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

31 May 2009

BOBGRAM7 issued 31 May 2009

WEATHERGRAM
YOTREPS
Issued 31 MAY 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

I welcome those who have recently signed on to these weathergrams.

TROPICS/SUBTROPICS
The South Pacific Convergence Zone SPCZ is settling down now after a few
active weeks. It is slightly north of normal from Solomons to Tuvalu to
Northern Cooks and has been affecting French Polynesia. A southern
band, which has really been the northern end of mid-latitude systems
coupled with rain maintained by the subtropical jetstream, has been
bothering Fiji and southern Tonga last week - this branch is expected to
stretch eastwards towards Southern Cooks by Wed/Thu, and then a small
low is likely to form on this branch near Niue on Fri/Sat and then move
off the southeast. Avoid.

The Galapagos to Marquesas route is all quiet now.

SUBTROPICAL RIDGE STR / NZ WEATHER
Last week I scoffed at the models for producing a 960 Low south of
Chathams for this weekend, and indeed it is just a normal 980 hPa low
that has turned up. Even so, the 1040+ HIGH has turned up to South of
Tasmania, and that is about as high as they get ... so that is around 60
hPa of southerly isobars for the South Tasman Sea and NZ today and
tomorrow, another polar-blast-weekend, but not as windy or snowy as
last weekend.

The swell now travelling north with this weekend's southerlies is
expected to peel off NZ on Monday and reach Tonga around Wed/Thu and
fade there this weekend.

That 1040+ High is a Big Fat High BFH and should be able to extend
eastwards across the Tasman, something which none of the other May Highs
were able to do.

And so it marks a change in weather pattern, for the coming week anyway,
into something more settled around NZ. Even so, it is likely to fizz as
it moves east, crossing southern NZ at less than 1025 central pressure
on Thu 4 June, followed by a front and SW change on 5-6 June (avoid) and
then another HIGH from 7 to 9 June, whilst a new LOW is now expected to
form in west Tasman Sea near 37S on 5-6 June and then fade over Northern
NZ from 9-11 June.

All this traffic means that there will NOT be any strong squash zone on
the north side of this HIGH, so the trade winds around Tonga, Fiji, and
New Caledonia are NOT going to be enhanced this week. It also means the
subtropical ridge between NZ and the tropical Islands is likely to
expand and broaden this week-a larger than normal zone of light winds
for anyway sailing northwards from Northland to have to motor through.
And so it is a week without swell for voyaging, and that's an enjoyable
change.

The next dose of wind over Tasman Sea/ Northland MAY be a period of NE
winds ahead of that Low mentioned in the Tasman, so may not start
arriving until Mon 8 June. This feature will need updating.

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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