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

05 September 2010

BOBGRAM7 issued 5 sep 2010

WEATHERGRAM
YOTREPS
Issued 05 September 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
There is a strong Walker/Hadley cell combo across the Pacific equator,
with rising air strongest between Malaysia and Taiwan and sinking air
strongest over Fiji to French Polynesia.

The South Pacific Convergence zone SPCZ is looking normal along 10S
across Papua New Guinea/ Solomons, but fades off over Tuvalu and to
north of Samoa. It is expected to extend east at times this week as far
as Suwarrow.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds as far as 15S. One of these
brought some welcome rain to Fiji mid last week (not enough to have much
impact on their dominating dry spell). This has faded somewhat is should
wander east across Southern Cooks on Mon 6 Sep and southern parts of
French Polynesia on Tue 7 and Wed 8 Sep. It is followed by a period of
strong trade winds with a bumpy south to southwest cross-swell. The
next such trough is likely to cross the North Tasman Sea and reach New
Caledonia on Sat 11 Sep.

SUBTROPICAL RIDGE: STR
A big fat high with central pressure over 1030 is moving east along 30S.
It has a squash zone of enhanced trade winds on its northern side over
the next few days. Tonight, Sun 5 sep it is over the dateline.
On Thu 9 Sep it should have wandered east as far as 160W, to south of
Southern Cooks, where it will be reinforced by another High wandering
along 35/40South and the combo high should get to way south of French
Polynesia on the Sat/Sun 11/12 Sep weekend.
Yes, this high is enjoying some of that curtain of sinking air referred
to above in the combo Walker/Hadley cell combo.

TASMAN/NZ
The Tasman Sea/NZ area is slowing moving into a long wave ridge area
after being under the influence of an upper trough for weeks. Even so
there is one more low to come ... This is the one that flooded Victoria
late last week and on Saturday (some places getting heaviest rain in 50
years). Its Northwester and front is crossing most of NZ tonight and on
Mon 6 Sep - heavy rain for Southern Alps and gusty east of the divide.

The low itself should follow and is expected to split into two across
central NZ on Tue 7 Sep--- that will allow a cold southerly to possibly
bring rain/sleet/snow to southern NZ on Monday night to Tuesday night,
and may be some heavy rain to Bay of Plenty during Tuesday.

Thanks to the new long wave ridge, these lows are likely to fade on Wed
8 Sep as a new HIGH crosses southern NZ, but this does bring onshore
flow and rain to eastern North Island.

On Thu/Fri 9/10 Sep, as that new High moves off, a northerly flow is
likely to develop over NZ, but there may be a trough over the North
Island. Then the forecast is for a front on Sat 11 Sep followed by a
westerly flow on Sun 12 Sep. The outlook next week 13-18 Sep is for
West to southwest winds over Northland, so those days may not be good
days for sailing towards 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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