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

18 April 2010

BOBGRAM7 issued 18 April 2010

Issued 18 April 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.

Best latitude with west-going current is just north of the equator from
Galapagos to 120W and then head direct for Fatu Hiva - winds are mostly
from SE around 10 to 15 knots. Reasonably clear of showers this week.

South Pacific Convergence Zone SPCZ is having a slight boost from a
passing Madden Julian Oscillation MJO. There was a burst of convection
over Vanuatu and New Caledonia last Friday 16 April, it is over Fiji
tonight 18 April and is forecast to go southeast, caught in an upper NW
flow. This burst should develop a small low south of Niue and northeast
of Kermadecs on Mon 19 April and that should wander off to the southeast
and deepen for the remainder of the week.

It is getting late in the cyclone season now and the chance of a cyclone
forming in the SW Pacific is starting to decrease, but raises up a touch
as this MJO has influence. Current indications are that there may be a
build up of convection in the Coral Sea during the coming week. This
cloud build-up may sweep onto Queensland and get captured by a trough
crossing New South Wales around Fri 23 April and then wash across
southern NZ as a warm/cold front combo on the 25 April. Or it may
follow last week's pattern and form a low over New Caledonia by 25 April
- maybe a bit of both. If a low does form and move onto New Caledonia,
then that raises the eyebrows to the possibility of a last gasp of a
seasonal cyclone.

HIGH cell in Tasman Sea has a squash zone of enhanced trade winds on its
northern side between New Caledonia and Queensland coast. It also
maintains a waxing-waning-southwester over New Zealand until Thu 22
April, which is when it should nose across the North Island. It then is
expected to wander off to east of NZ mainly along 40S from 23 to 30
April. That should be helpful to the sailing waka that have departed
Auckland heading for Tahiti.
The STR is expected to weaken after this high passes by, allowing a
pattern change in the Tasman Sea/NZ area to troughs, fronts and southern
ocean lows from 23 April to end of the month. People in the dry lands
around NZ have their tongues hanging out of this pattern change. And it
looks as though it'll arrive for some in time for ANZAC day.

Next HIGH in the STR is expected to cross inland Australia rather than
the Aussie Bight. The longer nights are helping for colder denser air
over inland Australia. This high is thus likely to get knocked inland by
the lows of the Southern ocean and so cross Australia from 28 April to 1

Repeating the details above: a waxing-waning-souwester until Thursday,
then a warm/cold front combo over the South Island on Fri 23 April,
followed by a larger, more widespread, warm/cold front combo on Sun
25/Mon 26 April and a third on 28/29 April, possibly drawing in some
cloud from north Tasman Sea onto North Island around 30 April.
Unsettled enough to delay departures.

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