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

31 May 2009

BOBGRAM7 issued 31 May 2009

WEATHERGRAM
YOTREPS
Issued 31 MAY 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.

I welcome those who have recently signed on to these weathergrams.

TROPICS/SUBTROPICS
The South Pacific Convergence Zone SPCZ is settling down now after a few
active weeks. It is slightly north of normal from Solomons to Tuvalu to
Northern Cooks and has been affecting French Polynesia. A southern
band, which has really been the northern end of mid-latitude systems
coupled with rain maintained by the subtropical jetstream, has been
bothering Fiji and southern Tonga last week - this branch is expected to
stretch eastwards towards Southern Cooks by Wed/Thu, and then a small
low is likely to form on this branch near Niue on Fri/Sat and then move
off the southeast. Avoid.

The Galapagos to Marquesas route is all quiet now.

SUBTROPICAL RIDGE STR / NZ WEATHER
Last week I scoffed at the models for producing a 960 Low south of
Chathams for this weekend, and indeed it is just a normal 980 hPa low
that has turned up. Even so, the 1040+ HIGH has turned up to South of
Tasmania, and that is about as high as they get ... so that is around 60
hPa of southerly isobars for the South Tasman Sea and NZ today and
tomorrow, another polar-blast-weekend, but not as windy or snowy as
last weekend.

The swell now travelling north with this weekend's southerlies is
expected to peel off NZ on Monday and reach Tonga around Wed/Thu and
fade there this weekend.

That 1040+ High is a Big Fat High BFH and should be able to extend
eastwards across the Tasman, something which none of the other May Highs
were able to do.

And so it marks a change in weather pattern, for the coming week anyway,
into something more settled around NZ. Even so, it is likely to fizz as
it moves east, crossing southern NZ at less than 1025 central pressure
on Thu 4 June, followed by a front and SW change on 5-6 June (avoid) and
then another HIGH from 7 to 9 June, whilst a new LOW is now expected to
form in west Tasman Sea near 37S on 5-6 June and then fade over Northern
NZ from 9-11 June.

All this traffic means that there will NOT be any strong squash zone on
the north side of this HIGH, so the trade winds around Tonga, Fiji, and
New Caledonia are NOT going to be enhanced this week. It also means the
subtropical ridge between NZ and the tropical Islands is likely to
expand and broaden this week-a larger than normal zone of light winds
for anyway sailing northwards from Northland to have to motor through.
And so it is a week without swell for voyaging, and that's an enjoyable
change.

The next dose of wind over Tasman Sea/ Northland MAY be a period of NE
winds ahead of that Low mentioned in the Tasman, so may not start
arriving until Mon 8 June. This feature will need updating.

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

24 May 2009

FW: BOBGRAM7 issued 24 May 2009

WEATHERGRAM
YOTREPS
Issued 24 MAY 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 was more active than normal as
it moved across New Caledonia and Vanuatu last few days, activated by a
passing trough. This week the Coral Sea is starting off reasonably free
from SPCZ convection but this is expected to build up again by 1 June
mainly over Vanuatu.

The squash zone mentioned in the last weathergram as peaking near Coffs
Harbour on Friday managed to produce wind and rain damage over southern
Queensland and northern New South Wales. Erosion is being reported as
worst in years. This squash zone is already moving off to the east and
is attached to the south side of the LOW now south of New Caledonia.

The squash zone mentioned in the last weathergram for northern NZ this
weekend has been bothering Cook Strait over last few days (11 metre
swells cancelled the ferries on Saturday) and is now likely t0 peak in
the Gisborne/Bay of Plenty area on Tuesday 26 May as the LOW from New
Caledonia makes its way southeast.

This LOW is expected to maybe do a loop between Raoul Island and North
Island on Monday/Tuesday. Anyone sailing north will find it OK to go
clockwise around this Low (light winds near the centre). All the strong
winds are tied up in the squash zones. There may be another subtropical
low forming just to south of Fiji too, and this low may grow as the one
south of New Caledonia dies. This is called a vorticity transfer.

There are a few showers on the Galapagos to Marquesas route.

SUBTROPICAL RIDGE STR / NZ WEATHER
The HIGH is the south Tasman Sea today is in a latitude more consistent
with LA NINA than the current neutral territory that is meant to be
affecting us. It intensified to 1038 and managed to draw a lot of
southern ocean air onto NZ over the past week, aided by a few lows near
Chathams. Well it is now starting to fade , decay, weaken, and filling
in-but will bring some clear skies/light winds and frosty dawns to the
South island on Mon/Tue, central NZ on Wed/Thu and the North island on
Thu/Fri. As this ridge spreads NE like this is brings a window of good
departing weather for anyone in NZ seeking to sail north--- just wait
for that squash zone to leave the North Island first. Wednesday
afternoon looks good at this stage.

The next trough from the west is expected to cross Tasmania on Tuesday
and then develop a LOW that move northeast across the South Island on
Friday, east across central NZ on Saturday and then southeast past
Chathams Island on Sunday, explosively deepening in the process, just
in time to muck up the Q Birthday weekend holiday in NZ.

Latest GFS model drops this low to below 960, and intensifies the next
HIGH, crossing to south of Tasmania on 1 June to 1040+ This seems to
be a computer wind-up to me, 80hPa of SW isobars attacking NZ on
Sunday??? Yeah, right. If so, avoid, but I'm sure this is model
extremism. The EC usually does a better job but seems to be offline at
this stage, so check again before your weekend sail.
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

17 May 2009

BOBGRAM7 issued 17 May 2009

WEATHERGRAM
YOTREPS
Issued 17 MAY 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
The South Pacific Convergence Zone SPCZ has been more active than normal
last week, stretching across the Solomons to Rotuma/Samoa and then
southeast roughly thru 20S 160W. It is expected to wander north and
south around that average position during the next week or two.

All looks normal on the Galapagos to Marquesas route.


SUBTROPICS
To east of 180 there is a subtropical HIGH near 40S 140W, somewhat
quasi-stationary, but it should be knocked more to 45S by the end of
this week, and extends out to 35S 100w.

This BIG FAT HIGH is maintaining two subtropical lows near 33to 35S at
present, one near 160 W (and this is killing the trade winds over
southern Cooks and French Polynesia, and t'other near 125W, these are
moving east or east southeast. Avoid their squash zones.

The subtropical ridge is very weak at present between 160E and 160W,
with troughs over NZ. Cold air has got into the subtropics and spun up
a small subtropical Low to east of New Caledonia today; this is the
"complication" mentioned in last weeks weathergram. It is expected to
cross the Kermadecs on Monday UTC and move off to the southeast. If you
are in the vicinity remember that it is easiest to clockwise around this
low.

The next BIG HIGH "upstream" is expected to move into the South Tasman
Sea on Thursday 21 May and then wander north into the northern Tasman
Sea over the weekend and early next week. As this High enters the south
Tasman Sea a Low is likely to form between Brisbane and Lord Howe Island
and then move east towards Kermadecs early next week. Avoid the squash
zone between Low and High--- looks like it'll bring big swells to Coffs
Harbour, peaking around Friday Saturday.
This Squash zone will extend eastwards along 33S reaching 180 by Fri 22
May--- anyone sailing northwards from Northland this week should try and
get north quick to beat this squash zone.

NZ AREA
Yes, It's been troughy. We've had heaps of rain over the Southern Alps
last week and the fronts have been thundery and proceeded by vigorous
northwest flows. One-more-of-these-fronts is likely on Monday and then
the flow should go more southerly thanks to rising pressures from the
incoming High.

These southerly winds may be snowy in the south on late Monday/Tuesday
and more so over eastern NZ on Wednesday (helped by a LOW that is likely
to deepen on Wed between Canterbury and Chathams. Later in the week
lighter winds and clearing skies should bring a dose of frosty dawns to
the South Island/central North Island.

Those waiting in Northland to sail off to the north should take off as
soon as the SW winds arrive around Tuesday. This is so as to try and
get north of 30S before the squash zone arrives--- then again, the
squash zone will mean hearty easterly winds and some will prefer that
perhaps.

From Friday 22 to Tuesday 26 May the squash zone between Low and High
will likely be over northern NZ-brace for it or be embarrassed.

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

09 May 2009

BOBGRAM7 issued 10 May 2009

WEATHERGRAM
YOTREPS
Issued 10 MAY 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
There is a weak Madden Julian Oscillation moving east across the central
Pacific at present. This enhanced convection on the South Pacific
Convergence Zone SPCZ - last week it was very strong from Solomons to
northern Vanuatu to Rotuma /Wallis /northern Tonga, and this week it
is expected to be extra active between Samoa and Southern Cooks and over
southern parts French Polynesia. Lowering pressures are forming a river
of surface NW winds from Tokelau to Northern Cooks, so LOWS are ;likely
to form on the SPCZ this week, maybe near Niue, and maybe near Gambier
Islands. Once these lows form they will travel south or southeast,
accentuating the squash zones between them and a large high further
south. Avoid.

West of the dateline it looks like a quiet week - the subtropical ridge
between Fiji and Australia is very weak at present due to the upper low
over NZ, and so the trade winds area are weak and it seems that the SPCZ
will have a quieter week here than last week.


There is nothing much to report on the Galapagos to Marquesas route.

SUBTROPICAL RIDGE STR
There is a lot of energy tied up in the long waves at present, so the
subtropical ridge has split into weak and strong longitudes. At present
there is a strong HIGH at 40S between 160 and 130W, blocking
proceedings. The High today approaching Tasmania is weakening away-so
the next strong High "upstream" is currently in the southern Indian
Ocean-and it will be moving across the Aussie Bight --- mainly along the
coastline---this week, and is forecast to reach the Tasman Sea NZ area
next week.

NZ AREA
So the STR is weak over NZ and the Tasman Sea, and a large multi-centred
Low is the result. The individual low centres are dancing around each
other in a clockwise reel, showing the dimensions of this large upper
low, what we call a cold pool. There has been too much swell and
occasional squalls around this system to allow any comfortable departure
north from Northland over the past few days. Things are expected to
ease over Northland on Wednesday/Thursday of this week, making a
reasonable departure window. There is likely to be another Low in the
Tasman Sea / South Island area from Friday, bringing another dose of
heavy swell to the Northland area on the 16-17 May weekend so that may
shorten this window.

Another complication for vessels heading north is that the GFS model is
picking that a weak trough may be crossing the Northern Tasman Sea area
from Thu 14 May and moving east across the Minerva area around Sun 17
May. EC and NOGAPS models are not so sure. This is a minor
complication, and might not actually appear, or it may require some
planning for comfort. Check the updates.

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

03 May 2009

BOBGRAM7 issued 3 May 2009

WEATHERGRAM
YOTREPS
Issued 3 MAY 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
South Pacific Convergence Zone SPCZ is in the zone from Tokelau
southeastwards to in-between Northern and Southern Cooks and French
Polynesia. Trade winds are weak east of the date line, with a
subtropical ridge near 15S (wow, that's way north), and huge LOWS
dominating between 30 and 40 S to east of NZ. So in this area the SPCZ
is behaving like the tropical end of the fronts attached to those lows.
That low I mentioned last week to be lurking to SE of NIUE has had some
fronts and gales around it is finally moving off, but watch out, it
looks as tho' another may behave much the same Thu 7 to Sat 9 May UTC.

West of the dateline, things are closer to the norm, with SPCZ from
Solomons to northern Vanuatu, and fresh SE trade wind sin the mid Coral
Sea between the SPCZ and higher pressures off Southeast Australia.
Conditions in this area are looking steady for good sailing.

That zone of convection mentioned last week mainly from 5 and 8S between
Marquesas and Galapagos is now fading away.

SUBTROPICAL RIDGE STR
A fortnight ago we had a big fat high in the STR. Now the STR is going
through a very weak week. There are highs moving across the Aussie
bight, but they are all weakening in the Tasman Sea - it may be a
fortnight (mid May) before we see the next high reach NZ.

NZ AREA
The first cold southerly in May is usually enough to shiver the timbers
of cruising yachts in Northland so they all want to leave for the
tropics. A similar autumn occurrence in the US is called the
Californian Flush, so it isn't a great surprise that our "OPUA FLUSH"
has begun.

Since the SPCZ is behaving itself and nothing much is happening in the
tropics (apart from those lows near Niue), the warmth beckons: it does
look as tho the cyclone season had faded fully, fingers crossed.

Those departing today on Monday should have a good voyage north. Others
should try and get away before noon Tuesday if they wish to get north
before some incoming heavier swells.

Those waiting til later may have problems--- between the weak STR and
lower pressures to the south the winds are dependable enough: west to
southwest, so no great problems there. The problems are to do with
some LOWS INTENSIFYING over NZ and bringing STRONG/GALE winds and bursts
of HEAVY SWELL. Those swells wander far away from their generating
winds and can be uncomfortable: they will, however, have long periods
once they get north of 35S.

This week there are two LOWS to avoid: The first brings wind and swell
to Northland on Wednesday as it crosses central NZ, then there's a brief
easing on Thursday, then the next brings wind and swell to Northland on
Friday and Saturday. At this stage it is likely to be followed by a
significantly cold outbreak over NZ on Sunday.

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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