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

12 May 2013

BOBGRAM issued 12 May 2013


Issued 12 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.

Some sailors have asked me recently how good are the Grib files they are downloading. Well, most of you are looking at data gleaned from the GFS model, and it's good, but not the best. A study done in 2012 shows ECMWF is usually the best. Grib data for EC is not easily available but when you have Internet you can check its latest for the next 10 days for

Second best is the UK model, again its Grib data is not easily available, but MetService pick the model of the day to feed their regional model , and it is usually EC or UK that they pick, and then it is further processed to give maps at

So check these web sites when planning your trip (and add to look at the likely swells). And if your Grib data agrees with these then all is good, but if not then you need to consider multiple scenarios.

Background influences

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 0.39. 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.

The Monsoon : The annual arrival of the monsoon over India usually sets in late in May and can set up weather which has downstream ramification going west (Sahel drought/ easterly waves for TC in the Atlantic) and east (MJO pulses from Indian Ocean to Pacific ocean triggering TC action in NW Pacific). At this time of the year mountaineers are watching jet streams and Monsoon indices to calculate the :"Everest climbing window"- it only lasts around 10 to 20 days so getting it right counts a lot.

This year there is talk of a summit climbing window around May 18 to 21. This is a few days early , and the monsoon index at indicates things are happening earlier than normal , but monsoon hasn't reached south India yet (usually arrives first over Sri Lanka around 25 May). Interesting to see an early tropical cyclone, MAHASEN, in the Bay of Bengal.

South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ has eased in activity a lot since last week. There is some activity between New Caledonia and Fiji tonight and this sis expected to result in the formation of a Low to SW of Fiji on Monday, L1, and this is then expected to move off to the SE.

Left behind are some discrete convergence areas like beads on a necklace mainly along 10 to 15S from Papua New Guinea to Marquesas.

Sub-tropical Ridge STR

This is generally in its winter position along 30S except for Tasman Sea.NZ latitudes where a high that is now over eastern Tasman Sea is expected to move only slowly over central NZ by Wednesday and then fade east of NZ on Thursday.

Roaring 40s and New Zealand

Main event is another Low L2. It is expected to move east across Tasmania on Monday and Tuesday, the Tasman Sea on Wed and Thu, then central and southern NZ on Friday and Saturday. It is likely to have gales near its rather complex centre and generate large swells in the Tasman Sea on Friday and Saturday. Avoid.

Route weather picks

Panama to Galapagos: A reasonable N or NW breeze to depart with from now until Thursday 16 UTC. Not a good week for the voyage overall— S to SW winds to south of 5N are getting persistent and may be over 15kt. Intertropical Convergence Zone ITCZ quite active between 2 and 5N and acting like a sort of barrier.

Galapagos to Marquesas: Good to go with 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.

NZ to the Tropics.

Departure son Monday and Tuesday should start with light winds and get some wind assistance around the west side of L1 but then encounter light winds north of 30S and finally get to trade winds around 20S. Wednesday departures may start in light winds but encounter fresh to strong NW winds on the way as L2 approaches NZ. Thursday Friday and Saturday, and maybe also Sunday are likely to be adversely affected by L2, with strong and squally W/NW winds ahead of a front that may not clear Northland until early next week, followed by a day or so of strong SW winds.

See my yotpak at for terms used.

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