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

24 August 2014

BobBlog 24 Aug 2014

Issued 24 August 2014
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

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 has been negative since July and has been hovering around -5 (Australian
units) for the past month - enough to be called a weak El Nino.

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.
Comparing the current evolution of Nino 3.4 with recent episodes shows us
that there is normally a drop around this stage of the episode and that
there is still plenty of time for this episode to grow stronger by early

It is all happening in the NE Pacific with Hurricane KARINA and Tropical
storm LOWELL and MARIE travelling NE/NW not expected to make landfall.
For the latest official data on these features see http://www.nhc.noaa.
The weekly rain maps for the past fortnight show a burst of rain around
India and heaps of convection between Hawaii and Mexico. Also the South
Pacific Convergence zone is becoming less defined.

SPCZ= South Pacific Convergence Zone
The SPCZ is expected to intensify this week and stay with one branch between
Solomons and northern Vanuatu, and another branch between Tuvalu and Tokelau
occasionally affecting Samoa, and a third branch mostly lingering over
Southern Cooks but expected to drift onto French Polynesia by end of this

STR= Sub-tropical Ridge
The STR has come south and intensifies since last week, The INTENSE High
(more than 1034 at centre) over South Tasman Sea and southern NZ today is
slow moving and should finally weaken and move off to the NE of South Island
from Thursday. This is a winter anticyclone so there is plenty of stratus
cloud under its inversionwhat some people call anticyclonic gloom , so it
maynt bring the 7 sunny days that some may have promised.
A squash zone of strong trade winds are found in the tropics on the north
side of this high These strong SE winds are already affecting Fiji/parts of
Tonga, and should continue there until around Thursday. The squash zone
should spread east to Samoa by local Monday, lasting there to local
Thursday, to Niue for local Wednesday to Friday, and Southern cooks for
local Friday and Saturday.

Departing westwards from Tahiti:
There are three challenges/complications: 1) the SW winds that are likely
around Southern Cooks until local Monday; 2) The convergence zone that is
expected to develop and linger over Southern cooks by local Tuesday and
linger there a few days and then travel east to French Polynesia by local
Saturday; and 3) the squash zone that is travelling east later this week.
It may be possible to make short trips and dodge around these factors.
Anyway none of these challenges are likely to be too intense this week 
they may make things uncomfortable at times but should still allow sailing.

Between NZ to the tropics
A subtropical Low is now deepening off Brisbane and expected to remain
slow-moving there for a few days, finally weakening and travelling east
along 30S across to north of North Island by the weekend.
Associated front should spread strong to gale easterly wind and heavy rain
onto northern North Island from the north during Wednesday to Friday.

See my yotpak at for terms used.
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