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

21 October 2012

BOBGRAM issued 21 Oct 2012

WEATHERGRAM
YOTREPS
Issued 14 Oct 2012
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.

The Ocean: Equatorial sea surface temperatures of our widest ocean act something like a thermostat for our weather engine—when they are sufficiently warmer than normal it is called an El Nino episode. Well they have been above average across much of the tropical Pacific over the last few months but are now relaxing. An El Nino may still occur over the next few months but it now doesn't look as though it will be a big one.
There seems to be a warming/cooling couplet occurring in the North Pacific.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is relaxing after a bounce back from its low of -1.01 back on 25 Aug. It has been hovering near zero for the past few weeks at around plus 0.2 so far during 11 October. It's now in neutral mode, and may swing back into El Nino mode over the next few weeks.

In the NIWA rainfall outlook for the South Pacific for October to December a zone of drier than normal conditions stretches from Papua New Guinea to Tonga and for Southern Cooks (Fiji is greyed out meaning no clear indication). An above normal wet season is being forecast for places from Solomons to Northern Cooks and for Kiribati.

The South Pacific Convergence Zone SPCZ
This has weakening into scattered clumps in the past week and is mainly along 10S between Solomons and Northern Cooks and Marquesas.
During the coming week a trough is likely to stretch into the tropics from the mid-latitudes, crossing Fiji/Tonga on Thursday/Friday 25/26 Oct. This may help part of the SPCZ to shift southwards towards Vanuatu/Fiji around Sat 17 Oct and to New Caledonia on Sun 28 Oct. AVOID planning any departing voyages when the SPCZ is visiting. It brings troublesome tropical showers.

In the NIWA outlook for the SPCZ from October to December , the computed output (using dynamical models) – shows a slight shift to the north of normal near the Solomons and to the south of normal east of 180.

SUBTROPICAL RIDGE STR
One High is expected to move east along 30S to east of 180 from Monday marking the STR.
The next High in the STR should form in the mid Tasman Sea on Tuesday and then travel north into the North Tasman Sea by Thursday and Friday and then travel east along 30s crossing north of NZ on Fri/Sat 26/27 Oct. Using rhythm as a guide, the next HIGH to cross NZ should do so around Thu/Fri 1/2 Nov.


NZ/Tasman Sea
Between the highs in the STR there are lows and troughs.

One worrisome Low is being picked by the GFS model to deepen in the Tasman Sea near Norfolk island on Monday and then to wander eastwards along 30S, bringing an easterly gale to northern NZ on Tuesday night/Wednesday, and being followed by a vigorous SW/S flow reaching as far north as New Caledonia on Tuesday/Wednesday 23/24 Oct and Tonga on Thursday 25 Oct. The GFS model is the main one used for GRIB data so anyone watching these will see that a voyage to NZ from New Caledonia/Fiji/Tonga over the next few days will be uncomfortable.

Another very good model for checking weather patterns is the ECMWF as at http://bit.ly/ecoz, this has a slightly different scenario than the GFS, and the real pattern may lie between the two. Both models show uncomfortable voyages from the tropics to NZ this week so that I am compelled to advise everyone to simply take the week off and focus on enjoying the tropics.

Comfortable voyages to NZ MAY start opening up again on or after Thu 25 Oct. Too far away to be sure at this stage, so if you are ready to go then seek a weather update first. There may be another low crossing northern NZ around Sun/Mon 27/28 Oct something worth avoiding.

SAILING TO/FROM NORTHERN NZ.
If you are planning to sail from Tonga/Fiji/Vanuatu/ New Caledonia to NZ in October/ November, and looking to buddy with someone, then you may be interested in checking out the ICA's "All Points Rally" to Opua, see http://www.islandcruising.co.nz/. The group in Tonga are assembling for Big Mama's Birthday party late this week and will by then be able to tell if a departure around 29/30 Oct to NZ will be comfortable or not.
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See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at http://metbob.wordpress.com/
Weathergram text only http://weathergram.blogspot.co.nz
Website http://www.metbob.com
Feedback to bob@metbob.com

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