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

18 July 2010

BOBGRAM7 issued 18 July

WEATHERGRAM
YOTREPS
Issued 18 July 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.

TROPICS
South Pacific Convergence zone SPCZ is showing signs of drifting south
after a few weeks of hovering along 10S to 14S. One branch extends from
Solomons to Wallis/Futuna, and another branch is hovering between 10 and
14S between Tokelau and Samoa and around Northern cooks.

The SPCZ is occasionally enticed southeastwards to merge with
mid-latitude troughs. As this combination wanders east along around 20S
it swings and then kills the trade winds for a day or so, but that's
about all this week.

For those in Tahiti looking to go west: One of these troughs is
expected to pass by Rarotonga on Mon 19 July. The next is departing
Queensland at present, should pas by New Caledonia on wed 21 July, Tonga
on Sat 24 July and Rarotonga around Tuesday 27 July. Otherwise this is
a week of trade winds.


SUBTROPICAL RIDGE: STR
A lot can be gleaned by watching the rhythm of the highs as they wander
east in the subtropics. Their shifting isobars mould the troughs
inbetween them.
This week the STR is becoming better defined in the pacific than it has
been for awhile.

One High with central pressure around 1025 is wandering along 30 to 35S
, across Northern North Island on Mon 19 July, then slowly and steadily
east or southeast to get to 40S well to south of French Polynesia around
Friday 23 July. This High is mediocre, so will maintain moderate trade
winds and settled weather in the tropics on its northern side. Good for
sailing to the west.

Next High is likely to be more intense, building to 1035+ between 35 and
40S in South Tasman Sea around Mon 26 July. Too far away to be sure,
but this high is likely to cross central NZ on Wed 28 July and should
have a squash zone of enhanced trade winds on its northern side - and
these strong winds will likely start in Coral Sea from Wed 21 July and
spread east to reach Vanuatu/New Caledonia by weekend of 24/25 July and
further east next week. Be wary.

TASMAN/NZ
It is between the highs that the lows can flourish, scrum together, and
elbow outwards.

There's a trough departing the Australian eastern seaboard tonight (18
July) and as this trough crosses the Tasman Sea on Monday/Tuesday 19/20
July a Low is forecast to form within it. This low will likely have
colder than normal air in its upper parts and that help to deepen its
central pressure, slow it down and breed squally showers/strong
winds/heavy swell. It may deepen to below 995 while still west of the
North Island on Wed 21 July. Avoid. Should then start to weaken , but
may still be around 1000 hPa as it crosses North island on Friday 23
July, and then it should wander off to southeast of NZ and expand to
cover the broad area between the two highs mentioned above.

So anyone sailing to NZ should best arrive by Tue 20 July, and this
isn't the best week to depart NZ.

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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