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

18 April 2009

BOBGRAM7 issued 19 April 2009

WEATHERGRAM
YOTREPS
Issued 19 APR 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

TROPICS
It is the planetary scale features that best explain what is going on at
present-a deviation from the normal weather pattern with a large scale
upper low in the north Tasman Sea 30S 160E and a twin large scale upper
low near 30S 160W . These two features are a magic 40degrees longitude
apart, which means that the upper high in-between them , near 40S 180,
is blocked and that surface feature rotate around these blocked upper
features for a few days.

To see this , if you have access to full fast Internet go to
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking/real_time_s
h/real_time_index_nrm.shtml
and scroll to the 500 hPa map.

The South pacific Convergence Zone SPCZ links the subtropical fronts of
those two upper lows..., so it snakes from near ne Caledonia/Vanuatu to
Tuvalu/Samoa (this section is weak this weekend but may some back) and
then southeastwards across the zone between Southern Cooks and French
Polynesia.
Over the past week, there has been some noticeable convection along
mainly 5 to 8 South between Marquesas and Galapagos but this is
weakening.

The trade winds easterlies are robust in most parts of the South
Pacific, but are lacking in the zone from Solomons to Fiji/Tonga. A
zone of weak wind is forecast to move along 20S from Queensland to New
Caledonia/Fiji this week, not good for sailing but OK for motoring.

SUBTROPICAL RIDGE STR
This has been displaced well south of normal at present, sort of a La
Nina trait and indeed, the 30day running mean of the SOI has bounced up
in the past two weeks after a falling trend. SO the Big Fat High BFH
that moved to east of NZ during the past week is now moving along 40S.
That LOW which popped up south of Niue in the past few days deepened
more than originally expected but is weakening away now. However, there
may be a few another small low developing near 30S 150W on 19-20 April
UTC and then moving off to the SE-all happening on the "western
shoulder" of the BFH.

The next BFH is crossing Tasmania at present and forecast to cross the
South Tasman Sea along 45S on 21-22 April and NZ on 23 April, and then
move off to the east of NZ along 40S.

NZ AREA
The Low near Lord Howe Island today has a weaker companion near Norfolk
Island. These two features are both caught in the circulation of an
upper low and thus are likely to rotate clockwise around each other,
with the eastern one fading away, its fronts moving onto northern NZ
tonight and Monday. Then the western Low should move south and weaken
on the western flank of the BFH, finally crossing the South Island on or
around ANZAC day Sat 25 April, preceded by its fronts and a NE flow on
Friday and followed by a NW flow over the South Island on the Sunday.
SO, looks to me that there are no-good-days for departing from northern
NZ for overseas this week.

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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