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

09 May 2009

BOBGRAM7 issued 10 May 2009

Issued 10 MAY 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.

There is a weak Madden Julian Oscillation moving east across the central
Pacific at present. This enhanced convection on the South Pacific
Convergence Zone SPCZ - last week it was very strong from Solomons to
northern Vanuatu to Rotuma /Wallis /northern Tonga, and this week it
is expected to be extra active between Samoa and Southern Cooks and over
southern parts French Polynesia. Lowering pressures are forming a river
of surface NW winds from Tokelau to Northern Cooks, so LOWS are ;likely
to form on the SPCZ this week, maybe near Niue, and maybe near Gambier
Islands. Once these lows form they will travel south or southeast,
accentuating the squash zones between them and a large high further
south. Avoid.

West of the dateline it looks like a quiet week - the subtropical ridge
between Fiji and Australia is very weak at present due to the upper low
over NZ, and so the trade winds area are weak and it seems that the SPCZ
will have a quieter week here than last week.

There is nothing much to report on the Galapagos to Marquesas route.

There is a lot of energy tied up in the long waves at present, so the
subtropical ridge has split into weak and strong longitudes. At present
there is a strong HIGH at 40S between 160 and 130W, blocking
proceedings. The High today approaching Tasmania is weakening away-so
the next strong High "upstream" is currently in the southern Indian
Ocean-and it will be moving across the Aussie Bight --- mainly along the
coastline---this week, and is forecast to reach the Tasman Sea NZ area
next week.

So the STR is weak over NZ and the Tasman Sea, and a large multi-centred
Low is the result. The individual low centres are dancing around each
other in a clockwise reel, showing the dimensions of this large upper
low, what we call a cold pool. There has been too much swell and
occasional squalls around this system to allow any comfortable departure
north from Northland over the past few days. Things are expected to
ease over Northland on Wednesday/Thursday of this week, making a
reasonable departure window. There is likely to be another Low in the
Tasman Sea / South Island area from Friday, bringing another dose of
heavy swell to the Northland area on the 16-17 May weekend so that may
shorten this window.

Another complication for vessels heading north is that the GFS model is
picking that a weak trough may be crossing the Northern Tasman Sea area
from Thu 14 May and moving east across the Minerva area around Sun 17
May. EC and NOGAPS models are not so sure. This is a minor
complication, and might not actually appear, or it may require some
planning for comfort. Check the updates.

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