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

06 May 2012

BOBGRAM issued 6 May 2012

Issued 6 May 2012
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 Southern Oscillation Index or SOI is now diving more into negative/ nearly El Nino territory. Its 30 day running mean was mainly -0.6 in April, and had a dive to -0.8 around mid-April. It has started May with a dive to -1.1 on 6 May. If it can maintain this for three months 9and that's a big IF at this stage) then we' can call it an El Nino with weak trade winds and lots of SW winds over the Tasman during winter.

South Pacific Convergence Zone SPCZ had an interesting week with a build in activity across the northern Coral Sea, and between equator and 5South from 170E to 170W. Another branch has been active across French Polynesia and Pitcairn and to the southeast

SPCZ is likely to drift south across Fiji on Tue/Wed and Tonga /Minerva on Thursday 10 may in response to a passing mid-latitude trough. Then it should revert to a northern position.

Galapagos to Marquesas: All clear of squally showers now. Depart quickly SSW from Galapagos until 3S then WSW to 5S then west to 125W then direct.

A northerly drift of the STR is noticeable, as is typical of this time of the year.
The High departing from NZ is starting off at 42S and then trending NE to near 32S when south of Rarotonga by Thurs 10 May.
The Next High of the STR is in two latitude bands. The one that is near 30S struggles to make impact. It is over inland New South Wales at present and gets to cross Tasman on Fri/Sat 11/12 May then fades. Its companion now near 55S to SW of Tasmania is expected to start going NE when it gets past Campbell island on Thursday 10 may and then expand to NE of NZ.
NZ /Tasman Sea troughs
Mid-latitude trough currently in mid-Tasman Sea has several features doing their own things. The LOW east of Tasmania should go SSE into southern ocean. The feature east of Lord Howe should turn into a LOW and cross central NZ on Tue and Wed 8/9 May then go off to the east—so there will be a squash zone between it the southern high.
Another southern Ocean LOW is expected to affect Tasmania on Friday 11 May and its cold NW then westerlies should spread onto NZ Sun to Wed 13 to 16 May followed by a southerly Thu 17 May .
NZ to Tropics
It's safe to say that this trough moves off North Island in dribs and dabs on Wednesday and that Thursday/Friday are looking to be OK days to depart Northland. This window 'closes; on Saturday 12 May IF something does form in coral Sea on Wed 16 may as predicted by GFS … but the EC model has a wider window, with no Coral Sea development .. see – we should be able to say more in this later this week.
The terms used are more fully explained in the METSERVICE Yacht Pack.
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