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

21 February 2009

BOBGRAM7 issued 22 Feb 2009

Issued 22 FEB 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.

That LOW we mentioned to avoid last week was briefly named TC INNIS as
it moved off to south of New Caledonia last Wed. It provided an oomph
of tropical moisture to the low that was stalled near Brisbane (the one
that flooded New South Wales early last week). The frontal zone on the
eastern side of this new family of lows did indeed bring briefly intense
rain to the North Island as it passed over on Friday. And the Lows are
still taking their time to cross southern NZ, raining out at around
2mm/hr. Yuck.

So the South Pacific Convergence Zone SPCZ has been drained and is now
There is a "left over" trough over the Queensland area. Not really SPCZ
stuff, so more about that later.

The SPCZ then is reforming over Solomons and Tuvalu. East of the date
line there is little coherence - scattered convection over a wide area
from Tokelau and Northern Cooks in the North to Tonga, Niue, Southern
Cooks in the South and out to French Polynesia in the east. Winds here
are mainly from north of east and reaching a peak of around 20 knots
around 20 South. Convection/ Shower activity is expected to shift
southwards this week but no Lows are expected east of the date line.

This is the main area of interaction and development this week.

SO we start with a "left-over" trough near the Queensland Coast, still
generating weather. At surface level it is marked mainly by a shear
zone between lighter easterly winds on its northern side and a squash
zone of enhanced SE winds on its south side maintained by a HIGH that
will be crossing the South Tasman Sea next few days.

This week there is expected to be a procession of lows and highs across
the southern Tasman Sea and NZ. That family of Lows still crossing
southern NZ today and Monday will be followed by a cool SW flow over
all NZ on Tuesday. Then we get the mid week HIGH, just as last week,
only this time not as intense. Its likely to split into two cells, one
at 35S and another at 55S. When the Sub-tropical ridge is weak like this
NZ gets more weather.

SO the next LOW should be interesting... models have it forming over
Tasmania on Monday and then wandering almost due east across the Tasman
reaching NZ on Thursday and Friday. This is a different path from last
week's low from the north, and less wind and rain is likely.

Models are at present producing different scenarios after this LOW...
some development is likely on the "left-over" trough in the North Tasman
Sea. For next weekend NOGAPS has output which develops a low near New
Caledonia, EC has a Low near Lord Howe, and GFS and MetService models
are quicker and have an output with a series of lows between Norfolk and
northern North Island. Too early to be sure, to be sure.

Anyway, the next High moving in from the west should cross the south
Tasman sea on 2-3 March , preceded by a cool SW flow over NZ, and then
a weak trough around 5 March and then a more substantial HIGH over
Tasman Sea/NZ on 6-8 March with a strong squash zone on its northern
side. Next lot of tropical lows will probably form when this HIGH moves
off to the east around 10 March. Enjoy a break in the cyclone season.

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