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

08 September 2013

BOBGRAM issued 8 Sep 2013

WEATHERGRAM
YOTREPS
Issued 08 Sep 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.

Background influences
Jet streams
This week, and for the next few weeks Lows and Highs in the NZ area are expected to be more extreme than they have been lately, assisted by jet-streams which visit the NZ area at this time of the year.

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has relaxed to around -0.07 (1 to 8 Sep).

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet.
This parameter has been near average so far this year, slightly on the cool side.

GLOBAL TROPICAL TOPICS
There are three possible areas of formation in the North Atlantic at present, and the low now over the Azores is likely to develop into a cyclone by Wednesday and then peel off into the mid-Atlantic.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has increased in intensity over the Solomons, and extends as a trough which during the weekend has shifted from Fiji/Tonga to Samoa/Southern Cooks. This trough and the westerly wind that follows it is likely to skirt the Austral group on Monday UTC, and the main part of the SPCZ is expected to weaken.
There is a weak part of the SPCZ along 10S between 140 and 170W and this is expected to intensify during the coming week but should stay north of Suwarrow – this week anyway.

Sub-tropical Ridge STR
High in north Tasman Sea tonight is expected to travel eastwards along 30S past 180 on Monday and then E/SE along 30/35S for the rest of the week. This High is expected to intensify to over 1032hPa, creating a squash zone of enhanced SE winds between Tahiti and Samoa from Wed to Fri UTC (Tues to Thu local). This can bring some excitingly fast sailing but the roughened seas are likely to be uncomfortable.

Roaring 40s and New Zealand
It is to be a troughy week over NZ, with jet streams intensifying the surface winds, NW on Monday to Wednesday, and cold southerly on Friday 13th and Saturday.

Route Briefings
Tahiti to Tonga:
Squash zone over the route from local Tuesday to local Thursday with some enhanced trade winds. These conditions should ease from local Friday onwards.

Between NZ and Fiji/Tonga
Squally trough is expected to cover the area between northern NZ and 30S on Fri 13 Sep local and then a broader trough is expected on Sat 14 Sep.

See my yotpak at http://lnk.ie/INNO/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/INNP/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/INNQ/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/INNR/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
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