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

28 June 2009

BOBGRAM7 issued 28 June 2009

WEATHERGRAM
YOTREPS
Issued 28 June 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/SUBTROPICS
The South Pacific Convergence Zone SPCZ stretches across the South
pacific in two main bands ... one is a continuation of the northward
moving band which has been around for a few weeks and was mainly
stretching form Solomons to Tuvalu to Tokelau to Northern cooks this
wee. Another band has been forming from around Rotuma to Samoa. The
recent activity over New Caledonia and Loyalty Island sis due to an
incoming trough that mixes the tropics with the subtropics and mid
latitudes, and isn't part of the SPCZ.

During the next week these two branches of the SPCZ are likely to merge
into once band from Solomons to Tuvalu to Tokelau, with a side branch
extending south to Fiji. According to some models Fiji is likely to
bear the brunt of some accumulated high rainfall by early next week.
Prepare.

The Galapagos to Marquesas route is all quiet now, but easterly winds to
Marquesas are less than normal and this trend may continue, yuck.

SUBTROPICAL RIDGE
Last weeks high has turned into a sausage along 30S between 130 and
170W, and this is expected to pull out further east and wander to 25S
this week, ending over south-eastern Islands of French Polynesia. Its
associated squash zone of enhanced trade should also move north and
reach Marquesas and extend as far as 90W - so those on the Galapagos to
Marquesas voyage should have a good week if the get south of 5S. What
with the warming winds in that area, it will be wise to take these winds
while they are there, for the outlook is that they will not last.

Subtropical ridge is going thru a weak period this week in the Tasman
Sea. This allows sub-polar ridges to form instead--- that HIGH
currently over Campbell Island area is expected to head for the Chathams
by Tuesday and then fade as another High cell to the south buds off the
ice shelf - this is likely to feed chilling southeasterlies onto
southern NZ on the 4-5 July weekend.

Next subtropical ridge is forming in the Aussie Bight this week, it is
expected to get to Tasmania around 7 July and NZ around 10-11 July.

TASMAN /NZ AREA
So this week is going to be a troughy week in the Tasman Sea/NZ area.

First Low formed between Lord Howe and Norfolk Islands today, on cue,
and its front and easterly-wind squash zone is over central NZ. Low is
forecast to move east along 35S, crossing northland on Monday. Wind and
rain over central NZ should ease by Wednesday as this low goes away.
Avoid.

Second Low is forecast to deepen over Tasmania on Thu/Fri and then cross
the Tasman Sea /Central NZ on Sat/Sun 4-5 July - bringing heavy swells
to western North Island and followed by chilling SE then S and SW winds.
Avoid.

Associated front comes first and should cross Tasman Sea/NZ on Wed/Thu
preceded by a period of strong NW winds, accompanied by a burst of rain
for western areas, and followed by showery westerlies-but this front
might trigger a brief secondary low off Wairarapa on Friday.

All these troughs crossing the mid latitudes of the Tasman Sea are
making for generally westerly winds between Brisbane and New/Caledonia/
Tonga, not the other way. Use the flow, Luke.

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

20 June 2009

BOBGRAM7 issued 21 June 2009 (winter solstice)

WEATHERGRAM
YOTREPS
Issued 21 June 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/SUBTROPICS
The South Pacific Convergence Zone SPCZ has recently been active in a
zone from northern Coral Sea to northern Vanuatu to Fiji - activity (not
quite the low mentioned in the last weathergram but wet enough to cause
flood warning in Fiji on Saturday) is now sliding over Wallis/Futuna
(Sunday UTC) and should slip SE and fade over Southern Cooks on Monday
UTC. This zone is now moving slowly NORTH and is likely to stall in a
zone from Solomons to Samoa from Thursday onwards.

The other weaker SPCZ zone has been between Tuvalu and Tokelau, and this
is expected to fade over ne few days.

The Galapagos to Marquesas route is all quiet now, but easterly winds to
Marquesas are less than normal and this trend may continue, yuck.

SUBTROPICAL RIDGE
A winter HIGH (caused by cold air) in the Tasman Sea is supported
strongly aloft by a planetary wave and a medium wave, so will be slow to
change this week. Its southern parts decay fastest and this causes an
apparent northeast displacement of the system across North Island on
Wednesday. Then there should be a steady eastwards movement of the HIGH
"sausage" along 30S.

Winter Highs bring frost and, as they fade, fog.

On the northern side of this is a squash zone of enhanced trade winds.
This currently stretches all the way from southern Tonga to Lord Howe
Island and should move east with the HIGH and be from French Polynesia
to Fiji on Sat 27 June. Avoid.

NZ and TASMAN SEA
In the western shoulder of the HIGH there will be a series of troughs
and Lows. First LOW is likely to form east of Tasmania on Tuesday with
a trough crossing Lord Howe on Wednesday, and weaken over NZ on Friday.
A new low is likely to form between Lord Howe and New Caledonia on
Friday /Saturday and MAY deepen quickly as it moves towards central NZ
on Sunday and Monday, making for a strong wet NE flow over Northland
this weekend for that start of the NOUMEA 2009 yacht race. And then
another trough is likely to cross New Caledonia on Monday/Tuesday 29/30
June. All worth avoiding.


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 June 2009

BOBGRAM7 issued 14 June 2009

WEATHERGRAM

YOTREPS

Issued 14 June 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/SUBTROPICS

The South Pacific Convergence Zone SPCZ spent last week in two, each at a limit.

 

The eastern part moved to the northern limit of the SPCZ and stretched from Tuvalu to French Polynesia.  It isn’t often that the SPCZ moves north across French Polynesia, but that did happen last week.  That zone is now weakening away.

 

The Western part ran along the southern limit, and stretched fro Coral Sea to NZ.  It watered the paddocks we parked in during National Fieldays and I was somewhat concerned my 2-wheel drive would get bogged, as some did. However, parking sensibly worked and I’m back in Auckland now.

 

During the coming week that western part is expected to become the main SPCZ.   It is now stretching from northern Vanuatu to Fiji/Tonga and should peel off to the southeast on Tuesday /Wednesday, but redevelops over Fiji/Tonga on Friday /Saturday.

 

The main development in the SW Pacific topics this week is expected to be the formation of a new LOW between New Caledonia and Lord Howe Island (sort of over Chesterfield reef) on Friday.  This LOW is expected to then take a SE track and be between Raoul Island and North Island by mid next week, steeling wind from the New Caledonia to Tonga area   Avoid.

 

The Galapagos to Marquesas route is all quiet now. .

 

SUBTROPICAL RIDGE STR / NZ WEATHER

HIGH currently in N Tasman Sea should fade away by Tuesday, and a new HIGH is expected to gel from along the Aussie Bight coast over Tasmania at 1030 on Tuesday.  This HIGH peaks as 1032 in mid Tasman on Saturday and then weakens slowly to 1020 over central NZ next week, whilst that sub tropical low swings by further north.  Avoid the squash zone of enhanced easterly winds between HIGH and Low.

 

Over NZ I suppose the highlight of the week comes from the south …we have a SW flow over the country at present, and a low in the southern Ocean is combining with that gelling HIGH in the Aussie Bight to shovel a southerly onto South Island tonight and Monday.  Not much moisture but cold enough to bring some snow to our higher roads and ski fields.

This cold southerly gets focussed onto eastern North Island on Tuesday as a new Low forms near 44S 170 to 175W whilst that new HIGH hits Tasmania. For the remainder of the week we should have a easing S to SW flow as that HIGH approaches.

 

 

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

 

06 June 2009

BOBGRAM7 issued 7 June 2009

WEATHERGRAM
YOTREPS
Issued 7 JUNE 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/SUBTROPICS
The South Pacific Convergence Zone SPCZ has been in its normal June
position over the past week or so - over Solomons to Rotuma to Samoa to
South Cooks area. There are signs that it will slowly spread NORTH
wards and thus eastwards this week, reaching the northern position from
Solomons to Tuvalu to Tokelau to Northern cooks to French Polynesia by
the 13-14 weekend.

Low near 35S 155W is moving south and deepening. It has temporarily
sucked the guts out of the wind over the Cooks, and is rucking up a lot
of southerly swell for the Cook area from 9-14 June.
The SPCZ is likely to be moving from Southern cooks on 7 June to French
Polynesia on 11-12 June. Around 10 June, a low may form near 25S 150W
and deepen as it goes south-- sort of a repeat of the pattern of the
first low, only all happening further east.

A similar pattern may also occur further west---- As a trough crosses
the Tasman Sea on 11-12 June its tropical extension may produce a small
LOW over the Loyalty group around Thu 11 June. And this Low could then
spit out southeast and deepen as it crosses the Minerva/Kermadecs area
on 13-15 June, with a squally southerly change. Not all models are keen
at this stage on this scenario, so this all needs updating closer to the
time.

All quiet on the Galapagos to Marquesas route but winds are weakening
due to a warming sea.

For the record : the SW Indian Monsoon has made it to 15N this weekend
in the Arabian Sea and 20N in Bay of Bengal, and is running around
normal, but slightly behind normal over inland parts of India.

SUBTROPICS
Large HIGH 1028 over Chatham Island area will slowly move east along 40S
this week, reaching 160W by the 13-14 June weekend. There will be a
squash zone of enhanced trade winds on its northern side and this will
affect Samoa/Tonga/Niue on 8-10 June and Samoa to Southern Cooks from
10-12 June. Not so good for fishing this week.

TASMAN /NZ AREA
Northeasterly flow over NZ on Monday is followed by a front on Tuesday.
On Wednesday and Thursday a LOW is expected to deepen and widen in the
Tasman Sea, bringing a NW /northerly flow to NZ. This LOW is forecast
to weaken as it crosses southern NZ on Friday/Saturday, but it could
well be accompanied by a thundery trough, with some heavy rain in
northern NZ on Friday. Avoid.

Anyway, it should be followed by a west/SW flow over NZ on the 13-14
weekend as the next HIGH fills in the North Tasman Sea along 30S, taking
the standard winter track.

Note: It is annual National Fieldays time again... I'll be busy
talking with farmers 9-13 June, maybe able to briefly check emails at
night.

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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