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

06 June 2010

BOBGRAM7 issued 6 June 2010

WEATHERGRAM
YOTREPS
Issued 6 June 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.

Whoops - I see that last week's email heading was dated 30 July 2010
instead of 30 May 2010.

TROPICS
South Pacific Convergence zone SPCZ is still gradually reforming along
10S, from Papua New Guinea to east of Solomons to Tuvalu/Tokelau and
extending to Samoa at times, also extending south between Vanuatu and
Fiji at times. During this week it seems that there will be more
convective activity building in the Coral Sea, and this may culminate in
a trough/low forming west of New Caledonia by around 19 June (end of
NEXT week).

There has also been more rain clouds than normal in a zone from just
south of New Caledonia onto northern NZ . This zone isn't part of SPCZ,
rather it is a combination of the northern end of mid-latitude frontal
systems + jetstream intensification + a large upper low stuck in the
western Tasman Sea and developing a series of surface lows to throw at
NZ, like a bowler player cricket. This "low index" pattern started in
mid May just in time to harass this years Auckland to Fiji race. It
seems to me that the pattern is about to change this week to the more
familiar "Roaring 40s" pattern.


SUBTROPICAL RIDGE: STR
The STR has been absent over NZ since mid May. There have been thin
ridges squeezing along 20 to 30S - north of the normal latitude, making
for lighter than normal trade winds between Fiji and French Polynesia.
This is about to change. The next STR is over central New South Wales
today (sun 6 June) and should wander east between 30 and 40S, crossing
NZ on Wednesday 9 June, and then settle between 25 and 35 South as it
wanders east of NZ.

The following STR should be a large HIGH in the Aussie Bight on Fri 11
June. As it crosses Bass Strait on the weekend of 12 /13 June it should
shovel polar air onto southern NZ producing a solid dump of snow there.
Models indicate this High will not be able to get into the Tasman sea at
40S , so it should go N across eastern OZ on 14/15/16 June and then
move east along 30 to 35S - across northern Tasman sea and north of NZ
on 17/18/19 June--- with enhanced easterlies on its northern side and
enhanced westerlies on its southern side --- the "Roaring 40s" will be
back.


TASMAN/NZ
Low crossing central NZ on Monday should go southeast across Chathams on
Tuesday. There is a sting in its tail, as one more squally trough with
W to SW winds is expected to be crossing northern NZ on Tuesday as well.


Light winds with a ridge on Wednesday.

Trough/front is expected to move from Tasman Sea across NZ on Thursday
and early Friday. The SW winds following this trough should reach
Northland Friday morning, producing a reasonable looking window for
sailing northwards.

Additionally, there is a brief window late Tuesday and early Wednesday,
with the minor impediment of some pre-frontal northerly winds on
Thursday and - note well- following the Friday front - there is
expected to be a period of heavy westerly swell over the seas north of
Northland.

I'm gearing up now to attend to MetService display at Fieldays from Tue
15 to Sat 19 June. There should be a briefer weathergrams issued Sun 13
June, then I'll mostly be unavailable until 19 June,


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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