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

24 October 2010

BOBGRAM7 issued 24 Oct 2010

Issued 24 October 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.

When the moon fades into its last quarter there may be a rising of
Pololo (or, in Fijian, Balolo). These coral worms choose this time of
the year to spawn. Ask the locals about them, may be fun to watch and
perhaps catch and cook.

LA NINA and the coming cyclone season
First, a recap on our weather zones: The weather engine starts with
energy from the sun. The warmest seas are near the equator and sun on
them causes evaporation which rises to form the Intertropical
convergence zone. Air rises as far as the tropopause and then travels
pole-wards. In the southern hemisphere a lot of this air sinks around
30S and returns along the surface as trade winds back to the equator---
this is the Hadley cell. The zone of sinking air is called the
subtropical ridge. Further south are the westerly winds of the roaring
40s. These weather zones move about, causing seasons. By the time we
get to the longest day, around 22 Dec, the subtropical ridge is usually
"following the sun" southwards and gets to around 40S. This is all part
of the annual cycle.

The second strongest cycle for seasonal weather is the ENSO = El Nino
Southern Oscillation (there are others). When the seas along the
equatorial Pacific Ocean are cooler than normal, as the are now, we call
it a La Nina episode. The weather engine turns over more slowly. The
peak upward motions in the Pacific occur over the Australian side rather
than the Peru side. This has the impact of, in the Southern Hemisphere,
encouraging the weather zones to go further south than normal. Already
the "anticyclones of summer" are reaching northern NZ, and there are
signs of an early start to the wet season in northern Australia. This
also tugs the South Pacific Convergence zone SPCZ to the south and west.

When we look at the sea surface temps, the Oceanic Nino Index ONI for
Sep was -1.0 and when we look at the weather maps the SOI (Tahiti to
Darwin normalised pressure difference) was +2.5. Both these point to a
La Nina that is already moderate to strong. The sub-surface temps in
the central Pacific are cool as well, so the consensus is that this La
Nina will continue through the coming South Pacific Cyclone centre.

NIWA has been able to come up with 8 similar cyclone seasons: Nov 1970
to April 1971 or 70/71, also 71/72, 73/74,75/76, 88/89, 98/99, 99/00,
and 07/08. Using these years as a guide, the forecast for the Nov 2010
to April 2011 season is for 9 to 12 named storms (9 is average). 3 are
forecast to reach at least category 3, and 1 is forecast to reach at
least category 4 (average winds near centre of 64 knots or more). The
Coral Sea and surrounding places west of 180 have an elevated risk of
Cyclone impact.

Although there appears to be a reduced risk for places east of 180, all
communities should remain alert and prepared. In the 8 similar seasons
chosen there have been some cyclone impact in southwest parts of French
Polynesia and in the Southern Cooks. During previous moderate to strong
La Nina's, cyclones have been able to leave the tropics and cross the
Tasman Sea onto southern NZ.

TROPICS (this week)
South Pacific Convergence zone SPCZ is rather spread out, active, and
wide, from the Solomons to Fiji to Northern Tonga and then stretches
southeast across Southern Cooks. There is an active upper trough just
NW of New Caledonia tonight Sun 24 oct. This should cross New Caledonia
on Mon 25 Oct, southern Vanuatu on Tue 26 oct, and then Fiji and Tonga
on Tue/Wed 26 and 27 Oct. Another upper trough is being picked by the
GFS model to cross New Caledonia on Sat/Sun 30/31 Oct. Avoid.

On Mon 25 Oct a new, replacement, HIGH is expected to develop over NZ as
the older one wanders off to NE and fades away. This new High then
should migrate east along 40 to 45S--- that's further south than normal,
but there will still be an zone of enhanced trade winds on its northern
side - a weak squash zone-- mainly between Fiji/Tonga and NZ from around
27 Oct to 1 Nov. This will provide spirited and bumpy reaching

Another replacement trough is expected to travel NE across NZ on
wed-Fri, 27-28-29 Oct , stalling awhile over central North Island ...
light winds and showery. This should be followed by an intense high
migrating east along 50S, making for a broad zone of easterly flow
between tropics and NZ, that then rotates into a northerly flow in the
Tasman Sea. This is called low-index*.

Conditions are OK this week, if you are happy with the enhanced trade
winds on the way and if you dodge the squalls of the SPCZ and its
passing upper troughs. Too early to say much about next week yet,
but the weather pattern does seem to be becoming so much of a "low
index" that the next likely step may well be a deepening low in area
south of New Caledonia in first week of November. So it may be better
to leave early rather than linger.

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