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

29 May 2011

BOBGRAM7 issued 29 May 2011

WEATHERGRAM
YOTREPS
Issued 29 May 2011
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.

TROPICS
La NINA is now starting weakening rapidly in the atmosphere - the
Southern Oscillation Index 30day running mean dropped from 1.21 on 8May
to 0.3 by 22May and has remained that low ever since. We are now in
neutral territory.

TC SONGDA is now re-curving and weakening, and should soon fade as it
sideswipes eastern parts of Japan. So the Northern Hemisphere cyclone
season has started.
USA's tornado season this year has been extremely intense. I'm unsure as
to the contributing factors, one may be the fading La Nina - 2008 was
the last time we had a La Nina fade around May/June and it too was an
intense and early tornado season (but not as intense as 2011).

Galapagos to Marquesas: More of the same. Avoid the Equatorial current
as it is against you this year. Dive south to around 5S 100W and then
head almost due west to 7S 133W and then head for landfall. There may
be some tropical showers (~120W), nothing organised.

South Pacific Convergence Zone SPCZ spent last week in a more relaxed
and spread out fashion. There was a section that linked in with a
mid-latitude trough and fed cloud from New Caledonia to NZ last
Thursday. The main SPCZ now seems to be from Coral Sea to across Vanuatu
then Southeast-wards to south of Fiji/Tonga, and is feeding off a
jetstream as well as converging surface winds. This zone is expected
to travel even further south this week and merge with a trough/low near
the North Island around Sat 4 June and then, next week, it is likely to
travel back northwards as the attached trough travels eastwards.

There is another convergence zone loosely hovering between Northern
Cooks and main Island groups of French Polynesia and this should stay
put without much change.

SUBTROPICAL RIDGE/ NZ/TASMAN SEA
High in the Tasman Sea is maintaining a SW flow over NZ. Around mid
week this system is expected to team-up with a trough forming in the
north Tasman Sea and then both should move east as a couplet onto NZ by
late week. The High is likely to progress faster than the low: so
that, by Sat 4 June, the High may be over Chatham Islands and Low may be
to west of Northland: there will be a squash zone of enhanced
northeasterly winds inbetween, along with frontal rain, over
northeastern North Island. A challenging start for this year's Auckland
to Fiji Yacht race.

That Low is expected to be collected by an incoming trough by late Sat 4
June and this combined system should move from west to east across NZ on
Sun 5 May, preceded by NW winds, accompanied by rain, and followed by
S/SW winds.
Another trough is likely to cross NE from west to east on Tue 7/Wed 8
June followed by colder southerlies. So, the weather over NZ is looking
to be unsettled for and following Queen's Birthday weekend.

WHEN TO DEPART NZ FOR FIJI/TONGA?
There is a squash zone forming near 33S in Tasman Sea by end of Mon
30May and this is likely to spread to 175E by Tuesday, ending the
current sailable pattern.
This squash zone should peak over Northland/Auckland on Fri 3/Sat 4 May.
Avoid. The SW change on Sun 5 June is too brief for a comfortable
voyage, for it may be followed too quickly by a trough on Tue/Wed 7/8
June. At this stage the next comfortable looking weather pattern may be
around 10 June. However, it is too far away to be sure.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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