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

25 December 2010

BOBGRAM6 issued 26 Dec 2010

WEATHERGRAM
YOTREPS
Issued 26 December 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.

La NINA is still strong. Average atmospheric SOI over past 30 days is
2.3 (an increase of 0.15 in the past week). Oceanic Nino Index for
Sep-Oct-Nov is -1.4

TROPICS
South Pacific Convergence Zone SPCZ is in its normal location for this
time of the year, stretching across the Solomons to east of Vanuatu,
moving north and south over the Fiji/Samoa/Tonga area, then weak between
the Northern and Southern Cooks, and then reasonably active again along
south end of French Polynesia.

A Madden Julian Oscillation MJO of extra convection is appearing over
the Coral sea ... it has already activated things as shown by the brief
appearance of TC TASHA that got to gale force before making landfall
near Cairns on Christmas Eve and then fragmenting inland over Queensland
on Christmas day. Models are producing differing scenarios for where
the next cyclone may form - and maybe not much is likely to happen this
week, except for a tropical low possibly forming near Northern cooks
around Fri 31 Dec and heading for Southern Cooks around sat 1 Jan UTC.
However the chances of formation of a low in the Coral sea are
increasing and GFS is currently producing something there by 4Jan -
EC is as well, but not so intense.

Subtropical jet is conveying the moisture fields from Queensland to NZ
when it gets a chance - when frontal zones allow this --- as is expected
on Mon/Tues 27/28 Dec. Avoid.

SUBTROPICAL RIDGE: STR.
High crossing Tasman Sea and central NZ on Tue/wed 28/29 dec, and next
one is likely from Fri 31 dec to Mon/Tue 3/4 Jan, followed by a strong
northerly flow

TASMAN SEA/ NZ AREA
Front crossing NZ tonight and Mon/early Tues 27/28 Dec is being assisted
by sub-tropical jet. Avoid.

Next trough, on late wed/thu 29/30 Dec, may have separate parts - one
over southern South Island and t'other over northern North island and
should just be a transitional trough between highs.

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

18 December 2010

BOBGRAM7 issued 19 Dec 2010

WEATHERGRAM
YOTREPS
Issued 19 December 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.

La NINA is still strong. Average atmospheric SOI over past 30 days is
2.05 (a drop of 0.05 in the past week). Oceanic Nino Index for
Sep-Oct-Nov is -1.4

TROPICS
South Pacific Convergence Zone SPCZ is in its normal location for this
time of the year, stretching across the Solomons to east of Vanuatu,
moving north and south over the Fiji/Samoa/Tonga area, then weak between
the Northern and Southern Cooks, and then reasonably active again along
south end of French Polynesia. There is a Madden Julian Oscillation MJO
of extra convection making its way eastwards across northern Australia
but weakening. This should wander into the Coral Sea during the coming
week. However, the accompanying equatorial westerly winds so far have
only reached 130E and computer models suggest that they may not be able
to get east of Papua New Guinea during the coming week. So, therefore,
then, there's a slightly increased risk of cyclone development in the
Coral Sea this week, but computers are not picking developments in this
region.

That low centre off the west coast of Australia has sea surface
temperatures around it that are rated to be below the threshold for TC
development. It should slowly wander west and turn into a trough. Of
more interest is what might happen in the Timor and Arafura Seas as the
MJO passes by. Some models are at this stage producing output showing a
TC around Darwin near Christmas Day (shades of Tracy 1974, I can still
remember), but details are all over the place, and the EC model is not
picking any development --- so if that's near you seek updates.

SUBTROPICAL RIDGE: STR.
High to southeast of Chatham Islands is quasi-stationary and should just
slowly wander north from 45S to 30S by Friday 24 Dec and then fade away.


The next high is expected to be able to get from Australia Bight across
Tasmania around Friday 24 Dec and then cross NZ around Sat 25
Dec/Christmas Day and then move quickly off to the east along 40S on
Boxing Day.

TASMAN SEA/ NZ AREA
There is a long wave trough over the Tasman Sea area helping convey
water-vapour from Indonesia, across Australia, to the NZ area. So this
is likely to be a week with three fronts.

The first went across today Sunday, bringing decent rain to the North
Island. The second is likely to be associated with a tight-centred low
tonight forming off the New South Wales Coast ... wind and rain mainly
for South Island on Tuesday, and then weakening over North Island on
Wednesday and Thursday.
The third front should mark the end of this series and lead in a SW
change (a heralding angel for the Christmas Day high), and should cross
the South Island during the day on Friday 24 Dec/Christmas eve, then the
North island that evening.

A note for those planning to sail from Auckland to Bay of Islands for
the holidays - SW winds for this are likely to be few and far between
during next few months, and this SW may only last for the 25th - light
northerlies returning as soon as Boxing Day afternoon.

SYDNEY-HOBART
Still somewhat uncertain, and depends on whether the Timor and Arafura
Seas can produce a tropical low for Christmas or not. At this stage the
idea is that a trough is likely to wander off the New South Wales Coast
around Boxing day, followed by a period of moderate southerly winds
that back to be from east and then northeast by Tuesday 28 Dec.

Check out moonrise on Tuesday 21st (if sky is clear). It is a full moon
right on the solstice and that does not happen often. It will also be a
total lunar eclipse--- the moon will rise into the earth's shadow and
appear red/brown-coloured, so may be particularly memorable.

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

11 December 2010

BOBGRAM7 issued Sun 12 Dec 2010

WEATHERGRAM
YOTREPS
Issued 12 December 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. Oceanic Index

La NINA: Still strong. Average atmospheric SOI over past 30 days is 2.10
(a rise of 0.41 in the past week). Oceanic Nino Index for Sep-Oct-Nov is
-1.4 and steady.

TROPICS
South Pacific Convergence Zone SPCZ returned to normal last week, draped
across the Coral Sea and Vanuatu with build ups over Fiji and Samoa. And
there is scattered activity along 15S to French Polynesia FP.

A aging low centre near 25S 170E (to SE of New Caledonia) has a
circulation that is drawing SPCZ activity southwards across Fiji and
Tonga. This centre is expected to fade as it wanders southwards
towards Norfolk Island, and SPCZ activity should continue to hug 15S
this week.

There is a Madden Julian Oscillation MJO of enhanced convection making
its way across the Timor Sea this week and into the Coral sea between 15
and 20 Dec. This should activate the SPCZ. Normally it increases the
risk of cyclone formation, but this MJO is likely to lack a zone of
equatorial westerlies so may not have much impact in the Coral Sea.

Some models are picking that a zone of weak equatorial westerlies may
trigger a tropical low or two to form off NW Australia. The latest GFS
model is picking one to form in Gulf of Carpentaria between Tuesday 14
and Friday 17 Dec and for this to then move SW then S inland towards
Alice as a wet Monsoon Low. In any event an Australian conveyor belt of
upward motion is being set up in the heat trough from North Australia to
Queensland, sometimes reaching Tasman Sea and then onto New Zealand, and
this NW flow in the upper air is likely to last for the next fortnight.

SUBTROPICAL RIDGE: STR.
The STR was knocked north last week but a weak Polar outbreak (polar
index went negative). Southern lows took the opportunity to expand
northwards and one passed by close to NZ - It is now near 45S 165W and
it seems destined to deepen as it wanders north getting as far north as
40S 150W by Wed 15 Dec then exiting to the southeast. Associated High
is making its way east along 30S. Another high is forming just east of
New Zealand tonight Sun 12 Dec. This should wander along 45S on Mon 13
Dec the slide around south-side of that low, but leave a cell near 30S
so that enhanced trade winds are expected all week from FP to Tonga.

Another High is expected to cross NZ Wed /Thu 15/16 Dec and then blossom
east of NZ and slowly shift north to 35S by Tue 21 Dec.

TASMAN SEA/ NZ AREA
Some more southern lows are likely to get near NZ this week. One is
expected to pass by south of Macquarie Island tonight Sun 12 Dec and an
associated front is expected to swipe by NZ on Monday/Tue 13/14 Dec,
followed by a showery southerly change.

A small low may form in the Australian conveyor belt off Queensland
around Wed 15 dec and bring some welcome rain to northern NZ in a NE
flow around Thursday 16/Friday 17 Dec. This may start a trend.

Then another southern low should pass south of Tasmania around Sat 18
Dec as an accompanying low may form off Sydney, feeding from the
activity in the Australian conveyor belt. These lows may dance around
each other around Sun 19 Dec, whizzing close by Southland and blasting a
strong northerly flow across most of NZ. Avoid. Its frontal band is
then expected to cross NZ on Monday-Tuesday 20-21 Dec.

Keep an eye on that conveyor belt as it is likely to continue producing
Lows in eastern Aussie seaboard to NZ area from 23 to 27 Dec, and this
may have an impact of the Sydney-Hobart.
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

04 December 2010

BOBGRAM7 issued 5 Dec 2010

WEATHERGRAM
YOTREPS
Issued 5 December 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.

La NINA: Average atmospheric SOI over past 30 days is 1.69, up a bit
from 1.5 last week.

TROPICS
South Pacific Convergence Zone SPCZ had a week off in the Coral Sea as
all that activity hat was over Fiji last week went across Tonga/Niue and
to south of Kermadecs. The remainder of this is expected to fade and
wander off to east on Mon/Tue 6/7 Dec.

A burst of convection occurred along the Queensland coast over the past
few days, but this is already fading and should be gone by end on Monday
6 dec.

A new SPCZ is likely to form over north Coral Sea this week and wander
south to be from Vanuatu to Fiji by 11/12 Dec. Enhanced convection
along this SPCZ is likely from 15 to 20 Dec due to an incoming Madden
Julian Oscillation--- more about that in next weathergram.

SUBTROPICAL RIDGE: STR.
High near Northland on Mon 6 Dec should fade away on Tues 7 Dec.
High crossing South Tasman Sea on Tue 7 Dec should expand to NE by Fri
10 Dec and then expand as it wanders east along 35S, making a squash
zone of enhanced trade winds on its northern side.

NZ/Australia AREA
Trough between the highs may bring showers- one of these troughs crosses
NZ on Mon 6/ Tues 7 Dec bringing a southwesterly change.

The next moves off Sydney on Thu 9 Dec, onto Brisbane and Southland on
Fri 10 Dec, central Tasman and central NZ on Sat 11 Dec, and then onto
North Island around Sun 12 Dec. By then there may not be much left in
this front, so it shouldn't put anyone off trying to approach Northland
from the north.

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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