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

22 May 2011

BOBGRAM7 issued 22 May 2011

WEATHERGRAM
YOTREPS
Issued 22 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 was 2.55 on 23 April, 1.21
on 8 may and 0.3 today. This is related to a DROP back to normal
pressures around Tahiti.

Now we are two months after the March 21 Equinox and the meteorological
year is building normally... Pre-monsoonal heat is developing in India,
the jetstream have moved off the Himalayas (it is that magic fortnight
called Everest climbing weather). Cyclones are starting in NW Pacific -
SONGDA should develop storm force winds as it re-curves just off the NE
end of Philippines.
And in the Southern Hemisphere the lows of the southern Ocean are
starting to expand their girth, so that a polar outbreak which hit South
Australia in mid May, gave strong winds to NZ from 16 to 18 May, and
brought huge swells to Fiji's South Coast ~20 May.

So, I suppose the Galapagos to Marquesas sailing season is now opening.
It looks OK to go now. Avoid the Equatorial current as it is against
you this year. Dive south to around 5S 100W and then head almost due
west to 6S 133W and then head for landfall. There may be some tropical
showers (~120W), nothing organised.

South Pacific Convergence Zone SPCZ spent last week in active mode from
Solomons to Vanuatu to southern parts of Fiji and then SE to south of
Niue where it mingled in with a passing mid-latitude front. This zone
is now retreating to the north and weakening.

A low is likely to form near 25S 155W by Monday 23 May and then go south
and deepen - may be used as a clockwise helper for vessels on the
NZ/Tahiti route

A new trough should rekindle SPCZ in southern Coral Sea on Mon 23 May.
This should cross New Caledonia on Wed 25 May and Fiji/Minerva area on
Sat 28 May then reach Southern Cooks around Wed 1 June (These features
travel with the upper westerly winds even if the surface winds with them
are modified trade winds). A new Low is likely to form on this trough
around Sat 28 May near 27S 160W and then deepen as it moves South
trough--- another clockwise helper for the NZ to Tahiti crowd.


SUBTROPICAL RIDGE:
Large High is moving east to east of NZ this week. Mainly along 40S.

Next High is expected to cross New South Wales around Fri 27 may and
reach central NZ around Mon /Tue 30/31 May, also along 40S.

These are mediocre anticyclones with average squash zones of enhanced
easterly winds north of them at around 20S. However, for tonight,
there is a squash zone with gale force SE winds near 25S from 165 to
170W between High and Low.


NZ/TASMAN SEA AREA
Between the Highs there is a transitional front and trough. Its clouds
are already active over Victoria and Tasmania tonight, along with some
preliminary troughs that are in mid Tasman. Front is expected to cross
NZ on Tue/Wed 24/25 May, preceded by a strong northerly flow. This
should be followed by the trough on Thursday 26 May--- might be a low
with this over North Island. And then a cold southerly change over NZ
on Fri 27 May - turning into a SE flow over the North Island for Sat 28
May.

WHEN TO DEPART NZ FOR FIJI/TONGA?
The High is now leaving NZ and there is a northerly flow on its western
flank, then the front and trough. SO, it is best to wait now for the
next trough to go by - maybe catch the southerly change on Friday 27 or
go with the SE flow on Sat 28 May.

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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