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

09 June 2013

BOBGRAM issued 9 June 2013

Issued 09 June 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 rose to over plus 1 briefly in April and dropped to minus 0.39 in early May and bounced back into the positive in May, and has now reaches plus 1.3 on 9 June. So the signal coming from the Atmosphere has been erratic and at present is tending towards La Nina.

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus the Sea surface temperatures SST across the equatorial Pacific may be thought of as a factor in the running of planetary weather engine. Its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet. In the past month there has been a growing area of cooler than normal conditions over the eastern equatorial Pacific. This is a possible sign of a trend towards La Nina conditions, but we still do not have enough evidence to say this trend will continue

ITCZ had strengthened across the Pacific. TC ANDREA was the first TC for the Gulf to the Atlantic this year and left a very marked path on the rainfall accumulation map. TC YAGI has formed in NW Pacific and is heading for Japan.

South Pacific Convergence Zone SPCZ
The SPCZ is gathering strength along 10S at present and may drift south onto temporarily across Vanuatu and Fiji on Thursday and Friday 13 and 14 June.

Sub-tropical Ridge STR
Large High travelling east along 40/45S well east of NZ has a strong squash zone of enhanced trade winds on its northern side tonight with swells over 5 metres.
High in the Tasman Sea is weak and is having its travel restricted; it should finally cross NZ on Fri/Sat 14/15 June. Its squash zone SZ is only in the Coral Sea.
Large high is southern Aussie bight is travelling along 45 to 55 S and should pass by south of NZ on Mon 17 June, preceded by very cold southerly winds onto southern NZ on Sun 16 June.

Roaring 40s and New Zealand
Between the Highs we have lows, and the one crossing northern NZ tonight is expected to the go SE and expand to cover a wide area east of NZ by Wednesday.
The one over Australia on Wednesday is expected to go SE into the Tasman, deepening rapidly on Thursday (avoid) and Friday in the Tasman and then be deflected NE across Northern NZ by Sun/Mon/Tue 16/17/18 June -- something like a near weekly trough cycle for NZ at present.

Route weather picks
French Polynesia to points west: There may be complications from the SPCZ between Southern cooks and Niue from Tuesday 11 to Sun 16 June--- could be a better voyage next week.

NZ to the Tropics.
Mon to Thursday are looking to be good days to go with a steady SW flow.
The weak winds of a passing ridge on Friday and northerly winds of an incoming trough on Sat/Sun 15/16 June are not good.

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