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

26 April 2015

Bob Blog

Issued 26 April 2015
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

Three Chilean Volcanoes have recently thrown ash into the atmosphere;
Calbuco, Caliaqui and Cordon Caulie.
The latest forecast for the VA (Volcanic Ash) shows it drifting into the
South Atlantic: for Volcanic ash advisory see

The Atmosphere:
The Southern Oscillation Index SOI sums up the weather pattern over the
South Pacific as one number and is based on the standardized difference in
the barometer readings between Tahiti and Darwin (30 day running mean).
It relaxed to zero in early March and then went negative, to be -10 units
for a few weeks in late March/early April, but is now relaxing again. See
If this index remains lower than -10 for four weeks in a row then we have a
fully- blown El Nino, but since it is now relaxing the trend is away from
an El Nino at present.

The tropics are quiet at present. The outlook is for a tropical LOW to form
over Marshall Islands from 4 May.
The rain map for the last two weeks show an increase in activity along the
ITCZ in the Northern Pacific and generally around the South Pacific, and a
decrease in activity in that convergence zone west of Galapagos which has
been a mirror image of the ITCZ and appeared soon after the equinox.
Also notice the intense rain around Sydney. Yuck.
Weekly rain signatures is seen at


Galapagos to Marquesas:
Good winds for departing on local Monday then fading winds around the
First of all head for 5S at around 97 to 100W, even though this involves
motoring into light SSE winds. Then the middle leg is westwards along 5 to
6S as far as 125W enjoying a west-ward going current. And then the third
leg is to go direct in SE winds.

SPCZ=South Pacific Convergence zone.
SPCZ is expected to be reasonably intense this week across Solomons/ Tuvalu
and Samoa and may visit French Polynesia for a few days. The part of the
SPCZ over Tuvalu is expected to drift southwards towards Vanuatu/Fiji by the
end of the week, 2/3 May.

STR= Sub-tropical Ridge
STR is reasonable north of NZ this week, Next significant HIGH is expected
to ridge across Tasmania on Wed 29 April and then across the central Tasman
Sea on Thu/Fri and across NZ on Sat/Sun/Mon 2/3/4 May.

Departing from Australian East coast for the tropics this week:
Not a good week for going to the tropics, as that HIGH moving east along the
STR from Wednesday is likely to have a squash zone of enhanced easterly wind
son its northern side. Also a LOW is expected to form off Brisbane on
Thursday/Friday and then move south along the western flank of the then
departing HIGH, the LOW may bring another gale to the Sydney area on Sat and
then weaken and go south.

Departing from Northern NZ for the tropics:
Northern NZ is experiencing a passing active FRONT tonight and on Monday,
then West to SW winds fresh to strong at times on Tuesday and Wednesday,
weakening away on Thursday, followed by light winds in that passing HIGH on
Sat/Sun/Mon 2/3/4 May. As that HIGH travels east, it bunches the isobars on
its northern side into a squash zone of strong SE winds (25 gust 35 knots
and 3 metre+ swells) over, and on the way to, Fiji/Tonga from 1 to 6 May.
Minerva Reef is likely to have strong trade winds from 3 to late on 5 May so
please time your trip to avoid these if you can--- maybe delay departure
from NZ until Thursday, or, if you do depart on Tuesday/Wednesday then go
slow so as to avoid getting north of 25S until after 5 May.

See my yotpak at for terms used.
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