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

29 August 2010

BOBGRAM7 issued 29 Aug 2010

WEATHERGRAM
YOTREPS
Issued 29 August 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.

TROPICS
The South Pacific Convergence zone SPCZ is strong along 10S across Papua
New Guinea/ Solomons and Tuvalu, and is likely this coming week to
extend to Tokelau, and maybe even affect Samoa to Suwarrow at times,
but otherwise is weak.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds as far as 15S. One of these is
expected to affect Fiji on Mon 30 Aug (some welcome rain!) Tonga on Tue
31 Aug, and then fade on its way to Southern Cooks by Thu 2 Sep.
Apart from this interruption, it looks to be a good wind for trade
winds in South Pacific.

SUBTROPICAL RIDGE: STR
Something to do with some extra energy in the subtropical jetstream
above around 30S. So the High that is expected to reach Lord Howe Island
by Wed 1 Sep should anchor its stern there and then rapidly extend
eastwards along 25/30S so that its nose gets to south of French
Polynesia by Fri 3 Sep. This is in response to a curtain of sinking
air, and it helps to feed the trade winds at 15 to 20S over the entire
region.

Next HIGH in the Tasman should build east of Sydney around Friday 3 Sep,
in good time to cross the North island during the 4/5 Sep weekend J

TASMAN/NZ

The long wave trough that has been bothering the Tasman sea is
retreating north for a bit. There is still a haphazard procession of
fast moving fronts bothering Tasman /NZ but the distance between troughs
is getting more, allowing for some voyage planning. And there may be a
break in this with next weekend's passing High.

Before then, today's trough is expected to settled into a couplet of
lows east of the North Island on Mon 30 Aug to Wednesday 1 Sep (stand
by for some rain in the Hawke's Bay ranges). The next trough is likely
to be preceded by a vigorous NW wind over South Island on Thu 2 Sep and
followed by a disturbed SW/S flow on Friday 3 Sep.

Anyone intending to head south to NZ this week can probably find
reasonable weather so long as an arrival around 6 or 7 Sep is targeted.
Plenty of south/southwesterly winds for anyone heading North, but avoid
the prefrontal conditions on Thu 2 Sep.

The parameter we use to measure the strength of the polar vortex has
indeed gone negative in the past few days, first time since May. This
increases the chances of a polar outbreak somewhere ... and indeed there
seems to be one near the Horn this weekend, but none, so far, anywhere
else.


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

22 August 2010

BOBGRAM7 issued 22 Aug 2010

WEATHERGRAM
YOTREPS
Issued 22 August 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.

TROPICS
The South Pacific Convergence zone SPCZ is slowly returning to normal
over Papua New Guinea and Solomons. It is putting in an appearance
around Tokelau, but otherwise is weak . Should continue to rebuild
from Tuvalu to Northern Cooks this week, and may start to drift south
for a while next week.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds as far as 15S over French Polynesia
and as far as 20S elsewhere. One of these is moving east off French
Polynesia today 22 Aug, and another should cross Tonga on 23 Aug, Niue
on 24 Aug, Southern Cooks on 26 Aug, and French Polynesia around 28-30
Aug. SO this week is something like a repeat of last week.

SUBTROPICAL RIDGE: STR
Upper air is such that Subtropical highs are being encouraged in north
Tasman Sea and in the broad zone east of NZ. There is a high cell in the
North Tasman Sea today 22 Aug: it should cross NZ on Tues 24 Aug and
then wander to the east slanting to 35S then 25S and reaching 150W to
South of French Polynesia by Sat 28 Aug. There should be a squash zone
of enhanced trade winds on the northern side of this high.

TASMAN/NZ
Weak frontal system is likely to cross NZ on Wed 25 Aug after that High
- preceded by a northerly flow and followed by a westerly.

A long wave upper trough is hovering around the Tasman Sea again this
week.

At ground level, we have a broad cold trough and a deep southern ocean
Low located today well to south of West Australia at 150E. This system
is the highlight of the coming week. Its Low centre should start moving
NE out of the southern Ocean when it gets south of Tasmania on Wed 25
Aug and reach NZ on Sat/Sun 28/29 Aug. However, there the High
following this system and it is expected to expand at 30 to 35S over SE
Australia on Sat /Sun 28/29 Aug. So that will limit the southerly fetch
this low can draw on, so it'll be wet and windy, but not a polar
outbreak.

SO it is OK to approach NZ until Tue 24 Aug. Then not OK until around 2
Sep.

The strength of the polar vortex was mentioned last week. Well, its
still holding its strength and is not weakening as much as the computer
models have been picking. Even if it does weaken to a stage that a polar
outbreak can occur in early September, the chance that this will impact
on our quarter of the world will be 1 in 4. So, while it is interesting
to watch for, it ISN'T another thing that yachties who are planning a
voyage from tropics to NZ or Australia need to worry about.


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

15 August 2010

BOBGRAM7 issued 15 Aug 2010

WEATHERGRAM
YOTREPS
Issued 15 August 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.

TROPICS
The causative factor for the cold air that brought floods to Pakistan
and slips to China AND the hot air bringing heat waves to Russia is the
same: meanderings in the Northern Hemisphere's jet streams. Over the
past few days NZ copped a burst of water vapour that had come from the
monsoon area, and this brought something like 200 to 300mm rain to our
mountains with some flooding and slips most notably around Whakatane,
but some drier air is now separating Asia and South Pacific.

The South Pacific Convergence zone SPCZ has faded recently over Papua
New Guinea and Solomons - should rebuild there by 20 Aug. It has built
up along 10S between Tuvalu and Tokelau, and is likely to slowly spread
south possibly affecting Rotuma to Samoa by Friday 20 Aug.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds along 20S. One of these is moving
east off the Tuamotus today 15 Aug, and another should cross Tonga on
16 Aug, Niue on 17 Aug and Southern Cooks on 18 Aug. Then the
subtropical ridge may be strong enough to block any northern extensions
of mid-latitude troughs, for a while anyway.

SUBTROPICAL RIDGE: STR
Small high cell in the north Taman Sea today 15 Aug is expected to move
east along 25S and fade near 180 on Mon 16/ tue 17 Aug. Next High will
be larger, forming in Tasman Sea on Wed 18 Aug and then moving east
along the 25 to 30S latitude band, reaching 180 around Fri 20 Aug, and
likely to be south of French Polynesia around Sun 22 Aug. There should
be a squash zone of enhanced trade winds on the northern side of this
high.

TASMAN/NZ
A long wave upper trough is hovering around the Tasman Sea this week.

At ground level, a broad cold trough, with squally thundery showers, is
now crossing the Tasman Sea and should cross NZ on 16 and 17 Aug.
Its SW flow is likely to linger over NZ on Wed 18 and much of 19 Aug.

Most settled day this week is looking to be Fri 20 Aug - for anyone
contemplating sailing to or from NZ.

Another broad cold trough is expected to cross Tasmania around Thu 19
Aug and reach the South Island on Sat/Sun 21/22 Aug.

There is a parameter that measures the strength of the polar vortex
(that ring of westerlies than encircles Antarctica). It is called the
AAO and can ne seen at
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/
aao_index.html
- and known locally as SAM (Southern annular modulus).
Not usually relevant here, but has been way out of whack recently with
record high values, up to 4 standard deviations from its mean last
month. This suggests that Antarctica's cold air has recently all been
locked up, no polar outbreaks anywhere. Well, computer modelling is
suggesting this will all change by end of August, and then polar
outbreaks will be on the menu again (not sure which longitude band yet).
--


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

07 August 2010

BOBGRAM7 issued 8 Aug 2010

WEATHERGRAM
YOTREPS
Issued 8 August 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.

TROPICS
More heavy rain is forecast in the Monsoon over Pakistan this week, and
lots of rain over much of Indonesia. A conveyor belt directed some
moisture from there to NZ over the past few days. If you have access to
Internet see this at http://www.ssec.wisc.edu/data/comp/wv/wvmoll.mpg

The South Pacific Convergence zone SPCZ is strong about the Solomons and
somewhat quiet elsewhere. It is expected this week to build in the
Tuvalu and Tokelau area, and this may spread towards Samoa by the end of
the week.

Passing troughs in the mid-latitudes are extending far enough northwards
to affect the southern parts of the tropical South pacific, but not to
reach the SPCZ. One of these is located south of Fiji tonight and
should wander east and reach Southern Cooks by Thu 12 Aug UTC...
Preceded by a swing of the trade winds so they come from the NE,
accompanied by some squally rain, and followed by southerly winds and
clearing weather. Another of these troughs is expected to form off the
east coast of Queensland on Tue 10 Aug and wander east across the Coral
Sea to reach New Caledonia by Thu 12 Aug. If sailing, try and plan
around these troughs - at least remember that the computers and GRIB
data can not resolve the details near them.

SUBTROPICAL RIDGE: STR
Long wave pattern continues to be conducive to forming troughs in the
Tasman Sea and helping the STR in longitudes east of the dateline.

The HIGH forming now/8 Aug UTC over western Tasman Sea is expected to be
rather slow and weak until it crosses northern NZ on Wed 11 Aug and then
should expand east of NZ until Sat 14 Aug. There is likely to be a
zone of enhanced trade winds on its northern side.

TASMAN/NZ
A southerly start to the week with an outgoing low- barometers are
rising. Rain was briefly intense on Saturday and there are still some
intense showers around- in Wellington slips are disrupting traffic.

Fronts following Wednesday's high are likely to cross NZ on Thu 12 and
Fri 13 Aug, followed by a low that may linger over Northland for the
14/15 weekend.

So it is OK to sail towards Northland if you can get there by
Wednesday, and anyone planning departure should consider waiting until
next week.


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

01 August 2010

BOBGRAM7 issued 1 Aug 2010

WEATHERGRAM
YOTREPS
Issued 1 August 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.

TROPICS
Asia is much wetter than normal, and in true Walker circulation style
this extra rising air is producing more sinking air in the subtropics,
accentuating the subtropical ridge especially around the eastern Indian
Ocean and Western Australia.

The South Pacific Convergence zone SPCZ has been knocked a little south
by this so that it has been DRY recently over Vanuatu, Fiji, and from
Samoa to Tahiti, and wetter than normal from Coral Sea to south of Niue,
with trade winds much stronger than normal over the whole South Pacific,
especially about Coral Sea where it is being drawn into a strong
Monsoon.

A passing upper trough in the mid-latitudes is likely to help accentuate
the SPCZ between Samoa and Southern Cooks on 2 Aug (UTC). There should
be some squally showers in this trough as it crosses Southern Cooks
around 3 Aug and Tuamotus by Thu 5 Aug. This offers something for the
puddle jumpers at Tahiti to take into account.

SUBTROPICAL RIDGE: STR
Long wave pattern is conducive to forming troughs in the west Tasman Sea
and helping the STR in longitudes east of the dateline.

The Large HIGH over 1036 east of NZ today (1 Aug) is expected to wander
east along about 40S this week, with a Low forming on its northwest
shoulder near 30S 150W around Tue 3 Aug UTC. As this pair wanders off to
the east, some smaller High cells are likely to form near latitude 30S
about and east of NZ by Fri 6 Aug, with fresh to strong trade winds on
their northern side.

TASMAN/NZ
One Low deepened in the mid Tasman Sea on 31 July and is expected to
track SE across Southland tonight. Next Low should deepen off New South
Wales on Mon 2, rotate clockwise around the Tasman Sea and weaken into a
trough that is likely to cross NZ on Wed 4 Aug UTC. Third Low should
deepen rapidly over Tasmania on Wed 4 Aug and rotate clockwise around
the Tasman Sea and cross North Island around Sat/Sun 7/8 Aug UTC.

There are brief ridges with slack winds between these lows but generally
too brief to allow much planning for a sail trip to or from NZ.


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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