BOBGRAM issued 25 Nov 2012

WEATHERGRAM
YOTREPS
Issued 25 Nov 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 Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) has a long time low of MINUS 1.01 back on 25 Aug. It has been gradually rising ever since and on 23 Nov was plus 0.5. Computer models are picking a neutral SOI for the cyclone season.


Tropical cyclones: The Indian ocean is now having the second cyclone of this season TC BOLDWIN. Another cyclone is likely to form near 5N 157E in the NW Pacific.

South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ has gathered in strength during the past week between Solomons /Vanuatu and Fiji/Tonga. A tropical depression formed over Vanuatu and took a path southeastwards to south of Fiji and close to Minerva and then off to the south of Niue. This is similar to the Low we had a few weeks ago at the beginning of November, and we are likely to get such systems in the tropics around once a fortnight for the remainder of this cyclone season. If you are still in the tropics seeking to get out then try and do so between these systems. This coming week is looking like one of those weeks, between depressions.

SUBTROPICAL RIDGE STR
The STR is likely to stay along 25 to 30S this week as one high cell stalls and fades in the north Tasman Sea and another High forms behind the path taken as that Low which is tonight south of Niue moves off to the southeast . It sits as a dependable zone of fickle winds between the trade winds of the tropics and the disturbed westerlies of the roaring 40s.

NZ/Tasman Sea
A deep LOW below 970 hPa is moving east tonight along 60S across the South Tasman Sea and should peak and move NE on Wednesday when it is expected to be southeast of Chatham Island. Several waves of cold air are expected to be shovelled northwards onto NZ as this Low goes east and a High forms in the South Tasman Sea. Avoid getting between Low and High, for the SW swell may build to over 9 metres there. Each front should be colder and followed by wind more southerly than the previous, with one front crossing North Island) mainly eastern areas) on Tuesday, another on Thursday and the main one late on Friday. That last one is likely to open out into a significant trough and form a low east of the North Island, bringing a southerly change all the way to Tonga on Saturday. Worth avoiding.

If you are sailing to NZ this week from Tonga, the trade winds are looking OK for starters. Be prepared to motor through the light winds of the sub-tropical ridge. That Friday front may be useful for a sailing breeze at 25 to 30S even if it is followed by SW winds. From Saturday 1 Dec for a few days it will pay to head for a spot North and somewhat West of Opua , such as 33S 173E so that you are in position for sailing a few days of SW winds.
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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/2012/11/04/weathergram-4/
Weathergram text only http://weathergram.blogspot.co.nz
Website http://www.metbob.com
Feedback to bob@metbob.com