BOBGRAM7 issued 10 Oct 2010

WEATHERGRAM
YOTREPS
Issued 10 October 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.

LA NINA
SOI (90 day) is now 2.19 and reasonably steady. This is a strong La
Nina, more than two standard deviations from its mean at present. We are
currently investigating analogue years - this investigation is still
being done with NIWA. A classic La Nina shifts the South Pacific
Convergence Zone to south and west of its normal Summer position---
this encourages cyclones in the Coral Sea area. Sea surface
temperatures in there are already warmer than normal, so the season may
start earlier than normal. More on this in next few weathergrams.

TROPICS
South Pacific Convergence Zone SPCZ is now in its normal October
position from Papua New Guinea to Rotuma to Samoa to Southern Cooks.
That surfacing upper trough (SUT) near Niue referred to last week,
managed to work westwards and activate some squalls about Fiji Tonga as
well as Niue over past few days--- First real rain for Fiji for a long
time and I think there may have been some jetstream assistance as well.
Anyway -it hailed in Sigatoka, and that's unusual. Maybe we can call
this an early start to the Fijian wet Season.

Another trough is on the map == this time it's a surface trough that
started off from the mid -latitudes and has been stretched out by a
jetstream between Australia and New Caledonia. It should cross Vanuatu
on Mon Tue 11-12 Oct and then Fiji/Tonga on Wed Thu 13 14 Oct and then
fade.

SUBTROPICAL RIDGE: STR.
Remains of last week's BFH Big Fat High is a centre near 1028+ at 40S to
south of French Polynesia FP moving off to the east, maintaining good
trade winds over FP.
High 1038 in south Tasman sea today near 42 S is shovelling some cold
air onto NZ, and has a squash zone with a little trough on top of it
near Brisbane. High is expected to split into two crossing the South
Island on Tue Wed 12/13 Oct - frosty! The western part should then head
NE for Norfolk and Kermadecs for the 16/17 Oct weekend, and the eastern
part should follow the path of last week's BFH and keep the trade winds
well supported over FP.

NZ AREA
The leading edge of the cold air being shovelled over NZ should cross
North Island tonight (sun 10 Oct). During Mon 11 and Tue 12 Oct the
trough associated with this air is expected to sharpen and form a low
between Northland and Kermadecs. AVOID. This low should then move SE on
Wed Thu 13 14 Oct. There's a ridge over central NZ mid-week and then
over the North island for the weekend. For South and central NZ, a
moist NW flow is expected for the 16 17 weekend.

Heading for NZ? Don't venture south of 25/30S until that LOW had gone
away - say, around Thu 14 Oct. Then try and time arrival to fit in
with a northerly flow over Northland early next week.

NOTE: I'll be unavailable 14 to 17 Oct, busy attending to MetService
display at Waikato Boat Show. So next weathergram may not be until Mon
18 Oct-or later.

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