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

17 April 2011

BOBGRAM7 issued 17 April 2011

WEATHERGRAM
YOTREPS
Issued 17 April 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.

TROPICS
La NINA is still active in the atmosphere. The Southern Oscillation
Index 30day running mean has been on a roller coaster ride: from a low
of 1.85 on 25 March to a high of 2.51 on 9 April, and, by 15 April,
relaxed 2.40.

Recent pulse of active convection, known as a MJO period, has been
lowering the isobars in North Australia and has helped form TC ERROL far
to west northwest of Darwin; this system is expected to stay offshore
and go west.

In our part of the world the South Pacific Convergence Zone has been
stretching from North Australia across the Coral Sea and Vanuatu to
south of Fiji and then off to the southeast. Over the weekend a
Tropical depression has formed about and just south of New Caledonia,
taking with it a lot of the activity that was in the Coral Sea.

Fiji Met has a gale warning on the W /NW winds associated with this
system at present. It is already showing signs of fading as it goes SE
--- some of its upper vorticity is expected to be consumed by the trough
that is now getting east of NZ, and help deepen a large LOW to BELOW 980
near 45S 170W on Mon to wed 18 to 20 April. Avoid.

Not much else happening in tropics this week--- but low pressure in
Coral Sea and high pressure in Tasman Sea are likely to keep fresh trade
winds along 20S from New Caledonia to Queensland.

SUBTROPICAL RIDGE:
New autumnal HIGH is crossing Tasman Sea along 37S on Mon 18 April and
then slowly across North Island from Tuesday to Thursday 19 to 21 April.


Next HIGH is expected to form at 45S in South Tasman Sea on Sun 24 April
and then to swing around South Island on Mon 25 (ANZAC Day) and build
over Chatham Islands on Tue 26.

On the northern side of both these Highs, there are likely to be squash
zones of enhanced easterly winds worth avoiding.

NZ AREA
A cold southerly flow dominates eastern areas tonight and Monday, with a
marine gale + heavy swells. Avoid

For this year's Auckland to Tauranga Yacht race starting Thursday
morning a passing High is likely to give light winds--- as that High
moves off on Good Friday, a northerly breeze may help.

Then the next trough should cross NZ on Sat/Sun 23/24 April as a weak
transition southerly change moving from south to north.

It should be Ok for anyone thinking of sailing north from Northland - to
do so Tue to Fri, so long as they route through the subtropical squash
zones properly. Over Easter and for much of the last week of April, NE
winds speak against a smooth voyage.

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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