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

11 December 2010

BOBGRAM7 issued Sun 12 Dec 2010

WEATHERGRAM
YOTREPS
Issued 12 December 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. Oceanic Index

La NINA: Still strong. Average atmospheric SOI over past 30 days is 2.10
(a rise of 0.41 in the past week). Oceanic Nino Index for Sep-Oct-Nov is
-1.4 and steady.

TROPICS
South Pacific Convergence Zone SPCZ returned to normal last week, draped
across the Coral Sea and Vanuatu with build ups over Fiji and Samoa. And
there is scattered activity along 15S to French Polynesia FP.

A aging low centre near 25S 170E (to SE of New Caledonia) has a
circulation that is drawing SPCZ activity southwards across Fiji and
Tonga. This centre is expected to fade as it wanders southwards
towards Norfolk Island, and SPCZ activity should continue to hug 15S
this week.

There is a Madden Julian Oscillation MJO of enhanced convection making
its way across the Timor Sea this week and into the Coral sea between 15
and 20 Dec. This should activate the SPCZ. Normally it increases the
risk of cyclone formation, but this MJO is likely to lack a zone of
equatorial westerlies so may not have much impact in the Coral Sea.

Some models are picking that a zone of weak equatorial westerlies may
trigger a tropical low or two to form off NW Australia. The latest GFS
model is picking one to form in Gulf of Carpentaria between Tuesday 14
and Friday 17 Dec and for this to then move SW then S inland towards
Alice as a wet Monsoon Low. In any event an Australian conveyor belt of
upward motion is being set up in the heat trough from North Australia to
Queensland, sometimes reaching Tasman Sea and then onto New Zealand, and
this NW flow in the upper air is likely to last for the next fortnight.

SUBTROPICAL RIDGE: STR.
The STR was knocked north last week but a weak Polar outbreak (polar
index went negative). Southern lows took the opportunity to expand
northwards and one passed by close to NZ - It is now near 45S 165W and
it seems destined to deepen as it wanders north getting as far north as
40S 150W by Wed 15 Dec then exiting to the southeast. Associated High
is making its way east along 30S. Another high is forming just east of
New Zealand tonight Sun 12 Dec. This should wander along 45S on Mon 13
Dec the slide around south-side of that low, but leave a cell near 30S
so that enhanced trade winds are expected all week from FP to Tonga.

Another High is expected to cross NZ Wed /Thu 15/16 Dec and then blossom
east of NZ and slowly shift north to 35S by Tue 21 Dec.

TASMAN SEA/ NZ AREA
Some more southern lows are likely to get near NZ this week. One is
expected to pass by south of Macquarie Island tonight Sun 12 Dec and an
associated front is expected to swipe by NZ on Monday/Tue 13/14 Dec,
followed by a showery southerly change.

A small low may form in the Australian conveyor belt off Queensland
around Wed 15 dec and bring some welcome rain to northern NZ in a NE
flow around Thursday 16/Friday 17 Dec. This may start a trend.

Then another southern low should pass south of Tasmania around Sat 18
Dec as an accompanying low may form off Sydney, feeding from the
activity in the Australian conveyor belt. These lows may dance around
each other around Sun 19 Dec, whizzing close by Southland and blasting a
strong northerly flow across most of NZ. Avoid. Its frontal band is
then expected to cross NZ on Monday-Tuesday 20-21 Dec.

Keep an eye on that conveyor belt as it is likely to continue producing
Lows in eastern Aussie seaboard to NZ area from 23 to 27 Dec, and this
may have an impact of the Sydney-Hobart.
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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