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

15 August 2010

BOBGRAM7 issued 15 Aug 2010

Issued 15 August 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.

The causative factor for the cold air that brought floods to Pakistan
and slips to China AND the hot air bringing heat waves to Russia is the
same: meanderings in the Northern Hemisphere's jet streams. Over the
past few days NZ copped a burst of water vapour that had come from the
monsoon area, and this brought something like 200 to 300mm rain to our
mountains with some flooding and slips most notably around Whakatane,
but some drier air is now separating Asia and South Pacific.

The South Pacific Convergence zone SPCZ has faded recently over Papua
New Guinea and Solomons - should rebuild there by 20 Aug. It has built
up along 10S between Tuvalu and Tokelau, and is likely to slowly spread
south possibly affecting Rotuma to Samoa by Friday 20 Aug.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds along 20S. One of these is moving
east off the Tuamotus today 15 Aug, and another should cross Tonga on
16 Aug, Niue on 17 Aug and Southern Cooks on 18 Aug. Then the
subtropical ridge may be strong enough to block any northern extensions
of mid-latitude troughs, for a while anyway.

Small high cell in the north Taman Sea today 15 Aug is expected to move
east along 25S and fade near 180 on Mon 16/ tue 17 Aug. Next High will
be larger, forming in Tasman Sea on Wed 18 Aug and then moving east
along the 25 to 30S latitude band, reaching 180 around Fri 20 Aug, and
likely to be south of French Polynesia around Sun 22 Aug. There should
be a squash zone of enhanced trade winds on the northern side of this

A long wave upper trough is hovering around the Tasman Sea this week.

At ground level, a broad cold trough, with squally thundery showers, is
now crossing the Tasman Sea and should cross NZ on 16 and 17 Aug.
Its SW flow is likely to linger over NZ on Wed 18 and much of 19 Aug.

Most settled day this week is looking to be Fri 20 Aug - for anyone
contemplating sailing to or from NZ.

Another broad cold trough is expected to cross Tasmania around Thu 19
Aug and reach the South Island on Sat/Sun 21/22 Aug.

There is a parameter that measures the strength of the polar vortex
(that ring of westerlies than encircles Antarctica). It is called the
AAO and can ne seen at
- and known locally as SAM (Southern annular modulus).
Not usually relevant here, but has been way out of whack recently with
record high values, up to 4 standard deviations from its mean last
month. This suggests that Antarctica's cold air has recently all been
locked up, no polar outbreaks anywhere. Well, computer modelling is
suggesting this will all change by end of August, and then polar
outbreaks will be on the menu again (not sure which longitude band yet).

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