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

02 October 2011

BOBGRAM 2 Oct 2011

Issued 2 October 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.

This edition comes from Ohope beach… "Is that a dark fin I see before me?"
I shall be discussing weather with local community groups here on Monday.

I hope you managed to avoid the stronger than normal tidal currents of the past week, all due to a Perigean new moon - currents should be quieter this week, allowing for better diving :)

Northern hemisphere cyclone season is still going strong. NALGAE is second destructive typhoon to visit Philippines in a week, and OPHELIA and PHILIPPE are still in North Atlantic, staying offshore.

The tropical ocean temperatures in west pacific are near normal, however there is a slow intensification of a cool anomaly around Galapagos --- so there is a tendency towards a La Nina, but it is more in the east than in the west.

From the atmosphere, the Southern Oscillation Index SOI is positive and increasing: its 30day running mean was 0.75 on 21 Sep and 1.2 on 2nd October. First time is has exceeded 1 since April, so yes there a change occurring in the air. Over next few weeks I shall explain what this may do to the coming South Pacific cyclone season.

The South Pacific Convergence Zone SPCZ has been in its normal zone from Solomons to Fiji then southeast across Tonga and to south of Southern Cooks. The eastern side to this zone is weakening at present as a new zone has formed – rather weak - over Coral Sea. This trough is at present linked via upper jetstreams with a mid-latitude feature and should cross New Caledonia on Monday and Fiji on Tue/wed 4/5 Oct and fade over Tonga on wed/Thu 5/6 Oct. Another tropical trough may form between Southern Cooks and French Polynesia this week, speaking up for the 'proper' SPCZ.

The STR seems to be south of its norm position of around 25S. That High around 1030hPa near 35 to 40S to east of NZ should continue to migrate along that latitude this week, out to way southeast of French Polynesia--- with a slight enhancement of the trade winds on its northern side.

The next High is likely to take a path along slightly more southern latitudes---around 45S to 47S—crossing Tasmania on Tue 4 Oct and skirting southern South Island on Thu 6 Oct, then spreading north be cover North Island by Sun 9 Oct.

The low that is currently in the Tasman Sea is under a slow-moving long wave trough and so is likely to take its time crossing the NZ area this week. Its main centre should move onto central NZ by Tuesday 4 Oct, and then form a secondary 'back' in the Tasman Sea on Wednesday 5 Oct. This complex should finally get to east of the North Island by Sat 8 Oct—mostly being pushed off its perch by the rising pressures of the High, rather than moving via its own pressure falling techniques. Some spats of heavy rain with the low's frontal bands are likely here and there Mon and Tuesday, and the cold southerlies arriving on Tuesday may bring a dash of snow to the Kaikoura ranges.

If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then you may be interested in following the ICA All Points Rally, see for this voyage.

The slow-moving Low crossing NZ is actually GOOD NEWS, as it provides a reasonably steady westerly flow over seas north of NZ from Tues 4 to Friday 7 Oct. OK there may be a mediocre front and southerly change near 30-35S on Saturday 8 Oct, but then the forecast is for light winds over Northland on Sun 9 Oct and steady northerly flow there on Mon/Tue 10/11 Oct. So if you can arrange to depart New cal/Fiji/Tonga at the right time (considering the incoming trough) then a relatively straight forward voyage is in the offering this week. Not 100% but not bad either.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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