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

25 December 2009

BOBGRAM7 issued 26 Dec 2009

WEATHERGRAM
YOTREPS
Issued 26 December 2009
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas come from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

TROPICS
This edition is earlier than normal and next one will be later than normal as I'll be travelling around during next week.

It looks to me as if we will have another reasonably quiet week in the South Pacific.
South Pacific Convergence zone SPCZ has split into three entities, but maybe they are still linked in some way.

Convergence zone1 is active in eastern Coral Sea, but this may weaken this week. Convergence zone2 extends from Tokelau to Southern Cooks. A tropical LOW within this zone is likely to cross the Southern cooks on Mon 28 Dec UTC and then deepen as it peels off to the southeast, stealing away the wind from the tropics.

A third convergence zone is draped over French Polynesia and will hover around there all week, this often happens in an El Nino.

SUBTROPICS
HIGH just north of NZ today is blocked and will fade where it is by Wednesday.
The enhanced trade winds on its northern side from Niue to New Caledonia and NE winds from New Caledonia to Queensland will also fade.

Next High rolling in from the eats is likely to cross Tasmania on Tuesday 29 Dec, killing the wind for the Sydney-Hobart racers, and then move northeast across the Tasman Sea and reach Northern North Island on 1 January. So that Tasman Sea pattern is almost a repeat if last week's.
This High should be able to get further east than its predecessor along 30 South during 1-2-3 January, so that then the enhanced trade winds on its northern side may extend from French Polynesia to New Caledonia, and the SPCZ may reform along 10 South.

NEW ZEALAND AREA:
Fronts have been side-swiping southern NZ during past week, knocking on the door but higher pressures have killed them.
Now that the High over northern NZ is expected to have dropping pressures, the next front should be able to open the door and spread across NZ – preceded by strong NW winds, crossing South Island on Tuesday and North Island on Wednesday, and be followed by a strong cooling SW flow until Thursday. Then the idea is for a disturbed westerly flow for a few days.

I'll leave you all with these last few verses from Robbie Burns to mark the finish of the first decade of the second millennium:

We two have paddled in the stream,
from morning sun till dine† ;
But seas between us broad have roared
since the days of auld lang syne.

For auld lang syne, my friend,
for auld lang syne,
we'll take a cup of kindness yet,
for the sake of auld lang syne.

And here's a hand my trusty friend!
And give us a hand o' thine!
And we'll take a right good-will draught, for the sake of auld lang syne.

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

19 December 2009

BOBGRAM7 issued 20 Dec 2009

WEATHERGRAM
YOTREPS
Issued 20 December 2009
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Cyclone MICK took with it a lot of the convective energy that had been
building up over recent weeks, and has left behind a very quiet South
pacific convergence zone SPCZ. This zone is now rebuilding along 10S
between Tokelau and Northern Cooks, and is expected this week to shift
more to 15-20S between Samoa and French Polynesia. Computer output
suggests a LOW forming on the SPCZ around French Polynesia on Tuesday 22
Dec UTC and then moving to the south of the group on Christmas Day.
Avoid.

There is still some of the old SPCZ convection around Vanuatu , and this
is expected to wander around the region all week in a disorganised
fashion.

SUBTROPICS
MICK dived into the subtropics last week and has "stolen" much of the
trade winds and disrupted the position of the subtropical ridge. The
next high roiling in from the west is over the Tasmanian area today and
is likely to take a track more common for highs at this time of the year
- we have the summer solstice on Tuesday 22 Dec= the longest day, and
meant to be the start of the southern most tracks for subtropical ridge.
This High is expected to cross the North island / 40S on Tuesday 22 Dec
and then nose into the South Pacific along 30 to 40S steadily holing
itself together as a single entity until 30 Dec. Reliable and
dependable.

As this High slides east it will bring the trade winds back from
southern French Polynesia all the way to the Coral Sea. There may even
be a small squash zone on the northern side of this high, along 25 to
30S between 160W and 180 from 23 to 28 Dec.

NEW ZEALAND AREA:
During a lead up to Christmas over NZ there is a dying southerly flow on
Monday 21 Dec, and then the passage of that High with light winds and
generally settled weather. On Wed 23 Dec, as the High is crossing the
North Island, some converging sea breezes may combine with cold air
aloft to make afternoon heat showers/intense downpours around Bay of
Plenty.

If people here can still remember the weather they had last Fri, 18 Dec,
then it looks like something similar will be happening to then Fri 25
Dec, for that's when the next front, in cahoots with the central
Australian heat trough, will be moving from Tasman Sea onto southern NZ,
preceded by a NW flow, kicking away the High further north. Some
differences also: the NW wind may not be as strong, and in central
Australia by then will be the remains of Cyclone LAURENCE.

Christmas reminds us of the miracle of new creation and the purpose of
living life to it fullest. Festive cheer is part of this process, so
feast cheerfully folks.

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

12 December 2009

BOBGRAM7 issued 13 Dec 2009

WEATHERGRAM
YOTREPS
Issued 13 December 2009
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

The past week was indeed busy with deep convection between Solomons and
Tuvalu, and all this has wound into the first tropical cyclone of the
season for the South Pacific. It has earned a name for itself: MICK.
This is expected to move southeast, across Fiji on Monday and Tonga on
Tuesday with gale winds, then further off into the southern Ocean, south
along 160W. AVOID.

Further east: the South Pacific Convergence zone SPCZ has been south of
its norm position lying along 20South between Niue and South of French
Polynesia, and is expected to stay in much the same place this week and
slowly fade. In tandem with this zone there is a subtropical ridge
between 35 and 40S.in the eastern end of the South Pacific.

Some computer models are picking that a tropical cyclone may form off
the NW Australian Coast from Tuesday onwards and it may go south and
inland on weekend 19/20 Dec-details still uncertain - if that may bother
you then stay up to date.

SUBTROPICS
A HIGH is expected to cross Tasmanian on Monday and then move northeast
across the Tasman Sea on 15-16-17 Dec, enhancing the trade winds on its
northern side and in the Cora Sea. This High should then move east,
and, once MICK gets out of the way, get past the dateline along the
subtropics at 30 to 35S from 18 to 21 Dec, enhancing the trade winds
between Fiji and Southern Cooks, and encouraging a new SPCZ to form
between Coral Sea and French Polynesia

NEW ZEALAND AREA:
Mediocrity: A trough is expected to cross NZ on Monday/Tuesday followed
by anticyclonic southwest flow on Wednesday/ Thursday, then, on Friday,
a strong NW flow followed by a front for the South Island. This front
should slow down and weaken over the North Island on the 19/20 weekend,
and another front is expected to move onto the South Island from the
southwest on Sunday 20 Dec.


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

05 December 2009

BOBGRAM7 issued 6 Dec 2009

WEATHERGRAM
YOTREPS
Issued 6 December 2009
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Sadly the Satellite that has been helping us out with sea winds
(Scatterometer) over the past decade has died--- the bearing which spin
its mirrors were only made to last 5 years, and they seized after 10
years , so no more data. I have not been informed of any replacement
plans at this stage, so in the meantime my ability to check wind over
the ocean is not as good as it was.

The wettest part of the planet last week has been along the Pacific
equator, between 160E and 180. There is a ship near by this region
reporting strong westerlies, so we can assume that is a surge of
equatorial westerly winds. Hold on to your hats in Kiribati, the swell
will possibly keep you in side harbour this week.

This could be an active week near the dateline:
Rain in the South Pacific convergence zone has also been active between
Solomons and Vanuatu and around the North Coral Sea. The SPCZ also lies
along a zone from Samoa to French Polynesia. Computer models are
latching on the area between northern Vanuatu and Tuvalu as a likely
breeding ground for Tropical lows this week, but lows breeding in this
region are likely to be slow-moving. There is already a large zone
below 1005 in that region. This is likely to feed off the equatorial
westerlies and bring strong NW winds across Tuvalu, Tokelau and Samoa.
Once these NW winds reach Tokelau-around Tue 8 Dec UTC, that may fire up
a rapidly deepening tropical low system between Tuvalu and Samoa on Wed
9 Dec, and then that system will likely travel SSE near Wallis /Futuna
on Thu 10 Dec and then across Northern Tonga on Fri 11 Dec where it may
well earn a name for itself. Avoid.

SUBTROPICS
A good sized high is dominating the subtropical ridge along 30S from 180
to 140W at present and this has enhanced trade winds on its northern
side. This high is expected to fade by Thu 3 Dec, allowing a trough to
spread east. Following this trough there is expected to be a new high
spreading east of NZ along 40S, thus taking the subtropical ridge
southwards this week.

TASMAN SEA / NZ AREA
Subtropical air brought some welcome but briefly intense rain to our
North Island last week. The SW Pacific subtropics are now shifting back
to around 30S again for at least the next fortnight.

SO it is back to the disturbed westerly pattern over NZ, but the fronts
are likely to weaken as the move across the country-one on Tue 8 Dec
just raining on southwest fringes, another on Wed 9 Dec more noticeable
over South Island, and a third on Fri/Sat/Sun 11/12/13 Dec bringing a
southerly wind change to eastern areas.

The fronts 8, 9 and 11 should be preceded by strong NW winds
concentrating rain on Alps and bringing some hot dry winds further east.


Around Sun 13 Dec, a High is likely to cross Tasmania and this should
shovel a cold front north on Mon/Tue 14/15 Dec feeding in a cold
southerly blast onto NZ and further north. If you are trying to sail
into Northland from the north then winds will be more useful for
landfall before Sun 13 Dec rather than after.

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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