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

24 September 2011

BOBGRAM issued 25 Sep 2011



Issued 25 September 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.


Northern Hemisphere is still at peak cyclone season, with HILARY west of Mexico, OPHELIA and PHILIPPE in Atlantic and NESAT about to cross northern Philippines.


The tropical ocean temperatures in mid pacific continue to be near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos---so much that it has exceeded USA's Climate prediction centre CPC's threshold and they have called it a new La Nina.  It is sort of neutral with bursts of La Nina at times.


From the atmosphere, the Southern Oscillation Index SOI is positive and increasing: its 30day running mean was 0.44 in 17 Sep and 0.75 on 21 Sep.  Some lingering La Nina weather patterns persist in the atmosphere, and the Subtropical ridge STR in the Australia/Tasman Sea area is now being taken south of its normal position.


The South Pacific Convergence Zone SPCZ is persisting from Solomons to Fiji then southeast across Tonga and to south of Southern Cooks.  The section across northern Vanuatu/Fiji/Tonga is weak at present but is likely to activate from Thursday 29 Sep to Sunday 2 Oct.



The STR seems to be south of its norm position of around 25S.  That High near 40S in the South Tasman Sea tonight is expected to travel northeastwards across NZ this week and may be located near 30S 170W by wed 28 Sep and then wander southeast to 38S 130W in early October.  Its intensity should stay below 1030hpa so that the trade winds on its northern side are likely to be enhanced a little but stay less than strong.

A large area of heavy SW swell associated with a low from the southern ocean is expected to reach NZ south coast by Wed 28 Sep and then move into the eastern flank of the High in early Oct – so avoid the centre of this high as it will have the combination of light winds and heavy swells – no good even for a power boat.



There is a developing Low between Sydney and Lord Howe Island tonight feeding off the jetstream aloft and the east Australian current from beneath. It should  move east along 30S until wed 28 Sep and then when it gets knocked a little to north of Norfolk Island,  but may redevelop near and to east of Kermadecs on Fri/Sat 30 Sep/1 Oct.   This low has strong to gale easterly winds on its south side, worth avoiding.

After that high crosses central an northern New Zealand on Tuesday 27 Sep, a North to NW flow is expected over the country for the remainder of the week : down-slope winds may be a problem for eastern coastlines, with temperatures foehning up into the 20s.


Looking further ahead a Low from the southern ocean is likely to reach Southland by Sunday 2 Oct, turning the flow over NZ to a squally westerly, then from Mon 3 to Wed 5 Oct next week, this low may deepen east of NZ,  bringing a polar chilled southerly outbreak to NZ eastern coastlines. Avoid.




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.


On Mon 3 Oct to Wed 5 Oct a polar chilled flow hits Northland with W then SW then S winds, so don't plan on departing this week unless you are quick enough to reach Northland by end of Sun 2 Oct – and if you come south from New Caledonia, then also watch the progress of that low wandering along 30S.   


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

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