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

05 May 2013

BOBGRAM issued 5 May 2013



Issued 5 May 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.

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the standardized difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number. SOI rose to over plus 1 briefly in early April but has since dropped to minus 0.24 on 5 May. So the signal coming from the Atmosphere has been erratic.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. 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 so its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet.

Over the past few months conditions have been near average, except for some cooling near South America. So the signal coming from the Ocean is mostly neutral.

Weather on Route:
Panama to Galapagos: Locally around Panama the best day this week to depart is Wednesday 8 May –along with some northerly winds. These will not last long but should be good enough to get away OK. The International Convergence Zone ITCZ is weaker than last week and mainly located between 8N and 2N and mainly to west of 80W.

Galapagos to Marquesas: Good trade winds over Galapagos between now and 11 May, so this is a good time to go. To get the most from wind and surface current head off for 6S110W and follow current to 7S124W and then go direct to Marquesas.

South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ was very active and broad this time last week, but much of its activity has been taken away by a tropical low that deepened over Tonga last Monday and then moved off first south then southeast into the mid-latitudes. The path taken by this low has upset several yachts heading east from NZ to French Polynesia last week. Some have been able to use the westerly winds on the north side of this Low to help them east. There is still some convection of this branch of the SPCZ over French Polynesia.

New SPCZ is now near its weakest and well north mainly between 10 and 12.5S from Solomons to 175E. It is expected to gradually shift south and may spread onto Fiji from Thursday 9 May and may form a tropical Low south of Fiji from Monday 13 May.

Sub-tropical Ridge STR
The Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is well north of its normal position in the eastern Pacific (south of French Polynesia) and this is helping to enhance strong trade winds over the Austral group.

However the STR is at its more normal latitude over the Australia/NZ area. A new high is expected to bloom in the Tasman Sea after Wednesday 8 May and may spread over NZ during the 11/12 May weekend. The STR seems to be doing a good job in keeping apart the polar chilled southern ocean air from the tropically warmed air, but for a few small excursions.

Roaring 40s and New Zealand
An active trough and its Low L1 is crossing the North Island on Monday and moving off to the E/SE on Tuesday, but weather is complicated by the likely formation of a secondary L2, which may cross northern NZ on Wednesday.

NZ to the Tropics.

It's likely to be squally with L1 over northern NZ on Monday and start of Tuesday. Winds should then briefly ease but then things get complicated by the likely passage of L2.

L2 may clear off NZ on Thursday or Friday and that could then make a good weather pattern with a settling southerly flow for anyone looking to sail away from northern NZ. Too early to tell at this stage what may happen during the remainder of such voyages.

See my yotpak at for terms used.

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