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

28 February 2009

BOBGRAM7 issued 1 March 2009

WEATHERGRAM
YOTREPS
Issued 1 MAR 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.

LAST WEEK
A jetstream over the South Island of NZ provided the impetus for the
rapid deepening of a low near Northland late Friday resulting in a wet
warm front crossing the North Island in Saturday. Accompanying easterly
winds were strong enough to dislodge several tents at Northland Fieldays
so that the (my) Saturday show was cancelled, also Lionel Ritchie's
Mission Estate concert in Hawke's Bay. Rotten timing, even Derby day in
Auckland was put off until today.

TROPICS
The South Pacific Convergence Zone SPCZ is only slowly reforming after
it was drained off a week or so ago. There are two main zones of
convection-one from Solomons to Tuvalu, weak over Samoa and then
stronger again over Northern Cooks and French Polynesia. T'other zone
lies from northern Queensland to New Caledonia and is currently
extending southeast towards the Kermadecs.

Not much action expected in the tropics this week, but southeast winds
should be strong at first on the south side of that SPCZ. that's roughly
from Kermadecs to Queensland until Wednesday (squash zone). Another
squash zone of enhanced easterly winds is likely between Southern Cooks
and Tonga from Wednesday to Saturday UTC.

TASMAN SEA
HIGH crossing the Tasman Sea on Monday and NZ on Tuesday is forecast to
intensify to over 1030 east of NZ by Wednesday. Light winds and settled
weather within this high, but a strong squash zone of easterly to NE
winds is expected over northern NZ when the HIGH moves off late
Wednesday and on Thursday.

On Thursday, a family of LOWS is likely to form off south-eastern
Australia and east of Tasmania. The frontal rain band (between the
departing HIGH and these new Lows) should reach northern NZ on Thursday
and cross NZ on Friday/Saturday.

This time it will not be jet assisted so shouldn't be as wet/windy as
the 27/28Feb, but something to interrupt the Auckland Boat show - I'll
be there and I have some free entry passes for Thursday/Friday , let me
know by return email if you are interested.

These Lows may amalgamate to one centre that may deepen to 970 as it
swings past Campbell Island on Sat 7 March - close enough to bring gale
to storm conditions for the start of the weekend to Foveaux Strait,
keeping the Bluff fishing fleet in port for a while at the start of the
new Oyster season.

OUTLOOK:
After this Low has moved off, on Sunday 8 March, another HIGH is likely
to cross the Tasman Sea / NZ area bringing a few days of settled
weather.
Next lot of tropical lows will probably form when this HIGH moves off to
the east after 10 March.

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

21 February 2009

BOBGRAM7 issued 22 Feb 2009

WEATHERGRAM
YOTREPS
Issued 22 FEB 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.

LAST WEEK
That LOW we mentioned to avoid last week was briefly named TC INNIS as
it moved off to south of New Caledonia last Wed. It provided an oomph
of tropical moisture to the low that was stalled near Brisbane (the one
that flooded New South Wales early last week). The frontal zone on the
eastern side of this new family of lows did indeed bring briefly intense
rain to the North Island as it passed over on Friday. And the Lows are
still taking their time to cross southern NZ, raining out at around
2mm/hr. Yuck.

TROPICS
So the South Pacific Convergence Zone SPCZ has been drained and is now
reforming.
There is a "left over" trough over the Queensland area. Not really SPCZ
stuff, so more about that later.

The SPCZ then is reforming over Solomons and Tuvalu. East of the date
line there is little coherence - scattered convection over a wide area
from Tokelau and Northern Cooks in the North to Tonga, Niue, Southern
Cooks in the South and out to French Polynesia in the east. Winds here
are mainly from north of east and reaching a peak of around 20 knots
around 20 South. Convection/ Shower activity is expected to shift
southwards this week but no Lows are expected east of the date line.

TASMAN SEA
This is the main area of interaction and development this week.

SO we start with a "left-over" trough near the Queensland Coast, still
generating weather. At surface level it is marked mainly by a shear
zone between lighter easterly winds on its northern side and a squash
zone of enhanced SE winds on its south side maintained by a HIGH that
will be crossing the South Tasman Sea next few days.

This week there is expected to be a procession of lows and highs across
the southern Tasman Sea and NZ. That family of Lows still crossing
southern NZ today and Monday will be followed by a cool SW flow over
all NZ on Tuesday. Then we get the mid week HIGH, just as last week,
only this time not as intense. Its likely to split into two cells, one
at 35S and another at 55S. When the Sub-tropical ridge is weak like this
NZ gets more weather.

SO the next LOW should be interesting... models have it forming over
Tasmania on Monday and then wandering almost due east across the Tasman
reaching NZ on Thursday and Friday. This is a different path from last
week's low from the north, and less wind and rain is likely.

Models are at present producing different scenarios after this LOW...
some development is likely on the "left-over" trough in the North Tasman
Sea. For next weekend NOGAPS has output which develops a low near New
Caledonia, EC has a Low near Lord Howe, and GFS and MetService models
are quicker and have an output with a series of lows between Norfolk and
northern North Island. Too early to be sure, to be sure.

OUTLOOK:
Anyway, the next High moving in from the west should cross the south
Tasman sea on 2-3 March , preceded by a cool SW flow over NZ, and then
a weak trough around 5 March and then a more substantial HIGH over
Tasman Sea/NZ on 6-8 March with a strong squash zone on its northern
side. Next lot of tropical lows will probably form when this HIGH moves
off to the east around 10 March. Enjoy a break in the cyclone season.

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

14 February 2009

BOBGRAM7 issued 15 Feb 2009

WEATHERGRAM
YOTREPS
Issued 15 FEB 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
Over the past week activity in the South Pacific Convergence zone has
generally declined, but over the weekend a tropical low and associated
wind and rain has formed just east of southern Vanuatu near 19S 171E.
It is moving west and is expected to weaken once it gets wets and
Aneityium and crosses the Loyalties on Monday and New Caledonia on
Tuesday. Avoid.

EC takes this low further south as a separate entity, crossing Norfolk
on 18th and central NZ on Fri 20 Feb, but GFS and NOGAPS models merge it
into a Tasman wide trough during the week. I think the merger idea is
the more realistic.

The low that was mentioned last week to form off Brisbane has indeed
done so, bringing Queensland more wind and rain, but it hasn't evolved
as fast as initially expected. The SPCZ has been strongly active in a
band across northern Australia coast and central Queensland (typical on
a La Nina). The computer models are all coming up with a forecast
that picks that the first LOW near Brisbane will fade. But, offshore, a
large trough with several new centres will form along with an active
rain band, and these will reach New Zealand on Fri 20 Feb preceded by
strong northerly flow and followed by west to SW winds on Sat and
Sunday. Avoid.

There is another weak branch of the SPCZ from Samoa to the southeast---
sometimes over Niue and sometimes over Southern Cooks. It isn't doing
much.

SUBTROPICAL RIDGE STR/ NZ
The STR is about as far south as it gets, indicating this current La
Nina at work. We start the weak with a HIGH in the STR about 1030 and
south of Tasmania. This HIGH should move northeast, peeling around
southern NZ on wed 18 Feb. On Thursday there should be a squash zone
between the departing HIGH east of NZ and the incoming trough then in
the central Tasman Sea. Avoid.

After the passage of the trough on Fri20th and the following west to
southwest winds on Sat and Sunday, an new HIGH should reach Tasmania on
Sat 21 and cross the Tasman Sea on Sun 22 and NZ on Mon 23/Tue 24 Feb
with settled weather.

So, at this stage, the forecast for the Patea Fishing competition off
south Taranaki next weekend is for strong West to SW wind, easing away
on the Monday.

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

07 February 2009

BOBGRAM7 issued 8 Feb 2009

WEATHERGRAM
YOTREPS
Issued 8 FEB 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 moon is at perigee on 8th (today) and is full on 10th so watch out
for king tides on 10th and 11th.

TROPICS
Thumping rain a month ago brought slips and flooding to Fiji. Thumping
rain in past two weeks brought slips and flooding to Solomons. And
thumping rain in past month has produced extensive surface flooding over
internal Queensland. For Queensland it's another week of the same, this
week.

South Pacific Convergence Zone SPCZ has been most active from Queensland
to eastern Coral Sea over past week, and its eastern end has partly
drifted south across New Caledonia over past few days and is tied up
with the LOW near Norfolk Island today. This low is expected to fade by
Tuesday but may bring a rain band onto the North Island.

Another LOW is expected to form offshore Brisbane around Wednesday and
partly move southeast and cross the North Island around Friday 13th.

There is another branch of the SPCZ between Samoa and Southern Cooks.
Rest of tropics are expected to have reasonable east to northeast trade
winds this week.

SUBTROPICAL RIDGE STR / NZ
The STR is about as far south as it gets, indicating this current La
Nina is at work. An intense HIGH is expected to build to over 1030 hPa
in the Aussie Bight at 45S by Wednesday. Its onset should allow cooler
easterly breezes to knock down the heat wave over southern Australia,
and hopefully bring a respite from the killing bush fires.

This big STR is expected to cross Tasmania on Thu-Friday and weaken in
the South Tasman Sea over the weekend. It is a bit of a toss up,
timing-wise, as to whether this High or the
"LOW-from-near-Brisbane-area" reaches NZ first, and there may be a
squash zone between these systems over central Tasman/northern NZ area
either on 13-14 Feb or 15-16 Feb, as picked by GFS. We are unsure at
this stage - EC model, at present, has the Low from Brisbane winning the
race and deepening on Fri 13th when it gets east of NZ and feels the
cooler condensing southerly that precedes the High. Maybe.

For Louis Vuitton Pacific Cup in Auckland the wind outlook is mixed:
Monday, NE, Tuesday NW fading away, Wednesday light and variable,
Thursday a front and mainly NW but may go SW, Friday Southerlies 20
knots plus prevailing, Sat- chance of strong easterlies, then a period
of prefrontal NE winds.

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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