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

22 August 2010

BOBGRAM7 issued 22 Aug 2010

WEATHERGRAM
YOTREPS
Issued 22 August 2010
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
The South Pacific Convergence zone SPCZ is slowly returning to normal
over Papua New Guinea and Solomons. It is putting in an appearance
around Tokelau, but otherwise is weak . Should continue to rebuild
from Tuvalu to Northern Cooks this week, and may start to drift south
for a while next week.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds as far as 15S over French Polynesia
and as far as 20S elsewhere. One of these is moving east off French
Polynesia today 22 Aug, and another should cross Tonga on 23 Aug, Niue
on 24 Aug, Southern Cooks on 26 Aug, and French Polynesia around 28-30
Aug. SO this week is something like a repeat of last week.

SUBTROPICAL RIDGE: STR
Upper air is such that Subtropical highs are being encouraged in north
Tasman Sea and in the broad zone east of NZ. There is a high cell in the
North Tasman Sea today 22 Aug: it should cross NZ on Tues 24 Aug and
then wander to the east slanting to 35S then 25S and reaching 150W to
South of French Polynesia by Sat 28 Aug. There should be a squash zone
of enhanced trade winds on the northern side of this high.

TASMAN/NZ
Weak frontal system is likely to cross NZ on Wed 25 Aug after that High
- preceded by a northerly flow and followed by a westerly.

A long wave upper trough is hovering around the Tasman Sea again this
week.

At ground level, we have a broad cold trough and a deep southern ocean
Low located today well to south of West Australia at 150E. This system
is the highlight of the coming week. Its Low centre should start moving
NE out of the southern Ocean when it gets south of Tasmania on Wed 25
Aug and reach NZ on Sat/Sun 28/29 Aug. However, there the High
following this system and it is expected to expand at 30 to 35S over SE
Australia on Sat /Sun 28/29 Aug. So that will limit the southerly fetch
this low can draw on, so it'll be wet and windy, but not a polar
outbreak.

SO it is OK to approach NZ until Tue 24 Aug. Then not OK until around 2
Sep.

The strength of the polar vortex was mentioned last week. Well, its
still holding its strength and is not weakening as much as the computer
models have been picking. Even if it does weaken to a stage that a polar
outbreak can occur in early September, the chance that this will impact
on our quarter of the world will be 1 in 4. So, while it is interesting
to watch for, it ISN'T another thing that yachties who are planning a
voyage from tropics to NZ or Australia need to worry about.


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

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