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

15 April 2012

BOBGRAM issued 15 April 2012

WEATHERGRAM
YOTREPS
Issued 15 Apr 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 continues its dive into negative territory. On 8 April its 30 day running mean was -0.6 and on 15 April it was -0.8; still way too early to call this a swing , but if it remains negative for another fortnight then we may be witnessing a swing into El Nino. And we may see trade winds easing, and the subtropical ridge may drift north.

South Pacific Convergence Zone SPCZ has been settling into more normal activity over the past week, compared to its extreme ran with an MJO event just before Easter. There's an extension of the zone in the Coral Sea, and the main SPCZ stretches from eastern Solomons or northern Vanuatu to Fiji. There has been a weak rather scattered zone from Tokelau to North Polynesia, and another weak branch just south of French Polynesia linking with a mid-latitude low that is tonight near 30S 140W.

Another mid-latitude low is likely to form at the SE end of the SPCZ to south of Niue on Tue 17 Apr. It should peel off to the SE by Fri 20 April

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Galapagos to Marquesas: There are squally patches near 3-4S 93-98W and near eq-2S 98-102W. Depart quickly SSW from Galapagos until you find the easterly winds they are near 5S, and then take the direct route to Marquesas.

SUBTROPICAL RIDGE STR
In recent weeks the STR has been taking on Indian-summer latitudes. The term 'Indian summer' has been applied in USA when autumnal anticyclones sit over the prairies and allow settled weather (after the first frost) to help Indians do some Buffalo hunting before winter sets in. Of course USA is now having a tempestuous spring. But NZ is having a large autumnal anticyclone this week, and some may well call its weather an 'Indian Summer'. It is forecast by some models to have a central pressure over 1038 in the Tasman Sea by Wed 18 April and to move slowly east over central NZ, not weakening until it gets east of NZ after Sun 22 April.

There is a zone of enhanced trade winds on the north side of this high,
But if you don't mind enhanced trade winds then this is an OK week for sailing from NZ for the tropical Islands.


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

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