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

24 May 2009

FW: BOBGRAM7 issued 24 May 2009

WEATHERGRAM
YOTREPS
Issued 24 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.

TROPICS/SUBTROPICS
The South Pacific Convergence Zone SPCZ was more active than normal as
it moved across New Caledonia and Vanuatu last few days, activated by a
passing trough. This week the Coral Sea is starting off reasonably free
from SPCZ convection but this is expected to build up again by 1 June
mainly over Vanuatu.

The squash zone mentioned in the last weathergram as peaking near Coffs
Harbour on Friday managed to produce wind and rain damage over southern
Queensland and northern New South Wales. Erosion is being reported as
worst in years. This squash zone is already moving off to the east and
is attached to the south side of the LOW now south of New Caledonia.

The squash zone mentioned in the last weathergram for northern NZ this
weekend has been bothering Cook Strait over last few days (11 metre
swells cancelled the ferries on Saturday) and is now likely t0 peak in
the Gisborne/Bay of Plenty area on Tuesday 26 May as the LOW from New
Caledonia makes its way southeast.

This LOW is expected to maybe do a loop between Raoul Island and North
Island on Monday/Tuesday. Anyone sailing north will find it OK to go
clockwise around this Low (light winds near the centre). All the strong
winds are tied up in the squash zones. There may be another subtropical
low forming just to south of Fiji too, and this low may grow as the one
south of New Caledonia dies. This is called a vorticity transfer.

There are a few showers on the Galapagos to Marquesas route.

SUBTROPICAL RIDGE STR / NZ WEATHER
The HIGH is the south Tasman Sea today is in a latitude more consistent
with LA NINA than the current neutral territory that is meant to be
affecting us. It intensified to 1038 and managed to draw a lot of
southern ocean air onto NZ over the past week, aided by a few lows near
Chathams. Well it is now starting to fade , decay, weaken, and filling
in-but will bring some clear skies/light winds and frosty dawns to the
South island on Mon/Tue, central NZ on Wed/Thu and the North island on
Thu/Fri. As this ridge spreads NE like this is brings a window of good
departing weather for anyone in NZ seeking to sail north--- just wait
for that squash zone to leave the North Island first. Wednesday
afternoon looks good at this stage.

The next trough from the west is expected to cross Tasmania on Tuesday
and then develop a LOW that move northeast across the South Island on
Friday, east across central NZ on Saturday and then southeast past
Chathams Island on Sunday, explosively deepening in the process, just
in time to muck up the Q Birthday weekend holiday in NZ.

Latest GFS model drops this low to below 960, and intensifies the next
HIGH, crossing to south of Tasmania on 1 June to 1040+ This seems to
be a computer wind-up to me, 80hPa of SW isobars attacking NZ on
Sunday??? Yeah, right. If so, avoid, but I'm sure this is model
extremism. The EC usually does a better job but seems to be offline at
this stage, so check again before your weekend sail.
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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