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

18 September 2011

BOBGRAM issued 18 Sep 2011



Issued 18 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.


North Atlantic, still boisterous with cyclones ROKE and SONCA – they seem to be staying offshore.


The tropical ocean temperatures in mid-pacific are 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.  Over the entire Pacific, it is sort of neutral with bursts of La Nina at times.


From the atmosphere, the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.49 on 10 Sep and 0.44 in 17 Sep.   There has been some lingering La Nina weather patterns in the atmosphere last month, but the sub tropical ridge STR is now near its normal position. 


In fact: the South Pacific this week seems to be a continuation of last week- with trade winds in the tropics, disturbed westerly winds to south of 30S and the subtropical ridge STR sitting mainly along 25S—something like the seasonal norm, and this week's disturbed westerlies are looking to be quieter than last week's.



A steady STR staying along about 25S.   STR in this position should maintain fresh to strong SE winds in the Coral Sea. 

One weak high cell should cross the North Tasman Sea on Monday 19 Sep and North Island on Tuesday 20 Sep then weaken away.  Another high cell is likely to weaken away in north Tasman Sea on Thursday and Friday 22/23 Sep.  Next high should advance across Tasmania on Sat 24 Sep, Tasman Sea on Sun 25 Sep and North Island on Mon 26 Sep.  These systems are not penetrating past the dateline at this stage.



Disturbed westerly flow in Tasman Sea latitudes is a good sign--- it is caused by the warmth of the extra sunlight in the Southern Hemisphere getting further south.  The stronger temperature difference between tropics and polar regions is what feeds these westerlies.  The equinox is around Friday 23 Sep, so this is THE week of the year when we have the strongest temperature difference.


Last week the disturbances were severe at times… hail in Wellington, a tornado in Auckland and, today,  more hail in Auckland and Bay of Plenty.  It seems that the coming week should be slightly quieter. The main disturbances should cross NZ on Mon 19 Sep (followed by cold air over southern districts), Thu/Fri 22/23 Sep and Sat/Sun 24/25 Sep (with more cold air in the south).


 The timing of these sequences may still change and anyone following the Rugby games in NZ can catch an update on the weather forecasts for the games from



If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then the next few months is the optimum time.  You can join the ICA All Points Rally, see, for this voyage.  My suggestion is that you try and meet a front at 30S – that way you will avoid meeting a front upon arrival in Northland. -


I'm just back this evening from a great Boat Show in Auckland.

Great new site-- thanks to organizers, and to Auckland City for helping sorting the parking!


The terms used are as explained in the METSERVICE Yacht Pack.

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