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

17 July 2011

BOBGRAM7 issued 17 July 2011

WEATHERGRAM
YOTREPS
Issued 17 July 2011
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.

TROPICS
Southern Oscillation Index SOI has jumped up a little in past week; its 30day running mean was 0.03 on 8 July and 0.5 on 16 July.

Galapagos to Marquesas: The seas around Galapagos are slightly warmer than normal. Even so, at present there seems to be little extra advantage is straying from the rhumb line/great circle.

South Pacific Convergence Zone SPCZ is active from northern Coral Sea (near Solomons) in bands --- one to Vanuatu, another to Rotuma area then Tonga, and a third to Tuvalu. That's about it at present and there isn't much shifting about either.

SUBTROPICAL RIDGE STR
After a long wave trough over the Tasman Sea / NZ area for the past 2 weeks, a subtropical ridge has finally managed to make into the Tasman Sea this weekend. This High should move east across NZ on Mon/Tues at 40S and then stretch northeastwards and weaken during remainder of the weak, so not much of a squash zone in the trade winds on its northern side.

Next High is starting to form in Australian Bight, and consists of some polar air – so is a frosty one, being modified as it encounters warmer water (brewing unstable showers). Should cross Tasmania on Sun 24 July and then move NE across Tasman Sea so that it passes by to north of Northland around Thu 28 July… a normal winter path, allowing a disturbed westerly flow over Tasman NZ by late next week.


TASMAN SEA/NZ AREA
A return to the pattern we had in June, with a moist mild northerly flow over NZ as a Low forms in the mid Tasman Sea. This Low may be complex and multi-centred, and there may be strong winds near some of its compact centres… especially one near Lord Howe on Tue 19 July. Its main frontal rain band should stall over the North Island on Thursday/Friday 21/22 July and an easterly flow should stay over central NZ as the Low weakens and wanders east across area north of NZ on the weekend 23/24 July. This should set the scene for a southerly blast over all NZ on Mon/Tue 25/26 July between the low (then moving off the SE) and the incoming High.

SAILING TO/FROM NORTHERN NZ.
Departing from Northland – There are challenges. Avoid that incoming front especially on Wednesday/Thursday, 20/21 July. There may be a window on Friday/Saturday 22/23 July, going clockwise around a passing weakening low. Then take care to double-check the strength of the southerly outbreak that is likely on Mon/Tues 25/26 July.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

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