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

16 October 2011

BOBGRAM issued 16 Oct 2011

Issued 16 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.

Northern Hemisphere cyclone season is easing off this week.

A La Nina episode is kicking in. This helps intensify the subtropical ridges STR and knocks then pole-wards. The stronger highs in the STR helps make stronger trade winds. There is one of these squash zones of enhanced trade winds over the seas of French Polynesia at present as a high moves east along 30S and this should last until Tuesday 18 Oct UTC. Another squash zone is expected to form in the Coral Sea on Tuesday 18 Oct UTC and this should spread east reaching New Caledonia /Vanuatu on Thu 20 Oct, Fiji on Fri 21 Oct and Tonga on Sat 22 Oct.

From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 9th it was 0.91, and on 16th Oct it was 1. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season. At this stage is expected to be a weak La Nina, and NOT as strong as the extreme La Nina we had last summer.

The South Pacific Convergence Zone SPCZ is expected to hover near its normal position this week between 10 and 15S—this means that Tuvalu and Tokelau may see some desperately needed showers for a change.

The week starts with a BFH (big fat high) near 40S 150W moving east – this is the cause of the squash zone over French Polynesia. It is expected to be followed by another High of weaker intensity near 35S 160W from Wednesday 19th October. And another High should form in the Tasman Sea on Tue 18 to Thu 19 Oct, crossing northern NZ on Sat/Sun/Mon 22/23/24 Oct (Labour weekend in NZ)

Of major concern to anyone sailing to NZ at present is a trough now in the Tasman Sea. One low in this trough is forecast to cross the South Island on Monday and another should cross northern NZ on Wednesday--- this brings vigorous SW winds and heavy SW swells to the zone between northern NZ and as far north as 30S, maybe 25S, on Wed 19 and Thu 20 Oct. AVOID.

For this year's COASTAL CLASSIC departing Auckland for Bay of Islands on Friday,
The forecast at this stage is for a ho-hum SW between 10 and 20 knots.

The main thing is to avoid 25S to NZ on Wed 19 and Thu 20 Oct, please

There is a squash zone slowly spreading east from Coral Sea, expected to reach New Caledonia around Thu 20 Oct and onto Tonga by Sat 22 Oct. It may be a good idea to depart before this zone arrives.

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