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

10 January 2009

BOBGRAM7 issued 11 Jan 2009

Issued 11 JAN 2009
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.

Last week we saw the MJO arrive in the Coral Sea. The South Pacific
Convergence zone splayed itself outwards as a broad trough from mid
Coral Sea across Vanuatu to south of Fiji/Tonga.

On its northern side: west to northwest monsoonal winds were able to
make their way east all the way to Southern Cooks. Typically wet and
squally, they were boosted by the mountains of Fiji, making Fiji the
wettest part of the world last week -- flooding rivers, closing towns,
and causing evacuation and two drownings. Over half-a-metre of rain in
a week:

On the SPCZ's southern side there is a squash zone in the trade winds
mainly between 25 and 30S.

Along the SPCZ's trough line are a few lows sometimes with weak winds
near their centres - one visited Vila on Wednesday and is now moving
southeast towards the Kermadecs, and another formed just south of
Rarotonga and has deepened near 33S 165W and is being steered to the
southwest. Yes , the southwest. This low has gales, avoid.

This week the MJO is expected to fade, and the SPCZ should stay in much
the same place, but lows that form on it will move off to the south.
One Low is expected to start near Vanuatu on Tuesday then deepen just
south of Fiji of Wednesday and the go southeast, crossing southern Tonga
on Thursday, Niue Friday and to south of Southern Cooks next weekend
(all UTC). Another Low is likely to form off to the SW of Fiji on
Friday. It's the monsoonal west/northwest winds on the northern side of
these lows that need to be treated with respect.

Over Australia a Low should briefly blossom in the Gulf of Carpentaria
on Tues/wed and then fade away.

The tropics seem to be at their wettest when the Subtropical ridge is at
its southernest. Last week we had a slow-moving high crossing NZ. NOT
this week. There is a HIGH moving quickly east off the South Island
today Sunday, and another is excepted to build in the Tasman Sea on
Monday/Tuesday-that will then shrink to northern Tasman Sea by Friday
and then zip like a flicked pip along 30S and past the dateline this
weekend. SO the Subtropical ridge is expected to reform near 30S by
this weekend and that should help the SPCZ to go north and weaken by
next week.

AT first the South Island should enjoy light winds and settled weather
with a ridge, and for the North Island there is a southeasterly flow.
Part of that Low heading southwestwards may reach the dateline around
Thursday to east of the North Island, turning the winds over the North
Island more southerly. A block has formed in the upper air just east
of the dateline and that's why that Low (or part of it) is taking this
unusual SW track. It means a whole week of surf for North Island
eastern beaches, with the wind swing making my pick for the best surfing
waves in Hawke's Bay to be on Wednesday and Thursday.
From Thursday a change of pattern is expected with troughs moving onto
NZ from the southwest - one of these may deepen into a large low
crossing central NZ by Sunday 18th.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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