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

18 August 2013

BOBGRAM issued 18 Aug 2013

Issued 18 Aug 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.
The Atmosphere: 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 0.5 (18 Aug).

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet. This parameter has been near average so far this year, slightly on the cool side.

Tropical Depression ERIN is weakening in the North Atlantic and it becomes subtropical. TC TRAMI is expected to take a path westwards into China to north of Taiwan this week, after TC UTOR killed at least four people and displaced more than 320,000 in Southern China las6t week. TC PEWA is also making its way to the NW in the NW Pacific. The Cyclone season is getting busy.
Monsoon seems to be weakening. In fact rain around the planet seems to have been below par last week, with the wettest zone in the South Pacific being from Samoa to Suwarrow.

South Pacific Convergence Zone SPCZ
The SPCZ currently extends from Solomons to Tuvalu/Samoa to west of Tahiti at present and is expected to divide into two branches during the week, with one near 10S and the other between Fiji and southern Cooks. A small but intense and squally LOW may form on the SPCZ and visit northern Vanuatu on Wednesday and then move SE. Avoid.

Sub-tropical Ridge STR
There is part of the STR between NZ and Fiji/New Caledonia with a weak extension further east. There is a strong and slow-moving High over Australia and this is expected to finally start moving east into the Tasman Sera after Sat 24 Aug-not before.
The High that is east of NZ by mid-week is expected to move northeast from out of the southern ocean, and so is a cold high. There should be a squash zone of strong winds on its northern side. The SAM (Southern Annular Mode) index became very negative last week, indicating that any southerly outbreak – such as that on the eastern side of this high- is polar-chilled.
A low crossed northern NZ on Saturday and is expected to continue to move east along 35 to 30S this week, between the two ridges.

Roaring 40s and New Zealand
A disturbed and sometimes showery northerly/westerly flow is expected over NZ this week.

Route Briefings
Tahiti to Tonga:
This week's weather pattern is looking Ok for sailing from Tahiti to Tonga or Samoa via the northern route (passing Suwarrow/Suvorov). There are not many troughs or convergence zones to worry about; however it is not expected to be entirely squall-free.
The southern route --- going via Southern Cooks /Niue—is second best this week because there are several troughs expected to make their way eastwards between 18 and 25S associated with the low that is expected to travel east along 35 to 30S.

Between NZ and Fiji/Tonga
The disturbed northerly/westerly flow this week is not a good weather pattern for going north or south. If a slow-moving high does enter the Tasman Sea this weekend then there may be an OK pattern for sailing north starting early next week.

See my yotpak at for terms used.
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