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

30 October 2011

BOBGRAM issued 30 Oct 2011

Issued 30 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 new La Nina episode continues in a weak to moderate fashion. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 22nd Oct it was 1.0, and on 30th it was 0.96. Computer modelling suggests this La Nina will continue into the southern hemisphere cyclone season. This tends to reduce the tropical cyclone activity around and east of the dateline, but may increase the risk around the Coral Sea.

Some Insurance policies take the start of the cyclone season to be 1 Nov- and indeed the risk rises rapidly during Nov and December.

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu to Wallis Futuna and to Samoa, then … slightly weaker … to Southern Cooks. This is its 'normal position' and it has shown a build up of activity during the last week.

It appears that a tropical Low is forming on the SPCZ around the Wallis/Futuna area during Monday and may move across northern/central Tonga on Tuesday, and then take a path ESE-wards to north of Niue on Wednesday and over Southern Cooks on Thursday, then southwards into the mid-latitudes. Avoid.

A BFH (big fat high, central pressure over 1036 hPa) has built well to east of NZ and is hovering around 45S 140W. There is a squash zone of enhanced trade winds on the north side of this high near 25S 150W also enhancing this squash Zone a sit moves slowly south. Avoid.

The next high is expected to form slowly in the mid Tasman Sea around 30S but its formation is expected to be delayed until Sun 6 Nov, and this high may just stay put all next week. During this time there will be the characteristic subtropical ridge link connecting this high to the BFH. The STR link should move north next week to 30S, fading out the squash zone.

An upper low is forming tonight near 28S 175E and it contains thundery squalls. This feature is expected to be collected by the remains of an old frontal system that is currently draped over southern NZ and a trough that is in the Tasman Sea . The combination is expected to move across NZ on wed 2 Nov and lead in a disturbed W to SW flow that looks likely to cover NZ and the area north of NZ from Thu 3 Nov until at least Fri 11 Nov. Drat.

If you have Internet access you may be able to check the weather patterns at or
Those in Tonga may as well wait for that tropical Low to go away on Tuesday before thinking of departure, just it case it changes track.
The STR will remain as a zone of light winds roughly along 30S from wed 3 Nov until at least Fri 11 Nov, with a disturbed westerly flow over Northland. This isn't the easiest pattern for getting to NZ but we may be able to sort out some zig-zags across the S/SW winds for a few boats with have enough fuel to motor across the STR light-wind-zone.

Note that I will be UNAVAILABLE from 8 to 12 Nov—attending to MetService display at Christchurch show.

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