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

01 June 2014

BobBlog issued 1 June 2014

WEATHERGRAM
YOTREPS
Issued 01 June 2014
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.

It is Queens Birthday weekend here in NZ, and this reminds me: It was twenty
years ago today that the Pacific Storm started. This summary of the weather
pattern for that event is taken from the Mariners Met Pack at
http://www.boatbooks.co.nz/weather.html#23104

a) On Thursday 2nd June 1994, conditions were almost perfect for the voyage
across the South Pacific from New Zealand to Tonga or Fiji. A high- pressure
area was stuck over New Zealand, with steady trade winds from the Kermadecs
to Tonga.

b) But on Friday 3rd, a tropical depression started forming between Vanuatu
and Fiji. It was NOT a tropical cyclone because it did not have the
symmetrical warm central core that characterizes such systems.

Only when this depression moved south out of the tropics did it deepen
rapidly. This is because it started entraining very cold air, which had been
brought all the way from 60: South by that high-pressure system over New
Zealand. When cold air meets warm air, the warm air is bumped upwards out of
the way. If, as in this case, the upper winds remove the rising air faster
than the lower winds can replace it, then the surface pressure in the
immediate area falls rapidly.

c) Central pressure dropped from 1001 hPa at 0000 UTC Friday 3rd (noon local
time) to 986 on Saturday 4th, making this system a meteorological BOMB.
Clockwise winds around the low centre accelerated to over 50 knots during
Saturday, creating a confused, tossing sea with steeply sloped waves.
Conditions near Raoul Island had turned from fair to foul within
24 hours.

d) The low pressure system reached its peak about 0600 UTC Sunday 5th June
(6pm local time) with central pressure about 978 hPa, generating a swell
judged by rescue aircraft to be 1014 m. A senior pilot of the New Zealand
Airforce No. 5 Squadron with 10,000 hours experience commented that he had
not seen anything like these wave conditions before.

>From Saturday to Monday, 16 yachts in trouble set off EPIRB beacons,
twenty-one people were rescued, but three lives and seven boats were lost.

The weather pattern for this years Queens Birthday weekend has some
similarities to what happened 20 years ago, but also some differences.

During Queens Birthday weekend this year we have a High 1030 in the
southern Tasman Sea (something like 20 years ago), but it is NOT blocked
and is free to travel to east of NZ along 40S. By Wednesday this High
should be east of NZ at around 45S, and central pressure over 1032. There
is an old saying amongst weather forecasters when it gets over1030 the
weather gets dirtyand indeed by Wednesday there should be a squash zone
of enhanced trade winds on the north side of this high, mainly along
around 30S between 170W and 180.

A main difference between this week and 20 years ago is the Low. 20 years
ago it formed in the tropics and deepened rapidly as it encountered cold
air when moving out of the tropics. This year we have had a Low form near
33S 178E, to NE of Northland, today, and this feature is expected to get
pushed off to the NE and weaken during Monday. The low that appears on
the Wednesday map above has come out of the tropics and is encountering
cold airbut this is happening south of the Southern Cooks and there is
little if any marine traffic in that area - anyone sailing from NZ to
Tahiti needs to try and avoid that low.

Once that high starts to move off to the east, and the pressure starts
falling between NZ and Vanuatu next weekend, it is likely that a trough
should deepen to a low in that area, forming a strong NE wind squash zone
between NZ and Fiji . This is likely to pose a challenge to the Sail Fiji
Race 2014 starting on Sat 7 June.

Scott Donaldson was making good progress but encountered a front and then
SE winds last week and these have knocked him into a backward loop. You
can track him or help his cause at http://doubleditch.co.nz/tracking.
I am hoping his progress will be better in the coming week with some
northerly winds as a Low forms off Sydney on Monday and then turns into a
trough that crosses the South Tasman Sea on Wednesday to Friday and then
reaches southern NZ on Saturday and Sunday, but stalls in the north
Tasman Sea.

TROPICAL TOPICS
Rain over the past week shows that the Indian monsoon is getting late and
that there is an increase in activity off the west coast of Mexico


SPCZ= South Pacific Convergence Zone
SPCZ is expected to stretch from Papua New Guinea to Samoa this week.

STR= Sub-tropical Ridge
The STR is strong and in its normal winter latitude across the interior
of Australia, south of its normal position over NZ, and then around
normal around 40S well to the east of NZ.
The next High coming off Australia on Sun 8 June is likely to be diverted
across Tasmania so that it moves into the far south Tasman Sea on Sunday
8 June, leaving room in the North Tasman Sea for a deepening Low.

Departing from NZ to the tropics
Squash zone of strong easterly winds is expected to form east of
Northland by Tuesday and then slowly drift north---a departure on Monday
may be able to escape this.
A trough is expected to form between NZ and Vanuatu by the end of the
week with a strong NE wind between NZ and Fiji.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at
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My website is at metbob.com  Feedback to bob@metbob.com
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