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

19 May 2013

BOBGRAM issued 19 May 2013



Issued 19 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.

Background influences

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the standardised 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 and dropped to minus 0.39 in early May and bounced to plus 0.6 by 18 May. So the signal coming from the Atmosphere continues to be 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, and in the past month there has been a growing area of cooler than normal conditions over the eastern equatorial Pacific. This is possibly a sign of a trend towards La Nina conditions.


The 5-day rain map ending 13 May shows some new life in the Intertropical Convergence Zone ITCZ. However, the ITCZ is staying slightly south of the Marshall Islands and so their long term drought continues.


South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ is weak and spread out at present, but there is an active branch between Tuvalu and French Polynesia. It is expected to drift south later this week and reach Fiji from Tuesday 28 May.

A small convergence zone CZ lies between Fiji and New Zealand tonight and is expected to cross Minerva on Tuesday and then ease away as it travels east/southeast.

Sub-tropical Ridge STR

This is rather weak at present. There is a High centre east of 180 in the normal winter latitude near 30S. This holds a weak and tenuous link to another high which is moving east along 45S from the Australian bight towards NZ. This High is preceded by a southerly flow that is shovelling cold air northwards. The southerly should reach southern NZ by Wednesday and the high is expected to cross the South Island on Thursday and the North Island on Friday.

Roaring 40s and New Zealand

Low1 in the Tasman Sea is expected to cross northern NZ on Wednesday. Low 2 is expected to move off Australia east coast on Thursday or Friday and then over northern NZ on Sunday 26th May followed by a strong SW flow.

Route weather picks

Panama to Galapagos: Not a good week for this voyage. S to SW winds to south of 8N are getting persistent and may be up to 20 kt. Intertropical Convergence Zone ITCZ quite active between 6 and 2N and acting like a sort of barrier.

Galapagos to Marquesas: Good to go this week. Light southerly winds over Galapagos and moderate trade winds trade winds from 2S onwards. To get the most from wind and surface current head off for 5S110W and follow current to 5S124W and then go direct to Marquesas. There may be some tropical showers between 5S and 8S.

NZ to the Tropics.

Departure on Monday is likely to be in light winds then fresh NW winds for a few days.
Tuesday and Wednesday departures are likely to get strong NW winds. No good.
On Thursday, strong southerlies are expected. No good.
Friday Sat Sun Monday departures are likely to be affected by winds from L2. No good.

So the next day with possible good enough weather for departure is Tuesday 28 May. Perhaps. More next week.

See my yotpak at for terms used.

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