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

15 September 2013

BOBGRAM 15 Sep 2013

WEATHERGRAM
YOTREPS
Issued 15 Sep 2013
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.

Background influences
Drought: Areas that miss out on rain for an extended time tend to have the ability to perpetuate themselves by diverting rain-bearing fronts around them. In our part of the world this has been the case for many months in northern New South Wales. However this week a rain-bearing low is expected to reach this region.

SOI: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has relaxed to around -0.07 in early September. This week is has risen to +0.11 (15 September).

GLOBAL TROPICAL TOPICS
Mexico has Cyclone MANUEL in the eastern Pacific and INGRID in the Atlantic. And there is also another Low heading west across the north Atlantic. In the NW Pacific MAN-YI is about to make landfall over Japan.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has a main branch from Papua New Guinea to approximately Samoa. There is a smaller side branch along about 10S to north of Suwarrow. The intensity in these zones is expected to remain weak the week. The main zone is expected to drift across Samoa at times, most likely on Tuesday and Wednesday UTC. The side branch may drift south across Suwarrow on Saturday UTC.
A trough is expected to develop over Vanuatu on Sun UTC and deepen over Fiji early next week.

Sub-tropical Ridge STR
High that is tonight in the South Tasman Sea is expected to spread NE across NZ on Tuesday and Wednesday and then expand as it goes east of NZ along 35 to 33S from Thursday to Monday, intensifying the trade winds on its northern side.

Roaring 40s and New Zealand
Jetstreams have gone north to around 30S over Australia and this has bred one Low that is tonight between Lord Howe and Norfolk Islands and is expected to roll past northern NZ on Tuesday.

Another Low is now moving from Victoria to New South Wales, and is expected to deepen off Sydney on Tuesday. It has the warmth and moisture potential to bring heavy rain to New South Wales, the first decent soaking since March. This Low is then expected to track off to the south-southeast, and another low may form in Tasman Sea on Friday and cross the South Island by Sunday.

Next week the outlook is for a disturbed westerly flow to spread from Australian Bight to Tasman Sea/New Zealand.

Route Briefings
Tahiti to Tonga:
This is likely to be a week with consistent trade winds. There is an increased risk of tropical squalls near Suwarrow on Sat 21 Sep UTC (local Friday).

Between NZ and Fiji/Tonga
Take care near northern NZ as that Low flies by on Tuesday, otherwise the week offers a mix of light winds and NE/NW winds, good for travelling southwards.

See my yotpak at http://lnk.ie/IV34/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/IV35/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/IV36/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/IV37/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

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