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

18 October 2010

BOBGRAM7 issued 18 oct 2010

WEATHERGRAM
YOTREPS
Issued 18 October 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.
I'm back in the office now from attending to a display for MetService at
the Waikato Boat Show over the weekend, but had to wait until tonight to
get any spare time to write this.

LA NINA
SOI (90 day) is now 2.2 and reasonably steady. This is a strong La
Nina, more than two standard deviations from its mean at present. More
on this in next few weathergrams.

TROPICS
South Pacific Convergence zone SPCZ is now in its normal October
position from Papua New Guinea to northern Vanuatu, then, weak at
present but hovering between Rotuma and Fiji, then move active over
northern Tonga and Niue/Southern Cooks.

An upper trough over South Coral Sea and New Caledonia tonight 18 Oct is
embedded in the upper westerlies rather than the surface trade winds, so
it migrating eastwards... it should activate the SPCZ over Fiji and
Tonga on Tue wed 19-20 Oct. On late wed 20 and during Thu 21 Oct, a
cold southerly / SW wind (from NZ) is likely to reach as far north as
Kermadecs, just as this upper trough reaches the Niue area and this
combination may deepen the pressure around Kermadecs, thus strengthen
the cold winds between NZ and Fiji Tonga for a day or so. Wait for this
to move on before sailing south.

SUBTROPICAL RIDGE: STR.
Remains of last week's BFH are 1028+ at 40S to south of French Polynesia
FP moving off to the east, maintaining good trade winds over FP, much
like last week.

High 1028 is expected to move east along 35S across New South Wales on
19 Oct and then slowly across the Tasman Sea to cross the Auckland area
on Sat 23 Oct, just in time for the NZ Labour Day holiday weekend. Also
in time to impact on this year's Coastal Classic, so that there should
be a good SW wind in Auckland for a spinnaker start on Friday and then a
fading left-turning breeze over night to challenge the heavier boats.
And as that High continue its eastward migration early next week, a
northerly flow is likely on Monday for the boats then returning to
Auckland.

This slow moving high is a good marker for those planning voyages from
Fiji or Tonga to NZ. Wait for that upper trough to pass by, and so
arrange to depart from Fiji on 21 or 22 Oct (or over weekend if you can
get Customs clearance) and arrange to depart from Tonga on 22-23-24-25
Oct period. The High is expected to be over 1030 and east of NZ from
24 Oct and will then have a squash zone of enhanced easterly winds on
its northern side, ending our departure window. With these voyages as
we sail south the high moves off to the east, so we need to time things
OK and arrange to reach Northland before the following trough and its SW
wind change. In this case that trough is likely to be delayed until the
end of the month, say around 30-31 Oct... but it's so far away this will
need to be updated both at the start and during the voyage.

Good voyages from New Caledonia to NZ or Australia are also on the menu
after that upper trough has got east of Noumea, say, around Wed 20
Oct--- there will be some squash zone impact on the north side of the
Tasman High but nothing major.

NZ AREA
There is a strong upper trough and associated thundery cold front
crossing South island on Tuesday 19 Oct and North Island to the
Kermadecs on Wednesday 20 Oct. On Thu 21 Oct, SW winds still likely to
still be strong and showery in the west, but should be easing. From
then on it's the Tasman High that commands proceedings.

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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