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

17 June 2012

BOBGRAM issued 17 June 2012

WEATHERGRAM
YOTREPS
Issued 17 June 2012
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 Southern Oscillation Index or SOI is becoming more negative. Its 30-day running mean was near zero at the start of June and was -0.68 on 17 June.
Indian Monsoon is about a week behind its normal position at present, but seems to be of normal intensity. It is taking a while to penetrate the Indian subcontinent interior. This may have ramifications downstream later this year.
In the South Pacific, the Convergence Zone SPCZ is active from north of the Solomons to Fiji to south of Tonga and south of Southern Cooks, with another band occasionally across the Vanuatu/New Caledonia area. A mid-latitude low dumped rain along 30S across the Tasman Sea. Conditions have been drier than normal elsewhere east of 180.
This week part of the SPCZ may move south over Fiji on Thursday 21 June, and the remainder of the SPCZ is likely to stay quasi-stationary. Early next week, the SPCZ may shift south across Coral Sea along with a drop in pressure.
SUBTROPICAL RIDGE STR
There is a narrow ridge over NZ tonight and this should expand and grow as it moves off to the east this week. A corresponding enhancement in the trade winds north of this high-that is , a squash zone- is likely over Samoa Tonga Niue on Monday to Wednesday and the Cooks then French Polynesia on Wednesday to Saturday. The squash zone may make diving conditions hazardous.
High over the interior of Australia is likely to bud off a ridge across the northern Tasman Sea along 30S from Thursday, fading to north of New Zealand by Sat/Sunday. This High is tracking north of the normal latitude-and that's an El Nino characteristic.

NZ /Tasman Sea troughs.
Welcome to the week with the least daylight - the winter solstice.
A Low and its associated front is set to cross NZ on Monday/Tuesday. A secondary Low is expected to cross NZ on Wednesday and Thursday after a brief break. There should be another break in the weather- good for departing off to the north – on late Friday and Saturday. Then another front is likely, with NW winds preceding it late Saturday and a good SW flow following it on Sunday.
Departing from NZ for the tropics:
It is probably a good idea to let the Lows go first and wait for a good SW. There is a brief window of suitable SW winds late Friday, and then another better looking one on Sunday 24 June.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback or requests for voyage forecasts to bob@metbob.com

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