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

09 October 2011

BOBGRAM issued 9 Oct 2011

Issued 9 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.

Cruising sailors in South Pacific may be in for an interesting delight this weekend /next week with the waning moon, especially around the third quarter- This usually triggers a spawning of a coral worm called Palolo (or Balolo in Fijian). Sometimes the overnight spawn may be large enough to colour the sea – stuff can be netted and eaten raw or cooked. Ask the locals.

Northern hemisphere cyclone season is still busy: PHILIPPE still in North Atlantic, staying offshore, and now IRWIN and JOVA in NE Pacific.

Equatorial ocean temperatures in west pacific are near normal, and in east pacific are below normal. A La Nina episode is kicking in. This helps intensify the subtropical ridges STR and knocks them pole-wards (in both hemispheres). The stronger highs in the STR help make stronger trade winds.

From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 2nd Oct its 30day running mean was 1.2, and on 9th it was 0.91. 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 gets knocked south by a La Nina. This was certainly the case early this year, and indeed SPCZ has not been visiting Tokelau or Tuvalu much this year… this explains their drought which is now getting desperate. The SPCZ over past week is now hovering between Solomons and inbetween NE of Vanuatu and SW of Fiji. Rather weak.

The STR is usually around 30S at this time of the year, but at present seems to be more near 35/40S. And some BFH (big fat high) specimens are on show this week, getting over 1035hPa near 35S 100W and near 37S 140W. These BFHs are also blocking highs so that troughs get directed around them rather taking a normal journey southeastwards. They also strengthen the easterly winds in the tropics on their northern side - be aware of the prospects of a squash zone of enhanced easterly winds between French Polynesia and Niue from Tue 11 Oct, with huge surf/swell over the Cooks. Avoid.

For Tasman Sea, next high should move in from west on Thu 13 Oct, but will be a slow-mover…should fade over Northland on Sun/Mon 16/17 Oct. The following High is likely to catch up with this and cross the North Island more sedately next week from 18 to 20 Oct. If this turns out to be the case, it may offer a few reasonable voyages from Tropics to NZ / Australia.

Broad multi-edged trough is expected to cross New Zealand this week. First trough already has a tight and intense low centre moving south along eastern Tasman Sea, and this should cross southern NZ on Monday 10 Oct, preceded by strong NE flow over NZ. Second trough has a low that should move east over southern NZ on Wed and then the trough line should cross remainder of NZ on Thu 13 Oct, followed by cold SW flow that may be thundery.

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.

If you time your voyage to depart Tonga/Fiji/New Caledonia before the squash zone of an incoming high arrives (New Caledonia already sat 15 Oct, and Fiji /Tonga by Tue 18 Oct. Also time it to arrive in NZ after the SW winds following Thursday trough have faded away (around 16 Oct) – then that has the offerings of a reasonable voyage.

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