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

11 August 2013

BOBGRAM issued 11 August 2013

Issued 11 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 0.7.

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.

The Atlantic hurricane season is on track to be above normal from mid Aug to late October.

At present there are two TC's. One, called UTOR, is tracking across northern part of Philippines and is expected to make landfalls to SW of Hog Kong on Wednesday. T'other, called Henriette, is traveling from east to west along path south of Hawaii. The winds of the SW Monsoon over India are starting to ease, compared with last week.

South Pacific Convergence Zone SPCZ
The SPCZ is expected to spend the first half of this week along roughly 10 to 12S. After Wednesday 14th UTC it is expected to drift southwards across Samoa, then it should cross Suwarrow on Friday and Southern Cooks on Saturday forming a Low near 23S 160W by Mon 19th UTC that is then expected to deepen and move off to the SE. This is likely to be a squally trough, one to avoid if possible.

Sub-tropical Ridge STR
The STR starts the week off strongly at its winter latitude of round 25/30S.
The High cell over central Australia on Wednesday is expected to cross the northern Tasman Sea on Friday and then the North Island on Saturday and move off to the east from Sunday.

Roaring 40s and New Zealand
A disturbed and boisterous westerly flow covers the mid-Tasman Sea and North island until Wednesday, albeit with the brief calm of a passing ridge on late Monday UTC. Thursday may be an OK day for departing to the north, but the NW/SW winds of the next trough are then likely to be encountered by Tuesday/Wednesday 20/21 Aug.

Route Briefings
Tahiti to Tonga:
Avoid that trough that is expected to cross Samoa on Wed/Thursday UTC and Suwarrow on Friday, Southern cooks from Saturday 17th UTC. If you depart early in the week you should be able to reach Suwarrow or Palmerston before the trough does, and wait out the passing trough there.
I am unsure how to compare the sheltering conditions of Suwarrow and Palmerston… I think Suwarrow will have a briefer impact than Palmerston from this trough, so maybe if you do intend to start this voyage this week, then leave early and go to Suwarrow and no further.

Between NZ and Fiji/Tonga
Travelling south arrange to arrive between Sat 17 and Tue 20 Aug.
Going north, there may be an OK departure on Thursday/Friday 15/16 Aug, but after that these voyages are likely to encounter Northerly winds by Tuesday 27th.

See my yotpak at for terms used.
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