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

15 May 2011

BOBGRAM7 issued 15 May 2011

WEATHERGRAM
YOTREPS
Issued 15 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.
Tonight's edition has been delayed by my attending and giving talks at
local Boat Show past 4 days, then catching up on email this evening..

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
last week and 0.49 today. This is related to a DROP from higher than
normal pressures around Tahiti.

South Pacific Convergence Zone SPCZ spent last week in active mode from
Solomons to Vanuatu to a zone between Vanuatu and Fiji, with another
branch from Niue to Southern Cooks area. There is now a convergence
zone over Northern Cooks.

The zone of convergence that has been along 5S between 100 to 140W has
started to fade-Puddle jumpers can now set off from Galapagos to
Marquesas

Low tonight 995hpa near 35S 135w is moving off to the SE.

A small low is likely to form on the SPCZ when it is over southern
Tonga/Niue around Thu 19/Friday 20 May. This low may weaken into a
trough as it wanders off to the SE-it should form a zone of strong winds
and heavy swells near Minerva for Thu/Fri 19/20 May, avoid.

SUBTROPICAL RIDGE:
The Big Fat High that was labelled H1 in last week's (and the week
before) weathergram has finally faded. Another HIGH H2 has been
spending last few days crossing the interior of Australia- this
indicates a radical change of season-Highs before this one found the air
over central Australia too thin and warm, but H2 was preceded by cold
air from the Southern Ocean dropping temperatures to abnormal levels
first in Melbourne and, last week in Sydney. SO H2 has found dense
enough air to take the typical WINTER track across central Australia. It
is expected to cross New South Wales on Tuesday 17 May and then cross
Tasman Sea NZ Area along 37 to 42 S from Wed 18 to Sat 21 May- then off
to the east of NZ next week.

NZ/TASMAN SEA AREA
H1 has been feeding a moist NE flow onto the NZ region since Easter,
energizing the local troughs and producing a frontal graveyard that
spent much of last week neat the dateline.

The collapse of H1 has allowed a REGIME CHANGE over NZ: no more NE
flow, now it's a SW flow. The cold front that crossed NZ on Fri 13/Sat
14 has been followed by an unstable/cold west to southwest outbreak.
This flow is likely to stay in place until it is weakened away on
Wednesday by incoming H2.

WHEN TO DEPART NZ FOR FIJI/TONGA?
Winds around Northland/ Auckland may be around 20 gust 30 knots on
Monday Tuesday
--- but are also likely to be 30 gust 40 knots in passing showers, so
that's one reason to delay until Wednesday.
Also the Tasman Sea is filling up now with large swells, probably
peaking in mid Tasman on Tuesday 17 May at around 8 metres! Heavy
swells are likely as far north as New Caledonia and Fiji from Wed 18 May
to Friday 20 May.

Departing from Northland on Wednesday/Thursday seems to be the best -
avoids the possible squally showers on Monday Tuesday and, if your trip
is into part of the north Tasman Sea, these departure days mostly avoid
the big swells.
Those big swells do not have much impact on a trip to Tonga, but we are
expecting some strong winds and heavy swells around Minerva on Thu/Fri
19/20 May due to a brief trough/low forming near Southern Tonga to Niue.


Departing from Friday 20 May onwards puts you in the easterly winds on
the northern side of H2. These winds may be enhanced into a squash zone
near 20S next week, but at this stage things are looking OK - more about
this next 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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