Bobgram issued 4 Aug 2013

WEATHERGRAM
YOTREPS
Issued 04 Aug 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
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. During early July it dropped to around 0.7 from kid to late July and rose to 0.9 in early August.

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.
So far this year NINO3.4 has been on the cool side, but rather weak.

GLOBAL TROPICAL TOPICS
There has been some unusual tropical action over Hawaii, FLOSSIE made landfall, and now GIL is weakening as it takes a track south of the Islands followed by another tropical depression.
Over Asia, SW/W Monsoon wind and rain have relaxed a little over Indonesia There has been some active convection over Philippines. No TC as yet, however, the risk remains moderate to high in this region this week.
In the past week there was a squash zone over the Marquesas area but trade winds were below par over remainder of South Pacific.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ rebuilt last week from Solomons to Suwarrow and there was a convergence zone around the centre of a low that formed near 30S to South of Southern Cooks.
This week the SPCZ is expected to intensify and move south, reaching from Samoa to Papeete by Tuesday then splitting and relaxing over French Polynesia but shifting south in-between Vanuatu and Fiji on Thursday and forming a LOW between Fiji and NZ on Friday that then rapidly moves SE and deepens to east of NZ by Saturday 10 August .

Sub-tropical Ridge STR
High cell in the STR to east of NZ is currently 1035+ near 45S and expected to weaken back to below 1030 by Wednesday. To west of 180 STR is reforming along 30S and remains weak, so that there is no squash zone in the trade winds to the north.

Roaring 40s and New Zealand
W/NW flow over NZ on Monday and Tuesday, and a trough crossing NZ on Wed to Friday, then a Low is expected to deepen to NE of NZ on weekend Sat/Sun 10/11 Aug UTC with a weak ridge over NZ.

Route Briefings
Tahiti to Tonga:
Wait until the trough clears Tahiti and the winds revert to be from S/SE either on Tuesday or Wednesday. The next trough is expected to cross Aitutaki on Tues 13/14 Aug, so it may be better to go via Suwarrow.

Between NZ and Fiji/Tonga
Keep an eye on that Low from Fid 9 to Sun 11 Aug.

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