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

10 February 2013

BOBGRAM issued 10 Feb 2013

WEATHERGRAM
YOTREPS
Issued 10 Feb 2013
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.
The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) dived to almost minus 1 during December, and relaxed to near zero during January. Around 3 Feb it was minus 0.4 and on 10 February it had dived to minus 0.9, so things are tilting in the favour of SW winds over the South Island now.

The Ocean: Sea surface temperatures across the equatorial Pacific act as a thermostat for the planetary weather engine. When they are different from normal we get a change in clouds around the equator and subsequent changes in the earthly wind zones has an impact on the latitude zones of weather across the whole Pacific.
Well, in the past month the SST in this target zone has mostly slightly below normal, indicating a tendency towards La Nina in the Ocean, and the ocean seems to be leading the atmosphere, perhaps.


WEATHER ZONES
There was a Madden Julian Oscillation of enhanced convection across the Pacific Ocean late January/early February and this has gone now… looking at the OLR measurements below it looks as though the next (and probably the last) chance for tropical cyclones this season is going to be with the next lot of below OLR (blue) shading in our region, during late February early March.


The South Pacific Convergence Zone SPCZ is mainly east of the 180 line, from Samoa to Tahiti and is expected to have a weak week ahead. Good for anyone wanting to do an island jump in the middle of the cyclone season.

The situation map shows a trough over Fiji on Sunday and cloud/rain in a low between Tonga and NZ. This Low is expected to weaken as it goes off to SE, from Wed 13 to Sat 16 February trade winds should then dominate the whole South Pacific north of the STR.

The Sub tropical ridge STR extends from Aussie Bight to north and then east of NZ with another branch along 30S from 160 to 120W.
The cell east of NZ is expected to be a slow-mover along 43 S expanding and travelling off to the east, leaving a lingering ridge over North Island until end of Monday.

A new High Cell is in the Aussie Bight and should squeeze around south end of Tasmania on Wed and Thu 13/14 Feb then travel NE across the Tasman Sea on Fri /Sat 15/16 and across NZ on Sun 17 Feb then off to the east.

TASMAN SEA/NZ TROUGHS
There is one front between those two highs. It is over Fiordland on Sunday night and should weaken as it travels east (NE across NZ) across South Island on Monday and off to the east of the North Island on Tuesday.
Acting as a herald for the new high, a southern ocean Low is expected to zip east on Wed/Thu 13/14 Feb, and on its northern end is cold front and southerly wind flick for eastern NZ. Heaps of swell around this low as well.
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See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://metbob.wordpress.com
Weathergram text only http://weathergram.blogspot.co.nz
Website http://www.metbob.com
Feedback to bob@metbob.com

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