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

20 December 2008

BOBGRAM7 issued 21 Dec 2008

WEATHERGRAM
YOTREPS
Issued 21 Dec 2008
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos, these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

On this the eve of the solstice, I wish seasonal greetings and glad
tidings to all of good cheer.

TROPICS
As forecast in last week's weather gram, TC BILLY formed northwest of
Darwin early last week and wandered southwest to west, making it onshore
near Wyndham on Friday /Saturday where it wound down. Those of you
with good Internet access can view it at
http://www.youtube.com/watch?v=oHQ48t50cTA. BILLY is expected to move
offshore and wind up to cyclone strength again, well offshore, peaking
Tuesday to Thursday this week then fading away.

BILLY formed as a Madden Julian Oscillation MJO wandered by across north
Australia. A MJO is a pulse of extra activity that wanders eastwards.
This MJO helped start the wet season in northern Australia and brought
them a humid NW flow. Equatorial westerlies have arrived over Papua New
Guinea and may penetrate eastwards towards Nauru for Christmas.

The MJO is expected to have peak impact over the Coral Sea between now
and Tuesday/Wednesday. There is a trough from northern Coral Sea across
Vanuatu to east of NZ North Island. The NOGAPS model output shows the
possibility of a low forming in north Coral Sea around Tuesday and
wandering off to the southwest. GFS, NOGAPS, UK and EC outputs all have
a low deepening to north of NZ on Monday and crossing close to East
Coast of North Island on Tuesday then whizzing off to the southeast.
That's about all we seem to be getting out of this MJO. After this it
should be quieter for a few weeks.

South Pacific Convergence Zone SPCZ is mainly extending from east of
Solomons across Tuvalu and Samoa and then to the southeast. It was very
active last week between Niue and Southern Cooks.

Not much expected elsewhere in South Pacific tropics, mainly weak NE
winds on eastern side of SPCZ and weak variable winds to west and south
of SPCZ; southeasterly between New Caledonia and Australia.

SUBTROPICAL RIDGE
HIGH in Tasman Sea on Monday is ridging eastwards towards central NZ,
and will be there on Thursday, Christmas Day, but may split into a
northern and a southern part. Then it should reform east of NZ along
dateline on Friday, Boxing day, and wander east along 40S. There will
be a good zone of enhanced trade winds on the north side of this HIGH
between 20 and 25S. This HIGH is likely to be followed by a week or so
of troughy weather, so kiwi should make hay whilst the solstice sun
shines.

TASMAN SEA/NZ
I've already mentioned the low from the north on Tuesday 23 Dec,
clearing Christmas Eve. Avoid. That should bring the last rain before
Christmas and then the outlook is for a generally dry period, albeit
occasionally cloudy, from Thursday to Sunday. Then, from 29 Dec, a
moist warm northerly flow is likely.

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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