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

28 March 2009

BOBGRAM7 issued 29 March 2009

WEATHERGRAM
YOTREPS
Issued 29 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
The "low from the Coral Sea" that was mentioned in last week's
Weathergram turned out to be JASPER which blossomed well last Tuesday
but then found the rising pressures over NZ were too much for it and did
a U turn and died.
I'd say that the New Caledonia highlands also had a hand in toppling
JASPER over; literally it was trapped between a rock and a high place.

TROPICS
The South pacific Convergence Zone SPCZ is still very active in the
Vanuatu area, with a LOW on Sunday near the Loyalty group/southern
Vanuatu. This Low should track off to the east-southeast, crossing the
Kermadecs/Minerva area on Tuesday and passing by 30S 160W, south of the
Southern Cooks, on Thursday. Avoid.

After Wednesday, the remains of the SPCZ in the Coral Sea will likely
travel northwards. It is likely to breed another tropical low near
Rotuma, to NW of Fiji Islands, and this may deepen a lot next weekend
and affect FIJI, Watch out for this.

There is a "twinning" of the Intertropical convergence zone (which lives
at around 3N) along around 7S all the way from 90 to 120W. This is an
equinoctial feature and should start to fade this week. Another branch
of the SPCZ from Tuvalu to Northern Cooks may last for a few weeks.

SUBTROPICAL RIDGE
The HIGH that is currently zipping across the Aussie Bight like a
squeezes orange pip is well positioned, thanks to some fresh dense cold
air from the southern ocean, to intensify to a peak of over 1040 hPa in
the South Tasman Sea on Tuesday. This is exceptional, and will result
in an intense squash zone somewhere between northern NZ and
Kermadecs/Fiji/New Caledonia that will likely last until this High moves
off NZ on the 4/5 April weekend.

NZ
After Sunday's warm westerlies (27 in Dunedin at local 6pm today), the
forecast is for some frosty southern Ocean air (dropping to 5C Tuesday
night and almost single digit daytime highs on Wednesday). This cold
southerly isn't very wet but may trigger some hail on Monday. On
Tuesday it should reach the eastern North Island. On Wednesday, it may
bring some rain as an "easterly onset" to Auckland and Northland. The
HIGH that follows on Thursday and Friday will be an autumnal one with
light winds and maybe some morning fogs. The front that follows next
should bring a strong NW flow to southern NZ on the 4/5 April weekend.
Sunday 5 April will be 25 hours long as NZ changes from Daylight saving
to Standard time.

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 March 2009

BOBGRAM7 issued 22 March 2009

WEATHERGRAM
YOTREPS
Issued 22 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.

The equinox was at 1144UTC last Friday. Weather systems around the
equinox are special; some say they set the pattern for the next few
months. Well, the cooler-than-normal sea temperatures along the Pacific
Equator are now slowly returning to normal, so the seasonal outlook is
that this LA NINA will weaken away over the next few months.

LAST WEEK
KEN came from that unorganised convection around the Cooks, and came and
went harmlessly enough. That LOW near New Caledonia today is the one
that EC was picking would form in the Coral Sea, and it seems to be
moving out the tropics without much drama (so far, anyway).

TROPICS
The South Pacific Convergence Zone SPCZ remains active over northern
Australia, and in the Coral Sea. There is a weak
Madden-Julian-Oscillation MJO arriving over the Coral Sea this week -
this is a tropical oscillation that tends to increase the SPCZ. It
looks as though this will be a week with low pressure areas forming in
the Coral Sea, extending as far as Vanuatu at times. These lows will
need watching and maybe worth avoiding.

The Low that is near New Caledonia on Sunday is forecast to be between
Norfolk and Kermadecs on Monday wandering to the SE, and helping another
low now east of NZ to deepen near 40S 160 to 150W on Tuesday and
Wednesday.

There is a secondary branch of the SPCZ along around 10S between
Tuvalu/Tokelau /Northern Cooks and Marquesas. This is typical of the
twinning that occurs near the equinox and is somewhat associated with
the ITCZ along 5 degrees N.

SUBTROPICAL RIDGE/TASMAN SEA/NZ
There is a Southerly over NZ to start the darker half of the year,
reminding us : just one fortnight of NZDT left.

HIGH 1018 over Tasman Sea/South Island should build to 1025 over North
Island on Wednesday (good fishing) and then weaken away to a thin ridge
near 30S by Friday. North of this high there will be a squash zone
between 30 and 20S initially in the North Tasman Sea/southern Coral
Sea-and gradually expanding eastwards with the high to get east of 180
by Friday and for the weekend. Avoid the squash zone.

For NZ there will be a interlude between highs with a frontal trough
over the South island on Thursday and the North Island on Friday,
followed by a mediocre southwest flow getting over NZ by the end of the
28/29 March weekend.

Next HIGH is likely to be in the central Tasman Sea by Mon 30th March,
along with another LOW in the Coral Sea - again I say: avoid the squash
zone in-between.


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 March 2009

BOBGRAM7 issued 15 March 2009

WEATHERGRAM
YOTREPS
Issued 15 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
HAMISH did as it was told. JONI was something of a surprise, springing
up out of that disorganised convection over the Cooks and shooting off
to the southeast.

TROPICS
The South Pacific Convergence Zone SPCZ remains active over northern
Australia. Today's output from the EC model is picking another tropical
cyclone in the Coral Sea from Sat 21 to Wed 25 March and the NOGAPS
model is picking two or three tropical develops in the Indian Ocean
during the coming week, and all this is in spite of nothing showing up
in the parameters measuring the MJO pulse. Basically it is still
cyclone season even if the equinox is happening on Friday (UTC).

SPCZ is also active from Solomons across Vanuatu and onto Fiji,
especially from Monday to Wednesday. Another branch of disorganised
convection is hovering over the Cook Islands and occasionally touching
parts of French Polynesia. Some computer models are picking that a
tropical Low may form over Tonga or New Caledonia on Thursday or Friday
- but I'll vote for the EC model which has a TC forming in the Coral
Sea.

SUBTROPICAL RIDGE/TASMAN SEA/NZ
A large HIGH 1030 is over Chathams today (15th) and moving off to the
east. It should extend along 40S and weaken when it is south of the
Cooks on Wednesday 18th. Between now and then there will be enhanced
easterly winds on the northern side of this HIGH, between 25 and 35S - a
squash zone.

Following this HIGH is a LOW --- on Monday 16th this LOW drops to around
998 hPa over Tasmania. It should track east the southeast and then it
deepens again on Thursday 18th over Campbell Island. Its associated
front crosses NZ on 18th and 19th. Avoid, but nothing major.

And following that LOW is another HIGH, rather weak at around 1022,
moving across New South Wales on 17-18 March. Days and nights are now
getting of equal length, and consequently the subtropical ridge is
starting its annual migration northwards, and this HIGH takes a track
further north than its predecessor. This HIGH is likely to weaken into
just a ridge and cross NZ on 20-21-22 March. There will be a strong
squash zone between this HIGH and that Low in the Coral Sea, from 19
March onwards, mainly 20 to 25S. Avoid.

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

08 March 2009

BOBGRAM7 issued 8 Mar 2009

WEATHERGRAM
YOTREPS
Issued 8 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.

Well I've just finished attending the Auckland Boat Show, so I'll keep
this brief.

LAST WEEK
I can vouch for the intensity of the squash zone over northern NZ that
I mentioned would hit last Wednesday/Thursday when the Chathams High
reached 1030+---winds were strong enough to briefly close one of the
Boat Show's pavilions and to turn off one of the coffee machines for 3
hours. Ouch. We got that front on Saturday too, but it was so weak when
it passed over Auckland it could only be seen by the discerning eye---
it re-intensified over Bay of Plenty on Saturday afternoon with
thunderstorms that took out power in Tauranga when many were watching
rugby.

In passing, I think the southwest change that reached Auckland after
that front is one that provides/announces a reasonable window for
sailing from Northland to Tonga- but it is still really too early to go.

I admit I wasn't expecting that convergence zone/squash zone combo over
the Queensland coast to cough up with a TC until maybe mid March... but
HAMISH has formed and has one of those CAT 5 eyes that can do a lot of
damage. It is moving southeast at present, parallel to the coast.
Hopefully it will maintain this track and stay offshore, and as it
encounters a cooler sea, it'll weaken - this may take til Wednesday.

TROPICS
The South Pacific Convergence Zone SPCZ is active with HAMISH near the
Queensland cost. There's another branch of lesser activity from
Solomons to New Caledonia, and another form Tuvalu to Tokelau, and some
disorganised convergence over French Polynesia. Once HAMISH fades these
zones are likely to amalgamate but the models are not expected any extra
activity this week.


SUBTROPICAL RIDGE
A large HIGH 1024+ is in the Australian Bight, slightly south of the
normal subtropical latitude for March, and that's a sign that this LA
NINA is still influencing our weather. This HIGH's centre is moving
slowly east along 50S - slowly rising to 45S - and should cross the
South Island on Sat 14 March. There will be a squash zone on the
northern side of this HIGH near 25S between it and HAMISH (OR THE
REMAINS OF Hamish). In the process of this slow migration, it is
expected to nose a ridge into the central and northern Tasman Sea on
Monday, but this nose should weaken away on Tuesday as a LOW below
980hPa moves northeast out of the southern ocean to east of Chathams,
deepening to below 960hPa by Saturday.

SO A POLAR BLAST of COLD SOUTH TO SOUTRHWEST WINDS (between HIGH and
Low) should cross the South Island late Tuesday/Wednesday and North
Island on Thursday. It is likely to bring a taste of snow to the
southern mountains, but soils are still so warm that the snow'll melt
away quickly.


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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