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

03 May 2015

Bob blog

Issued 3 May 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.

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, then relaxed in April and is now
SOI is shown at

As we move into the month of May, the NW Pacific fires up. This region is
likely to produce a cyclone per week for the next 26weeks (that s an
alphabet of Cyclones). Currently this week s cyclone is numbered 06W which
is a number it gets from the computer models, but as you can see from this
forecast from JTWC its gales are on their way to Philippines region by 8

The rain map for the last two weeks show not much change, but what is
apparent is an increase in extent and activity along the ITCZ, and a
decrease in activity around French Polynesia. The east Australian coast is
still a hot spot for rain.
Weekly rain signatures are seen at


Galapagos to Marquesas:
Best winds for departing are on local Wednesday, otherwise be prepared to
motor at the start.
As last week the best strategy is as follows: 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

SPCZ=South Pacific Convergence zone.
SPCZ is expected to remain active from Samoa to French Polynesia, triggering
a LOW just south of Australs Islands by Tues 05 May which is expected to
intensify a while as it goes southeast, finally weakening meat 40S.
SPCZ should also drift south across Vanuatu this week and may help form a
trough there on Thu/Fri/Sat. Strong NE/E winds on eastern and southern side
of this trough. Avoid.

STR= Sub-tropical Ridge
STR is expected to remain north of NZ this week. Also it is expected to
move into interior of Australia bring clear and therefore cold long nights
to their desserts. This allows the roaring 40s to spread north.
High over northern NZ this weekend is already travelling off to the east and
should do so along 35S for remainder of this week. Next significant HIGH to
reach Eastern Australia is NOT expected before 17 May, and may not reach NZ
until 20 May, so the Tasman Sea is being set up for a dose of disturbed
westerly winds.

Departing from Eastern Australia this week:
That LOW which was mentioned in last Weathergram to visit Brisbane area last
Thursday/Friday was a real damaging beast. It has already turned into an
area of slack winds which is crossing the north Tasman Sea from Monday
reaching 30S to north of NZ on Friday, and this might be a weather window
for motoring vessels to the tropics.
However, if you are intending to sail then beware of a series of fronts and
lows crossing the Tasman Sea, and filling up the Tasman with westerly gales
and heavy swells. One of these is expected on Tuesday to Thursday, another
from Friday to Sunday and a third on Sunday and Monday 10/11 May-followed by
several more the following week.

Departing from Northern NZ going north.
Those vessels which departed during the light winds of this passing HIGH
over the weekend should be able to pick a speed so that they arrive north of
25S after the squash zone of strong winds there finally ease by Thursday.
An approaching front is preceded by strong NW/N winds, so no good departing
between now and when that front passes on Thursday. Then Ok to go until next
front reaches northern NZ on Monday 11 May. Then a long period of strong
disturbed westerly winds is expected- stay and read a book unless you are

Departing from Northern NZ going east.
The long period of roaring 4os weather that NZ is now about to experience
is good for this voyage, but you need to be careful and try and run the
northern edge of the roaring 40s zone. Hard to do when this keeps buckling
with each passing Low and High. Email me at for more info.

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