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

20 June 2010

BOBGRAM7 ISSUED 20 June 2010

WEATHERGRAM
YOTREPS
Issued 20 June 2010
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.

Galapagos to Marquesas: trade winds are now strong enough so that
anyone sailing this route may as well go direct, rather than chase the
extra help of the west going current that is around 30 miles north of
the equator. Head for 4S and then hug that latitude to 120W - that will
avoid a counter just west of Galapagos and prolong time in a west going
current near 4S. No weather to mention, but note that early this week
there are TWO cyclones in the NE Pacific, BLAS and CELIA.


TROPICS
South Pacific Convergence zone SPCZ is building in activity. It spent
last week over Coral Sea/Vanuatu, Fiji, and Samoa, then weaker between
Northern and Southern cooks, then strong again over French Polynesia FP.
It should reform slightly further north over Tuvalu and Tokelau this
week, and some of the convection in the SPCZ is likely to be drawn to
the SE by passing mid-latitude troughs - so look for troughs turning to
lows in the subtropics at around 20 to 30S from Sat 26 to Sun 27 June:
one near 180 and another near 120W/ SE of FP.


SUBTROPICAL RIDGE: STR
The squash zone and heavy swells over FP on Sun 20 June are related to a
HIGH at 40S, to Southeast of FP. As this wanders off to the east it
will take its squash zone with it.

HIGH crossing Tasmania on Tue 22 June should go east and get across
South Island on wed 24 June and then should slide northeastwards and
reach 30S to south of FP by Monday 28 June. cross the Tasman Sea on
Tue/Wed 15/16 June, and North Island on Wed 16/Thu 17 June, and then
slide off to the NE and get to move smartly along 40S to be south of
French Polynesia by Mon 21 June. Its migration to the east will be
accompanied by a squash zone in the trade winds on its northern side,
mainly along 20S. There will be a squash zone with this High, but
mainly on its NW shoulder.

Next HIGH is likely o be delayed getting across Australia and, at this
stage, is expected to reach Tasmania around 1 July - so next week, with
no Subtropical ridge around NZ, the stage is set for a large LOW over or
east of NZ.

TASMAN/NZ
Disturbed W/SW flow over NZ is good for those wishing to sail off north
today/Monday/Tuesday 21/22 June, but on wed 23 June an SE change is
likely to reach northland. This will be counter productive to sailing
and, combined with the movement of the High to the South Island is
likely to result in the formation of a LOW that may loop and linger
around Northland for the remainder of the week.

The next good sailing window from Northland will then be after this low
has moved sufficiently off to the east - probably Mon or Tue 28/29 June.
And that low is likely to deepen and expand in area as it goes east,
because of the weakness of the STR. If you are interested in this
window, then be aware that it may be bothered by lows and troughs
lingering around 30S, so re-check closer to the time. Also, around 1
July, between the expanding low to the east and the incoming High in the
South Tasman Sea, there should be a polar outbreak with cold
southerlies over NZ.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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