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

04 September 2016

Bob Blog issued 4 Sept 2016

Issued 4 September 2016

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.

A few weeks ago I pointed out that a lot of the detail on Google earth over the
tropical oceans is missing as a data-saving tactic. See "Google lacunae" at

Since then Tom Partridge of Yacht Adina has seen my comments and been in touch.
He has written a much more detailed and thorough account of navigating with
satellite imagery for Yachting Monthly's May 2016 edition. This article is now
free online and he has kindly given me permission to share a link to his blog
with you at


Last Friday morning there was a M7.1 quake to NE of our East Cape.
This caused a 30cm Tsunami which was recorded via our tidal gauges network in
their "detided" data:
Remember this link so you can track the waves is the next New Zealand Tsunami.

The averaged isobar pattern for the past month may be seen at
And the variation from normal may be seen at
The most significant features are the lower that normal pressure over the arctic
and higher than normal pressure over the Antarctic.

The Tropics
TC NAMTHEUN is over Japan, LESTER is moving off to the north of Hawaii, and
HERMINE is travelling away from New York after bringing a deluge of rain to
eastern USA states, There are also a few possible tropical lows, labelled
GENESIS in this map, which may affect some areas later this week.
Cyclone tracks seen on

RAIN around the tropics:
Rain for the past fortnight from
The rain maps show an intensification in the intensity of rain in the South

SPCZ=South Pacific Convergence zone.
SPCZ this week is expected to remain strong in the Solomon Islands to north of
Fiji area, and may weaken further to the east.

Subtropical ridge (STR)
High that is shown to be in western Tasman Sea by Wednesday is expected to
travel northeast, crossing northern NZ on Sat/Sun 10/11 Sep and then moving east
along 30Sto east of New Zealand from Monday 12 Sept. There is likely to be a
zone of strong enhanced trade wind (over 35 knots) from Tonga to Vanuatu on 10
or 11Sep, associated with this passing High.

Travel from Tahiti to Tonga:
Voyage is likely to encounter the north end of a passing trough on local sat 10
Sep (variable winds and some showers), and then there may be a squash zone of
strong SE winds around Tonga on or around 11 Sep.
It is looking OK to venture via Suwarrow this week.

Travel between NZ and the Tropics:
Front part of trough went across North island Sunday night and is expected to
weaken as it goes north, it may bring a moderate wind change and some showers to
Tonga on Thursday.
Ahead of the rising pressures of the HIGH mentioned earlier, a TROUGH is
expected to cross Northern NZ on local wed night, followed by strong cold SW
winds. During Thu/Fri 8/9 Sep winds over NZ should swing from strong cold SW
wind to strong cold southerly, finally easing over Northland on Sat 10 Sep.
Try to avoid this strong wind change. it is expected to travel north on
Thu/Friday and reach Tonga as strong S/SE winds in a squash zone on Sun 11 maybe
late Sat 10Sep.

See my yotpak at for terms used.
See my website for information on tailor-made voyage forecasts-
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1 comment:

alexandra mollet said...
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