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

31 August 2008

BOBGRAM7 issued 31 Aug 2008

WEATHERGRAM
YOTREPS
Issued 31 August 2008 NZST
Bob McDavitt's ideas for South Pacific sailing weather.
(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).

TROPICS
South Pacific Convergence Zone SPCZ is building in strength over Papua
New Guinea and Solomons. Further east there are two branches-one from
Tokelau to Northern Cooks along around 10S, and t'other sort of in the
Tonga/Kermadecs region, stretching east along 20/25S to a peak south of
French Polynesia.

SUBTROPICS
The pattern is dominated by the Big Fat High BFH east of NZ this
weekend. It is rare to find a 1040+hpa anticyclone in our part of the
world - I can't seem to find a tele-connection between it and GUSTAV in
Gulf of Mexico, suffice it to say that planet Earth is going through
some extreme weather at present. We in MetService have a saying, when
High above 1030, weather will get dirty... so when High above 1040,
weather will get naughty. The squash zone between this High and the low
that will form on its NW shoulder (as they do) will be particularly
intense over the Kermadecs area on Monday and Tuesday. Fiji Met has
already issued a gale warning on this.

The Low that forms south of Fiji/west of Kermadecs on Monday is forecast
to wander south/southeast to east of Chathams by Sat 6 Sep as the High
wanders east. There's another squash zone on the north side of this
High, and it'll reach its peak over Southern Cooks on the 6/7 Sep
weekend UTC w and strong easterly winds from Tuamotu to Southern Cooks
and with 4m+ swells around Southern Cooks . AVOID.

NZ AREA
Next trough rolling in from Tasman Sea is a trough between two Highs, so
this is a new pattern for us - a change from those lows that have been
forming in Tasman Sea and dumping rain on us. The trough should bring
rain to western South Island and gusty NW to eastern NZ from Mon to Thu
---stalling a while as a secondary low forms near Brisbane--and finally
secondary low and trough should cross the North Island on Thu and Fri
and maybe Sat and Sun. Avoid departing/arriving Northland Thu-Sun.

Sailing NZ/Fiji, Tonga? Wait until the Low has gone, after 4 Sep.
Sailing NZ/Noumea, looks OK.
Sailing Eastern Aussie? Not good this week, wait for that Low off
Brisbane has gone.

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

29 August 2008

BOBGRAM7 issued 24 Aug 2008

WEATHERGRAM
YOTREPS
Issued 24 August 2008 NZST
Bob McDavitt's ideas for South Pacific sailing weather.
(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).

TROPICS
South Pacific Convergence Zone SPCZ had another quiet week and is most
active along 10South - Papua New Guinea to northern Vanuatu and southern
Tuvalu to Tokelau to Northern cooks to northern Tuamotu. This still
means that those hopping west from French Polynesia should go via
Aitutaki or Niue rather than the Suvarov way.
The subtropical ridge, to east of 180, is strongest along 30S, so the
trade winds are enhanced about half-way in-between, along 20S.

Some of the SPCZ occasionally connects with a mobile east-ward
travelling mid-latitude trough. One of these connection-troughs is
crossing New Caledonia today, 24 Aug, and should visit Tonga on
Tuesday/Wednesday, Niue on Thursday and Southern cooks on Friday UTC
this week. These passing troughs are preceded by a period of light
variable mainly NE winds, accompanied by a few tropical squalls, and
followed by a re strengthening of the SE trade winds. There has been a
near weekly rhythm to them recently.

SUBTROPICS
It is time for another HIGH to makes its way eastwards across the Tasman
Sea - That one over eastern Australian coast today (sun 24 Aug) is
expected to weaken away as it moves NE across the Tasman Sea towards
Norfolk Island on Monday to Wednesday - Just not enough oomph. A weak
front is forecast to move east across Tasmania on Tuesday UTC followed
by a HIGH with oomph, that should cross the south Tasman Sea at 45S on
Friday and the South Island on Sat and be east of North Island by Mon 1
Sep.
Early next week there are signs that a low /trough may form in the Fiji
area and wander southwest towards eastern New Caledonia.. Not sure about
this yet, but if it does happen then there will be a squash zone of
enhanced easterlies between this trough and that HIGH , mainly around
30S between Kermadecs and Norfolk Island. Maybe worth avoiding.

NZ/TONGA AREA
It is still winter here in NZ, but the deepest depth of cold was , it
seems, 9-10 August, just after the start of the Olympics if you can
remember back that far. The LOW crossing North Island Monday/Tuesday is
followed by a southerly affecting southern North Island mainly in
Tuesday/Wednesday. The remainder of the week is dominated by the
incoming HIGH.

Nothing much to watch out for, in sailing to/from Northland this week.
Just remember about the possible Kermadecs to Norfolk squash zone that
MIGHT form early Sept.

The terms used here are more fully explained in the METSERVICE Yacht
Pack. More info at http://weathergram.blogspot.com Feedback to
bob.mcdavitt@metservice.com

16 August 2008

BOBGRAM7 ISSUED 17 August 2008

WEATHERGRAM
YOTREPS
Issued 17 August 2008
Bob McDavitt's ideas for South Pacific sailing weather.
(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).

TROPICS
South Pacific Convergence Zone SPCZ is going through a quiet time at
present and has broken into a few disconnected branches. One stretches
from Papua New Guinea to Vanuatu and occasional feeds showers over
northern parts of Fiji. Another stretches from between Samoa and
Tokelau to the Northern Cooks and occasionally breeds showers between
Tuvalu and Wallis/Futuna in the west or over Tuamotu group in the east.
This means that those hopping west from French Polynesia should go via
Aitutaki or Niue rather than the Suvarov way.

Easterly winds dominating the whole South West Pacific, both north and
south of the SPCZ. There is likely to be a squash zone of enhanced
trade winds over western half of Coral Sea (and the Great Barrier Reef)
especially on Wednesday and Thursday in response to a HIGH then crossing
the Tasman Sea. Not comfortable.

Some of the SPCZ occasionally connects with the mobile east-ward
travelling mid-latitude troughs. One of these connection-troughs is
likely to visit New Caledonia on Friday and Saturday 22-23 August, and
may affect parts of Fiji/Tonga on Sunday 24 August. These troughs
bright light variable winds and a few tropical squalls. Today's
computer output is picking intense squalls around the NW of New
Caledonia on Friday 22 August but this may change by then.

SUBTROPICS
It is time for another HIGH to makes its way eastwards across the Tasman
Sea - and the next one is forecast to form in the Tasman near 30S on
Tuesday. This HIGH should cross Norfolk Island on WEDNESDAY and
Kermadecs on Friday and be south of French Polynesia on Sunday 14 August
UTC-there will be enhanced trade winds at 20S to north of this High.

The High is expected to be followed by a trough that will cross reach
Sydney on Thursday then cross the Tasman and then NZ on Saturday/Sunday
(yep another wet Saturday). AT this stage the HIGH following this
trough (next week's HIGH) is expected to take the 35SOUTH latitude.

So, there are reasonable weather patterns for crossing the Tasman
either way : next week's HIGH may be a slightly better choice for
sailing westwards to Brisbane, and this week's trough does offer an
opportunity from sailing eastwards across the Tasman.

NZ/TONGA AREA
Heaviest snowfalls in the Arthur's pass and Ruapehu areas of NZ since
the early 90s (and the Pinatubo eruption) - caused by a deep LOW that
stalled over southern NZ taking southern ocean air and swinging it north
into the Tasman Sea then in a westerly flow onto NZ. We meteorologists
call it mmP or modified maritime Polar (kiwi will get the pun about mmp
in an election year). But nope, this winter, in spite of all the wind
rain and cold doesn't get to break any records or hop on any medal
podium - we've had wetter, colder and windier winters in NZ. It's just
the snow that stands out, and that's due to mmp.

A system forming over the North Island today (Sunday) will deepen into a
LOW below 990 east of the North Island on Monday and this system is
forecast to LOOP and deepen further on Tuesday, exposing eastern North
Islanders to a sting of enhanced wet cold southerly winds (avoid).
Thursday is the best day of the week in NZ for light winds and a
reasonable sunny day for golfers.
The next trough is likely to start off on Friday with heavy rain for the
Southern Lakes with its NW flow on Friday, followed by west to SW
showery weather on Saturday and Sunday. Avoid.
Best days to depart/sail from northern NZ for Tonga are Monday/Tuesday.
Best day to depart/sail from Tonga to NZ is Tuesday (for a vessel
capable of up to 6kt).

The terms used here are more fully explained in the METSERVICE Yacht
Pack.
More info at http://weathergram.blogspot.com Feedback to
bob.mcdavitt@metservice.com

09 August 2008

BOBGRAM7 issued 10 August 2008

WEATHERGRAM
YOTREPS
Issued 10 August 2008 NZST
Bob McDavitt's ideas for South Pacific sailing weather.
(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).

TROPICS
South Pacific Convergence Zone SPCZ continues to linger at the northern
end of its range, along 10 South latitude from Solomons to Tuvalu, and
should spring to life from Tokelau to northern parts of French Polynesia
later this week.
This means it is lingering near Suvarov, so those hopping west from
French Polynesia should go via Aitutaki or Niue rather than the Suvarov
way.

Some of its moisture occasionally branches off to the south and mixes
with the mid-latitude troughs as they travel from west to east.
One of these mid-latitude troughs visited New Caledonia and Fiji last
week, and another is likely to reach New Caledonia on Monday 11 August
UTC and brush past Fiji on Wed 13 Aug UTC. These troughs are reasonably
mediocre, but they do bring periods of variable wind.

SUBTROPICS
Between these troughs we have subtropical ridges/Highs, these were weak
over past few weeks but are becoming more noticeable now. They are
migrating from west to east mainly along 30 s. One is over North
Island/ Kermadecs today/Monday 11 Aug UTC and should be south of French
Polynesia on Thu 14 Aug, and the next is likely to blossom sat 30S south
of Southern Cools around Sat 16 Aug UTC. North of these highs there is
a zone of enhanced trade winds in the tropics especially around 20S.

SO, trade winds in the tropics this week go up and down in speed as
ridge and troughs pass by in the subtropics Otherwise it is likely to be
a weak of trade winds in the SW Pacific.

TASMAN/NZ AREA
T'was good to see the sun over NZ today from that passing ridge - first
in many weeks. But it was a cold High and didn't last long. The LOWS
are still deepening in the Tasman Sea, but they are not blocked like
they were in late July.

Next Low is already deepening near Sydney today 10 Aug and its fronts
are expected to cross NZ late 11 and on 12 Aug UTC with the LOW crossing
the South Island. This should be followed by a disturbed westerly flow,
with another front on Thu 14 Aug UTC and a LOW affecting southern parts
of Tasman Sea/NZ on 16-17 Aug followed by cold southerlies. Avoid
these fronts and lows. The westerly to southerly winds in the Tasman Sea
again this week will make it hard for anyone wanting to sail to Sydney.

The terms used here are more fully explained in the METSERVICE Yacht
Pack.
More info at http://weathergram.blogspot.com Feedback to
bob.mcdavitt@metservice.com

03 August 2008

BOBGRAM7 issued 3 Aug 2008

WEATHERGRAM
YOTREPS
Issued 3 August 2008 NZST
Bob McDavitt's ideas for South Pacific sailing weather.
(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).

TROPICS
South Pacific Convergence Zone SPCZ is well no from the northern part of
Solomons to Tuvalu/Tokelau to Northern Cooks. It'll probably start to
extend southwards this week and get to Samoa next weekend.

Some of its moisture is likely to branch off to the south across the
Coral Sea on Thu 7 Aug (all dates UTC) and join with a trough then
crossing the Tasman Sea so that a wet trough reaches New Caledonia on
Fri 8 Aug (in time for start of the Olympics) - accompanied by variable
winds and squally showers -- and this trough will then move southeast
across Kermadecs on Sat 9 Aug. Also on 8 Aug UTC another trough
(rather weak) is likely to move eastwards across French Polynesia
accompanied by some squally showers and variable winds.

Otherwise it is likely to be a weak of trade winds in the SW Pacific.

TASMAN/NZ AREA
The LOW that passed over NZ last week was a slow-mover. It was blocked
by a zone of higher pressures to south and east of it. Blocking is
something that adds grunt to NZ's winter storms, and it looks as though
it may hang around for the next few weeks. So a few more Lows from the
Tasman are on the menu - but this week there'll first be a break in the
pattern with a passing High.

This incoming HIGH will bring us a welcome break in the recent wet windy
weather. It's sitting over eastern parts of Australia today (3 Aug) and
expected to peel off into the Tasman on 4th then wander over NZ on
5-6-and much of 7th with dry weather and light winds. Good for fishing.

On Thu 7th the next trough will start deepening in the Tasman Sea,
preceded by northerly winds, accompanied by rain, and followed by
squally SW winds. This trough is likely to cross NZ on 8-9 Aug,
followed by a disturbed west/southwesterly flow on 10-11 Aug, turning
colder southerly on 12-13-14 Aug.

The terms used here are more fully explained in the METSERVICE Yacht
Pack. Feedback to bob.mcdavitt@metservice.com

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