Tasman Sea This is the more normal latitude for the STR in September and more indicative of spring and more encouraging to the roaring 4os pattern.

TASMAN SEA/NZ AREA
Between every two highs there is a trough, and in fact there will be a run of troughs across NZ after the High moves off on Thu 8Sep. Firstly a small low may form between New Caledonia and Northland on Friday 9 Sep and then move southeast brushing past NZ's northeastern areas on Sat 10 Sep. This just marks where a trailing upper trough gets moistened by an upper jet - not expected to bring much.

The next trough should be more substantial and should build into a well-wound up low off Sydney by late Friday 89 Sep, and followed by gale force south to southwest winds in the Tasman Sea for Sat/Sun /Mon 10/11/12 Sep, along with strong wet mild nortehrlies over NZ on Sun 11 Sep. Avoid.

Considering the location of the next high being to north of most of NZ, it seems that roaring 40s or disturbed westerlies are likely over NZ during the week Mon-Fri 12 to 16th Sep--- This may change , so read in again next week.

SAILING TO/FROM NORTHERN NZ.
The passing high makes this an OK week to depart or arrive, but NE winds ahead of approaching trough rules out a good departure from Thu to Fri 8 0r 9 Sep.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

28 August 2011

WEATHERGRAM
YOTREPS
Issued 28 August 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI is hovering slightly above zero:
its 30day running mean was 0.47 on 20 August and 0.38 on 28 Aug.

Cyclones in the Northern hemisphere… NANMADOL intensity 85kt already responsible for around 10 fatalities in Philippines, heading for Taiwan on Monday 30 Aug, and likely to make landfall in China. And there is one called TALOS waiting in the wings and likely to make landfall on southern Japan later this week. The remains of IRENE (latest reported intensity 70kt) is moving north along US east coast...its dangerous right front quadrant is forecast to mostly miss New York and hit Boston.

South Pacific Convergence Zone SPCZ split into three during past week, one from Solomons and across Coral Sea to New Caledonia has triggered a small low near Lord Howe Island. Another zone lies along 15S from just N of Fiji to Northern Tonga to Southern Cooks--- this zone is likely to help trigger a Low to deepen near 25/30S 150W on Mon/Tue 29/30 Aug that will then move off to South, weakening the winds over southern French Polynesia for a few days. The third convergence zones lies roughly along 10S mainly about Tuvalu and Tokelau, and west of Marquesas. By the end of this week these are likely to all merge into one zone roughly from Solomons to Southern Cooks.

SUBTROPICAL RIDGE STR
STR had upper support over New Zealand area last week (long wave ridge), but there is likely to be a phase shift towards a long wave trough in this region this week.
The high that starts this week moving off to east of NZ along around 45S should split into weaker zones at 30S and 50S by Tuesday 30 Aug and these should move off to the east at different rates.

Of more intensity, a new high last of over 1030 hPa may move into south of Tasmania on Thu 01 sep and then northeastwards across the Tasman Sea to reach northern North Island by wed 7 Sep.

TASMAN SEA/NZ AREA
Remains of the rain /low pressure system that is today near Lord Howe Island should cross NZ area on Mon/Tues29/30 Aug. This should be followed by a westerly flow of increasing intensity on wed/Thu 31 Aug/01 Sep (wet in west/warm in east), and then a trough and southern change on Friday 02 /Sat 03 Sept. The GFS model currently has a deepening low east of the South Island along with this Fri/Sat southerly, making it something of a polar outbreak--- but EC model at bit.ly/ecoz, simply has it as a transition trough leading in the new high. As the day draws near these models may start to get more pieces of the puzzle and resolve the scenario more closely—if this affects you then keep checking. No point in hyping one idea yet when models still disagree.


SAILING TO/FROM NORTHERN NZ.
Avoid departure /arrival on Mon/Tue and reconsider Friday/Saturday, otherwise things are looking good.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

21 August 2011

BOBGRAM issued 21 Aug 2011

WEATHERGRAM
YOTREPS
Issued 21 August 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI is in the positive and relaxing this week: its 30day running mean was 0.87 on 12 August and 0.47 on 20 August. .

South Pacific Convergence Zone SPCZ is steady in the zone from Coral Sea to New Caledonia. This zone has been caught by a jetstream and is being wrapped into a small surface low that started off near the east Australia coast a few days ago. This system has a few more days to run, mainly between Lord Howe and Norfolk Island and then should move west towards the Queensland coast and fade. Avoid.

There is another convergence zone – strong to east of northern Vanuatu then weak and lying across Wallis and Futuna then southeast towards French Polynesia. This system is expected to remain slow-moving this week, and then is likely to activate early next week with a LOW forming between Southern Cooks and French Polynesia.

SUBTROPICAL RIDGE STR
STR had a few weak weeks in the South pacific and is about to go through a counter-balancing strong period. The BFH Big fat High in Tasman Sea is a slow-mover and should finally reach Northland on Sat/Sun 27/28 Aug and then next week move along 40S to east of NZ. There are enhanced trade winds in a squash zone in the tropics on north side of this high.

TASMAN SEA/NZ AREA
Westerly winds are expected to return to South island by Wed 24 Aug, warming up places east of the divide. A strong gusty NW flow should arrive over much of NZ during the Sat/Sun 27/28 weekend, and an active trough then may cross NZ on Mon/Tue 29/30 Aug.

SAILING TO/FROM NORTHERN NZ.
The squash zone in the tropics is only recommended for spirited sailors and is expected to start easing by end of the week. It is OK until Friday to depart from Northland to north or east, and Sat/Sun 27/28 Aug are looking good as target arrival dates for voyages to Northland.

Congratulations to Bruce Arms for wiping almost 3 days off the record for a solo circumnavigation of Australia. Way to go , bro!.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

14 August 2011

BOBGRAM issued 14 Aug 2011

WEATHERGRAM
YOTREPS
Issued 14 August 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI continues to hover in the positive: its 30day running mean was 0.79 on 5 August and 0.87 on 12 August.

South Pacific Convergence Zone SPCZ is steady in the zone from Solomons to Fiji. There is another convergence zone lying from west to east from Tokelau to Northern Cooks sort of along 10S.
That portion of the SPCZ which is over Fiji at present is expected to travel east (the upper winds are westerly), and reach Tonga on Wed 17 Aug UTC and Southern Cooks on Fri 19 Aug UTC and then fade over southern French Polynesia FP this weekend.
The portion of SPCZ near Solomons remains in place and should be able to spread south into Coral Sea by Sat 20 Aug and onto Vanuatu/New Caledonia this weekend.
Maybe some strong SE winds in the western Coral Sea this week.

SUBTROPICAL RIDGE STR
STR is weak over Southwest Pacific at present….a fading high south of FP extends a weak ridge westwards mainly along 25S to south of New Caledonia and then southwards across the Tasman Sea to a high over 1032hPa in the south Tasman sea
--- this Tasman High is centred near 50S at present, shovelling a cold southerly flow onto NZ. It should just slowly wander along 50S to be south of NZ by Thu 18 Aug and then move NE along eastern NZ this weekend and set off to east of North Island along 40S early next week, intensifying to 1038 for a while on Monday 22 August.

TASMAN SEA/NZ AREA
There is a polar blast over NZ today, shepherded by large areas of low pressure to east of the country and that 1032+ high in the south Tasman High. There may be a small low caught in these southerlies and sideswiping the east North Island Coast on Tuesday 16 August UTC intensifying the winds. The polar blast is making for large seas that may peak on Tuesday 16 Aug at around 5m in the eastern Tasman and around 9m between Canterbury and Wairarapa coasts. Avoid.

Southerly winds should turn SE then E and ease, first in the south Wednesday 17 Aug, reaching North Island by Sat 20 Aug.

A new LOW is expected to move across Australia and deepen off its east coast near Sydney on Thu/Fri 18/19 August and then wander to the NE near Lord Howe on Sun 21 Aug and Norfolk around Tue 23 Aug, preceded by an area of strong NE winds. Avoid.

SAILING TO/FROM NORTHERN NZ.
Polar blast winds and sea should ease enough for OK sailing from Northland on Thursday/Friday…after that the NE winds of the next Low may interfere.

It isn't really the right weather for coming south but if you intend to, then Friday/Saturday are looking OK weather for arrival days.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

07 August 2011

BOBGRAM issued 7 Aug 2011

WEATHERGRAM
YOTREPS
Issued 7 August 2011
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI continues to hover in the positive: its 30day running mean was 0.76 on 29th July and 0.79 on 5 August.

Galapagos to Marquesas: If you are still looking at this voyage, it seems that there is a slight advantage with the wind to first head off to 3dge 30min S at 100W and then to 6S at 133W and then to Marquesas. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ spent most of last week from Papua New Guinea PNG to Fiji/Tonga and has now shifted to be from PNG more to Tuvalu/Samoa/Southern Cooks. That part of the zone which is east of the dateline is likely to wander north and weaken over next few days. The portion west of the dateline will probably stay put.

SUBTROPICAL RIDGE STR
At present there is a dominant and extended example of the Subtropical Ridge STR along 30S from just north of NZ all to way to south of Southern Cooks.

In a weird way the 'Roaring 40s' westerly pattern has gone awry in the Australian Bight so that the STR there has been deflected SOUTHWARDS to near 50S. This part of the STR has one high cell within it that is likely to fade south of Tasmania on Fri 12 Aug and then another cell should wander along 50S to 45S in the South Tasman Sea by Monday 15 Aug.

TASMAN SEA/NZ AREA
NZ starts off with a strong disturbed southwest flow, easing away on Monday/Tuesday 8 Aug.

There is a weak tongue of high pressure across the Tasman Sea linking those two branches of the STR together. This weak link is expected to cross Tasman on Monday 8 Aug and NZ of late Tuesday 9 Aug/Wed 10 Aug. It will bring a period of light winds and clear skies good for frosts and fogs.

The LOW that is over South Australia/New South Wales at present is 'out of position' as far as the normal pattern is concerned. It started in the Southern Ocean and got knocked into the normal STR latitudes by some upper winds--- brought a shock of cold air to Melbourne. Anyway, it should cross the Tasman on Mon-Tue 8-10 Aug and then cross NZ on late wed/Thu–Fri 10.11.12 Aug, proceeded by some warmer NE winds.

This paves the way for a polar blast over NZ on Sat 13th/Sun 14th Aug, shepherded by low pressures to east of the country and an incoming High in the South Tasman Sea. Avoid.

SAILING TO/FROM NORTHERN NZ.
SW flow on Monday is still good for going north. NE on late Tue/Wed is good for coming south. Fronts on Thursday/Friday are probably no good for anyone. Strong SW/southerly wind changes Sat /Sun 13/14Aug worth avoiding. It may be good again for going north early next week.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com

30 July 2011

BOBGRAM issued 31 July 2011

WEATHERGRAM
YOTREPS
Issued 31 July 2011
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI continues to slowly rise; its 30day running mean was 0.5 on 16 July and 0.6 on 22 July and 0.76 on 29th July.

Galapagos to Marquesas: If you are still looking at this voyage, it seems that there is a slight advantage with the wind to first head off to 3dge 30min S 100W and then to 6S 133W and then to Marquesas. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ is slowly returning after a weak period. The main band is affecting Solomons Vanuatu and Fiji. There are weak convergence zones along 3 to 5S, and near 15S between Samoa and French Polynesia.

Not much change with the SPCZ during the coming week, except for a low budding off to south of Fiji on Mon-Tue 1-2 Aug then moving off to SE then E, possibly re-intensifying near 30S 150W on Sun 7 Aug, with its trough affecting French Polynesia FP on Sun-Mon-Tue 7-8-9 Aug UTC.

Another trough MAY bud off near New Caledonia on Wed-Thu-Fri 10-11-12 Aug— but unsure at this stage, tune in again next week for an update on that.

SUBTROPICAL RIDGE STR
Subtropical ridge is expected to be strong this week, getting support from aloft. The high that is crossing NZ may stall for most of this week in North Tasman Sea. There should be some enhanced trade winds north of this high cell—affecting the Coral Sea on Mon 1 Aug and this affect spreading east to reach the Fiji area from Fri 5 to Tue 9 Aug – this little squash zone should be OK for sailing but not so good on divers, and should continue east onto Samoa, etc., next week.

Next High cell should cross Australian Bight Thu-Sat 4-6 Aug and eastern Australia Sun-Tue 7-9 Aug.

TASMAN SEA/NZ AREA
High hanging around northern and central areas until Thursday should bring frosts and fogs. There should be enough westerly flow over southern areas to just bring showers in the west, but a Roaring 40s trough on Thursday may bring a cold southerly change to all of NZ on Fri-Sat 5,6 August .

SAILING TO/FROM NORTHERN NZ.
Not a good week for sailing southwards.
OK for sailing northwards from Tues to Friday 2-5 Aug --- depending on how next week's trough near New Caledonia develops.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

24 July 2011

BOBGRAM7 issued 24 July 2011

WEATHERGRAM
YOTREPS
Issued 24 July 2011
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI continues to hover slightly above zero; its 30day running mean was 0.5 on 16 July and 0.6 on 22 July.

Galapagos to Marquesas: The sea around Galapagos now seem to be reverting to normal temperatures. If you are still looking at this voyage, it seems that the direct path is as good as any other. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ is having a weak period. There is one band that is active over Solomons and then weakens as it extends SE to east of Vanuatu and about or to Southwest of Fiji. There is also a weak convergence zone in the French Polynesia FP region.

Not much change with the SPCZ during the coming week, except that a trough may form on it around or to south of Fiji/ New Caledonia on 31 July.

SUBTROPICAL RIDGE STR
The high that is east of Tasmania tonight 24 July is shoveling a polar chilled southerly flow onto eastern NZ. It is showery stuff but with enough snow and ice to impede traffic. This High is expected to travel NE across Tasman Sea on Mon and then, when it gets to 30S on Tuesday, to extend itself east along 30S from 160E to 150W, intensifying the trade winds in the tropics and the westerly flow on its south side.
The SE trade winds in the tropics are likely to intensify over the whole South Pacific and reach a peak around Wed/Thu 27/28 July due to this rebuilding of the STR.
The STR may be reinforced by another high cell making its way across central Australia and budding off at 30S around Thu /Fri 28/29 July.

This is a typical weather pattern.

Next weekend: as this STR goes east and when air-pressures start to fall between Fiji/ New Caledonia and NZ: a trough is likely to form – GFS has this by 31 July, and EC has it, at this stage, around 1 Aug. For the EC – see http://bit.ly/ecoz .

'Roaring 40s and a deep low' is the expected pattern for Aussie Bight next week —no sign of the next High until end of next week.

TASMAN SEA/NZ AREA
Polar chilled southerlies should turn to be from the SW and weaken on Monday as that high migrates across the Tasman Sea.

A disturbed westerly flow is then likely for Tue 26 to Sat 30 July, with an embedded front on Wed and Friday.

SAILING TO/FROM NORTHERN NZ.
Useful winds on Tuesday to Saturday--- but check again on the possible trough to north of NZ next weekend, and maybe avoid the fronts on Wednesday and Friday.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

17 July 2011

BOBGRAM7 issued 17 July 2011

WEATHERGRAM
YOTREPS
Issued 17 July 2011
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI has jumped up a little in past week; its 30day running mean was 0.03 on 8 July and 0.5 on 16 July.

Galapagos to Marquesas: The seas around Galapagos are slightly warmer than normal. Even so, at present there seems to be little extra advantage is straying from the rhumb line/great circle.

South Pacific Convergence Zone SPCZ is active from northern Coral Sea (near Solomons) in bands --- one to Vanuatu, another to Rotuma area then Tonga, and a third to Tuvalu. That's about it at present and there isn't much shifting about either.

SUBTROPICAL RIDGE STR
After a long wave trough over the Tasman Sea / NZ area for the past 2 weeks, a subtropical ridge has finally managed to make into the Tasman Sea this weekend. This High should move east across NZ on Mon/Tues at 40S and then stretch northeastwards and weaken during remainder of the weak, so not much of a squash zone in the trade winds on its northern side.

Next High is starting to form in Australian Bight, and consists of some polar air – so is a frosty one, being modified as it encounters warmer water (brewing unstable showers). Should cross Tasmania on Sun 24 July and then move NE across Tasman Sea so that it passes by to north of Northland around Thu 28 July… a normal winter path, allowing a disturbed westerly flow over Tasman NZ by late next week.


TASMAN SEA/NZ AREA
A return to the pattern we had in June, with a moist mild northerly flow over NZ as a Low forms in the mid Tasman Sea. This Low may be complex and multi-centred, and there may be strong winds near some of its compact centres… especially one near Lord Howe on Tue 19 July. Its main frontal rain band should stall over the North Island on Thursday/Friday 21/22 July and an easterly flow should stay over central NZ as the Low weakens and wanders east across area north of NZ on the weekend 23/24 July. This should set the scene for a southerly blast over all NZ on Mon/Tue 25/26 July between the low (then moving off the SE) and the incoming High.

SAILING TO/FROM NORTHERN NZ.
Departing from Northland – There are challenges. Avoid that incoming front especially on Wednesday/Thursday, 20/21 July. There may be a window on Friday/Saturday 22/23 July, going clockwise around a passing weakening low. Then take care to double-check the strength of the southerly outbreak that is likely on Mon/Tues 25/26 July.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

10 July 2011

BOBGRAM7 issued 10 July 2011

WEATHERGRAM
YOTREPS
Issued 10 July 2011
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

TROPICS
Southern Oscillation Index SOI remains in neutral territory: 0.08 on 3
July and 0.03 on 8 July.

Galapagos to Marquesas: The seas around Galapagos are slightly warmer
than normal and the winds there are now southerly 5 to 15knots. So the
optimum route is to head due west across these southerlies to 95W and
then head for a waypoint near 6S 134W.

South Pacific Convergence Zone SPCZ is active from northern Coral Sea
(near Solomons) to Vanuatu and then weakly active to the Fiji Area. The
part near the Solomons is expected to wander southwards into Coral Sea
this week.
There are strong SE winds over northern Queensland - associated with the
SPCZ and the high pressures stuck this week over central Australia.
There is a minor branch of the SPCZ coming and going from Tuvalu to
French Polynesia. This zone extends a trough to the southeast into the
mid-latitudes and a LOW is expected to form within this trough between
30S and 35S near 135W on Monday UTC.

SUBTROPICAL RIDGE STR
The High that is southeast of French Polynesia today, 10 July, has a
zone of enhanced trade winds on its northern side, but is now moving off
to the east and fading away.
There is a very weak Sub-tropical ridge along 25S across the South
pacific this week- typical of mid winter.
The next high is expected to stay over central Australia until Fri 15
July and cross the Tasman Sea on the 16/17 weekend and then hang around
central NZ during next week.

TASMAN SEA/NZ AREA
The disturbed westerly flow which arrived here last week is set to stay
for next few days. Between the 1020 isobar of the subtropical ridge and
the 960 isobar near 50S there are 80hPa and westerly gradient across the
area-so the embedded squall lines continue to be thundery - and snowy in
the SW mountains.
There is a SW storm near Tasmania tonight 10 July, and this should move
onto southern NZ on Wed 13 and remainder of NZ on Thu/Fri. There may
then be that new High in the Tasman Sea and a deepening low near (or
east of) Chatham Islands, with an eggbeater of a S or SW flow inbetween
harassing eastern coasts of NZ.
Heavy SW swell is likely in Tasman Sea until 14 July - avoid.

SAILING TO/FROM NORTHERN NZ.
Departing from Northland - wait a while: westerlies may be strong at
times until Thursday 14 July. Conditions should turn SW on Friday 15
July with barometer rising above 1012, and that may be OK for a
departure... but check again nearer that time as there may be a problem
low developing between Northland and Tonga next week.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

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