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

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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