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

18 December 2010

BOBGRAM7 issued 19 Dec 2010

WEATHERGRAM
YOTREPS
Issued 19 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.

La NINA is still strong. Average atmospheric SOI over past 30 days is
2.05 (a drop of 0.05 in the past week). Oceanic Nino Index for
Sep-Oct-Nov is -1.4

TROPICS
South Pacific Convergence Zone SPCZ is in its normal location for this
time of the year, stretching across the Solomons to east of Vanuatu,
moving north and south over the Fiji/Samoa/Tonga area, then weak between
the Northern and Southern Cooks, and then reasonably active again along
south end of French Polynesia. There is a Madden Julian Oscillation MJO
of extra convection making its way eastwards across northern Australia
but weakening. This should wander into the Coral Sea during the coming
week. However, the accompanying equatorial westerly winds so far have
only reached 130E and computer models suggest that they may not be able
to get east of Papua New Guinea during the coming week. So, therefore,
then, there's a slightly increased risk of cyclone development in the
Coral Sea this week, but computers are not picking developments in this
region.

That low centre off the west coast of Australia has sea surface
temperatures around it that are rated to be below the threshold for TC
development. It should slowly wander west and turn into a trough. Of
more interest is what might happen in the Timor and Arafura Seas as the
MJO passes by. Some models are at this stage producing output showing a
TC around Darwin near Christmas Day (shades of Tracy 1974, I can still
remember), but details are all over the place, and the EC model is not
picking any development --- so if that's near you seek updates.

SUBTROPICAL RIDGE: STR.
High to southeast of Chatham Islands is quasi-stationary and should just
slowly wander north from 45S to 30S by Friday 24 Dec and then fade away.


The next high is expected to be able to get from Australia Bight across
Tasmania around Friday 24 Dec and then cross NZ around Sat 25
Dec/Christmas Day and then move quickly off to the east along 40S on
Boxing Day.

TASMAN SEA/ NZ AREA
There is a long wave trough over the Tasman Sea area helping convey
water-vapour from Indonesia, across Australia, to the NZ area. So this
is likely to be a week with three fronts.

The first went across today Sunday, bringing decent rain to the North
Island. The second is likely to be associated with a tight-centred low
tonight forming off the New South Wales Coast ... wind and rain mainly
for South Island on Tuesday, and then weakening over North Island on
Wednesday and Thursday.
The third front should mark the end of this series and lead in a SW
change (a heralding angel for the Christmas Day high), and should cross
the South Island during the day on Friday 24 Dec/Christmas eve, then the
North island that evening.

A note for those planning to sail from Auckland to Bay of Islands for
the holidays - SW winds for this are likely to be few and far between
during next few months, and this SW may only last for the 25th - light
northerlies returning as soon as Boxing Day afternoon.

SYDNEY-HOBART
Still somewhat uncertain, and depends on whether the Timor and Arafura
Seas can produce a tropical low for Christmas or not. At this stage the
idea is that a trough is likely to wander off the New South Wales Coast
around Boxing day, followed by a period of moderate southerly winds
that back to be from east and then northeast by Tuesday 28 Dec.

Check out moonrise on Tuesday 21st (if sky is clear). It is a full moon
right on the solstice and that does not happen often. It will also be a
total lunar eclipse--- the moon will rise into the earth's shadow and
appear red/brown-coloured, so may be particularly memorable.

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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