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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

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