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

01 August 2010

BOBGRAM7 issued 1 Aug 2010

WEATHERGRAM
YOTREPS
Issued 1 August 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.

TROPICS
Asia is much wetter than normal, and in true Walker circulation style
this extra rising air is producing more sinking air in the subtropics,
accentuating the subtropical ridge especially around the eastern Indian
Ocean and Western Australia.

The South Pacific Convergence zone SPCZ has been knocked a little south
by this so that it has been DRY recently over Vanuatu, Fiji, and from
Samoa to Tahiti, and wetter than normal from Coral Sea to south of Niue,
with trade winds much stronger than normal over the whole South Pacific,
especially about Coral Sea where it is being drawn into a strong
Monsoon.

A passing upper trough in the mid-latitudes is likely to help accentuate
the SPCZ between Samoa and Southern Cooks on 2 Aug (UTC). There should
be some squally showers in this trough as it crosses Southern Cooks
around 3 Aug and Tuamotus by Thu 5 Aug. This offers something for the
puddle jumpers at Tahiti to take into account.

SUBTROPICAL RIDGE: STR
Long wave pattern is conducive to forming troughs in the west Tasman Sea
and helping the STR in longitudes east of the dateline.

The Large HIGH over 1036 east of NZ today (1 Aug) is expected to wander
east along about 40S this week, with a Low forming on its northwest
shoulder near 30S 150W around Tue 3 Aug UTC. As this pair wanders off to
the east, some smaller High cells are likely to form near latitude 30S
about and east of NZ by Fri 6 Aug, with fresh to strong trade winds on
their northern side.

TASMAN/NZ
One Low deepened in the mid Tasman Sea on 31 July and is expected to
track SE across Southland tonight. Next Low should deepen off New South
Wales on Mon 2, rotate clockwise around the Tasman Sea and weaken into a
trough that is likely to cross NZ on Wed 4 Aug UTC. Third Low should
deepen rapidly over Tasmania on Wed 4 Aug and rotate clockwise around
the Tasman Sea and cross North Island around Sat/Sun 7/8 Aug UTC.

There are brief ridges with slack winds between these lows but generally
too brief to allow much planning for a sail trip to or from 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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