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

27 July 2014

BobBlog issued 27 July 2014

Issued 27 July 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 of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

SOI. The Southern Oscillation Index SOI sums up the weather pattern over the
South Pacific as one number. It is based on the standardized difference in
the barometer readings between Tahiti and Darwin.
The SOI has dived negatively in the past month but needs to remain in the
pink area of the graph (shown in illustrated version of this blog) for more
than a month for this to be called an El Nino episode. Even though we are
not there yet, there have been some signs of El Nino like weather in the
South pacific recently: the westerly winds of the roaring 40s have extended
further north than normal over the New Caledonia area.
Looking at the 30 day averaged pressure anomaly map shows this clearly, and
also shows near neutral conditions around French Polynesia. For an El Nino
to kick in, Darwin needs to have a sustained above normal barometer, and
this is happening. Also Tahiti needs to have a sustained below normal
barometer and that is not happening yet.

Action this week seems to be shifting to the NE Pacific GENEVIVE approaching
the south side of Hawaii HI And HERNAN is lurking off Mexico These features
are expected to fade over the next few days. There are also a few features
that may develop into cyclones in the NW Pacific.
The weekly rain maps for the past fortnight show a burst of tropical
activity spreading eastward along the ITCZ International Tropical
Convergence Zone. The South Pacific Zone SPCZ has also shown signs of
redeveloping after a few quiet weeks.

SPCZ= South Pacific Convergence Zone
The SPCZ is intensifying and is expected to drift south this week so that it
visits Samoa on Tuesday UTC and the Southern Cooks on Wed/Thu UTC.

STR= Sub-tropical Ridge
The High moving east off NZ on Monday is expected to travel along 30 to
40 S. There should be a zone of enhanced easterly winds along 15 to 20S on
its north side from Wed to Sunday.

Departing westwards from Tahiti:
OK to depart before Tuesday local but any later in the week and there are
likely to be some strong SE winds encountered on the voyage, except maybe
for a trip to Suwarrow.

Departing from NZ to the tropics
A broad trough is likely to take all week to cross the Tasman Sea and NZ.
Its first feature should be preceded by northerly winds and followed by NW
winds and cross the South Island on Monday and the North Island on Tuesday/
Tuesday night. The second feature is likely to be more intense, preceded by
NW winds on Thursday and Friday, accompanied by a front on Saturday and
followed by SW winds that may be strong on Sunday 3 August.
SO it is not a good week to depart from NZ.

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