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

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

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