BOBGRAM7 issued 29 Aug 2010

WEATHERGRAM
YOTREPS
Issued 29 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
The South Pacific Convergence zone SPCZ is strong along 10S across Papua
New Guinea/ Solomons and Tuvalu, and is likely this coming week to
extend to Tokelau, and maybe even affect Samoa to Suwarrow at times,
but otherwise is weak.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds as far as 15S. One of these is
expected to affect Fiji on Mon 30 Aug (some welcome rain!) Tonga on Tue
31 Aug, and then fade on its way to Southern Cooks by Thu 2 Sep.
Apart from this interruption, it looks to be a good wind for trade
winds in South Pacific.

SUBTROPICAL RIDGE: STR
Something to do with some extra energy in the subtropical jetstream
above around 30S. So the High that is expected to reach Lord Howe Island
by Wed 1 Sep should anchor its stern there and then rapidly extend
eastwards along 25/30S so that its nose gets to south of French
Polynesia by Fri 3 Sep. This is in response to a curtain of sinking
air, and it helps to feed the trade winds at 15 to 20S over the entire
region.

Next HIGH in the Tasman should build east of Sydney around Friday 3 Sep,
in good time to cross the North island during the 4/5 Sep weekend J

TASMAN/NZ

The long wave trough that has been bothering the Tasman sea is
retreating north for a bit. There is still a haphazard procession of
fast moving fronts bothering Tasman /NZ but the distance between troughs
is getting more, allowing for some voyage planning. And there may be a
break in this with next weekend's passing High.

Before then, today's trough is expected to settled into a couplet of
lows east of the North Island on Mon 30 Aug to Wednesday 1 Sep (stand
by for some rain in the Hawke's Bay ranges). The next trough is likely
to be preceded by a vigorous NW wind over South Island on Thu 2 Sep and
followed by a disturbed SW/S flow on Friday 3 Sep.

Anyone intending to head south to NZ this week can probably find
reasonable weather so long as an arrival around 6 or 7 Sep is targeted.
Plenty of south/southwesterly winds for anyone heading North, but avoid
the prefrontal conditions on Thu 2 Sep.

The parameter we use to measure the strength of the polar vortex has
indeed gone negative in the past few days, first time since May. This
increases the chances of a polar outbreak somewhere ... and indeed there
seems to be one near the Horn this weekend, but none, so far, anywhere
else.

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