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

24 July 2010

BOBGRAM7 issued 25 July 2010

WEATHERGRAM
YOTREPS
Issued 25 July 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
South Pacific Convergence zone SPCZ is mainly along 8 to 11 South from
Solomons to Tuvalu/Tokelau. It occasionally peels away to the southeast
and makes a trough-an example of this should occur over Tonga Niue on
Sat 31 July, and this may develop into a LOW to south of Southern Cooks
early next week.

Passing mid-latitude trough occasional extend north in to the tropics.
As they pass, the trade winds weaken and get swung to be from the NE for
a while, then squalls appear, sometimes followed by a swing to southerly
winds. One of these events is near Niue tonight and should wander east
across Sothern Cooks by Tue 27 July.

SUBTROPICAL RIDGE: STR
This is stronger than normal now and further south than normal, signs of
an incoming La Nina.

HIGH 1034 budding off New South Wales on Mon 26 July should wander along
35S crossing northern North Island around Thu 29 July and reaching 150W
by Sun 1 August. A Squash Zone of enhanced trade winds is likely on
north side of the High, mainly between 15 and 10S, reaching Samoa on
Tuesday/wed 27-28 July. This squash zone is especially strong in mid
Coral Sea and across northern Vanuatu until Wed 28 July. Avoid.

TASMAN/NZ
With the incoming High, NZ is in a disturbed West to Southwest wind
flow, with fronts, on Monday/Wednesday 26-28 July; OK to good for
sailing off to the north.

After the Thursday HIGH, a disturbed flow from the NW is to be expected
over NZ, and this should wetten the West Coast and warm the East Coast,
and lead to a LOW crossing the South Island on the 31/1 weekend; good
for sailing towards Northland from the tropics, but please note this
good window closes quickly by Sun 31 July.

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

18 July 2010

BOBGRAM7 issued 18 July

WEATHERGRAM
YOTREPS
Issued 18 July 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
South Pacific Convergence zone SPCZ is showing signs of drifting south
after a few weeks of hovering along 10S to 14S. One branch extends from
Solomons to Wallis/Futuna, and another branch is hovering between 10 and
14S between Tokelau and Samoa and around Northern cooks.

The SPCZ is occasionally enticed southeastwards to merge with
mid-latitude troughs. As this combination wanders east along around 20S
it swings and then kills the trade winds for a day or so, but that's
about all this week.

For those in Tahiti looking to go west: One of these troughs is
expected to pass by Rarotonga on Mon 19 July. The next is departing
Queensland at present, should pas by New Caledonia on wed 21 July, Tonga
on Sat 24 July and Rarotonga around Tuesday 27 July. Otherwise this is
a week of trade winds.


SUBTROPICAL RIDGE: STR
A lot can be gleaned by watching the rhythm of the highs as they wander
east in the subtropics. Their shifting isobars mould the troughs
inbetween them.
This week the STR is becoming better defined in the pacific than it has
been for awhile.

One High with central pressure around 1025 is wandering along 30 to 35S
, across Northern North Island on Mon 19 July, then slowly and steadily
east or southeast to get to 40S well to south of French Polynesia around
Friday 23 July. This High is mediocre, so will maintain moderate trade
winds and settled weather in the tropics on its northern side. Good for
sailing to the west.

Next High is likely to be more intense, building to 1035+ between 35 and
40S in South Tasman Sea around Mon 26 July. Too far away to be sure,
but this high is likely to cross central NZ on Wed 28 July and should
have a squash zone of enhanced trade winds on its northern side - and
these strong winds will likely start in Coral Sea from Wed 21 July and
spread east to reach Vanuatu/New Caledonia by weekend of 24/25 July and
further east next week. Be wary.

TASMAN/NZ
It is between the highs that the lows can flourish, scrum together, and
elbow outwards.

There's a trough departing the Australian eastern seaboard tonight (18
July) and as this trough crosses the Tasman Sea on Monday/Tuesday 19/20
July a Low is forecast to form within it. This low will likely have
colder than normal air in its upper parts and that help to deepen its
central pressure, slow it down and breed squally showers/strong
winds/heavy swell. It may deepen to below 995 while still west of the
North Island on Wed 21 July. Avoid. Should then start to weaken , but
may still be around 1000 hPa as it crosses North island on Friday 23
July, and then it should wander off to southeast of NZ and expand to
cover the broad area between the two highs mentioned above.

So anyone sailing to NZ should best arrive by Tue 20 July, and this
isn't the best week to depart 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

11 July 2010

BOBGRAM7 issued 11 July 2010

WEATHERGRAM
YOTREPS
Issued 11 July 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
South Pacific Convergence zone SPCZ is still quiet but slowly activating
along 10S from Coral Sea, in clumps, all the way to Northern Cooks and
Marquesas. It has been hugging to 10S latitude, but occasionally links
south with mid-latitude fronts, making wet troughs --- one affected the
Cooks on 7 and 8 July and another affected Fiji on 9 and 10 July. This
latter trough is expected to continue eastwards and finally fade out
between Cooks and Marquesas on 15 and 16 July Thu/Fri. Anyone heading
west from Marquesas to the Cooks, or south from Tonga or Fiji to NZ will
need to factor this trough into their plans.

There is a low; lets call it L1, which has formed with the trough that
was over Fiji last night. This Low is near 30S 180 and should move
steadily south along the 180.

Total Eclipse: Clouds seem to be keeping away from the area that will
become a total eclipse at 111830UTC near Maria Island in the Tuamotus
(and Mangia Island in the Southern Cooks). Easter Island is currently
partly cloudy and clearing, but there is a cloak of cloud heading that
way, so fingers crossed there.

SUBTROPICAL RIDGE: STR
High over 1030 crossing NZ this weekend is likely to wander off to the
east between 40 and 45S getting as far east as 140W by the end of this
week. This is so far south that it isn't likely to enhance the trade
winds on its northern side-but there will be a squash zone between it
and L1 that is worth avoiding.

Next high upstream will be slow to appear, and is likely to bulge east
off southern Queensland around Fri/sat 16/17 July, stretching east along
25/30S to get to 180 by Tue 20 July. This northern latitude STR will
help enhance a westerly flow over NZ.

TASMAN/NZ
Yes, we got polar air over NZ this weekend, as mentioned in the previous
weathergram; it was mainly dry but did manage to bring us the lowest
temperatures we have seen this winter.

As L1 comes south over next few days, the eastern North Island are
likely to get a dose of cold southerly rain, especially on Wednesday
/Thursday 14/15 July.. But the low should be further away than last
week's, and hopefully will have less impact.

There are some nice tail winds around L1 for anyone wishing to sail
northwards from Northland. These winds will fade away by Wednesday and
then turn northwest and then strong westerly for the remainder of the
week.

A winter /Southern Ocean Low is expected to deepen off Tasmania on Wed
14 July and then head northeast out of the south Tasman and onto the
South island / central NZ on Sat 17 July, followed by a disturbed
southerly and then a Southwest flow.

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 July 2010

BOBGRAM7 issued 4 July 2010

WEATHERGRAM
YOTREPS
Issued 4 July 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.

Galapagos to Marquesas: It seems this week that the direct route using
the current near 4South is NOT any faster than the alternative route
using the strong west-going current at around 2 degrees North from 100
to 120W. If you do decide to take the northern route you have to get
away for the east-going current at the equator, and you will likely
experience some squally showers.

TROPICS
South Pacific Convergence zone SPCZ is reasonably quiet this week, and
is mainly in the northern Coral Sea and northern Vanuatu and from there
along 10S towards Tokelau and Northern Cooks. There is an interesting
dry slot of sinking air from Fiji to Southern Cooks, and another around
northern Australia.
Due to High pressures that are now blocked over southern Australia there
is a zone of enhanced trade winds in the Western Coral Sea and around
the north end of Queensland. This should ease after 7 July but isn't
going to go away this week.

SUBTROPICAL RIDGE: STR
High to east of NZ today Sun 4 July is set to move steadily along 30S
and fade on 5 July. Another associated High should expand at 45S -50S
and this High fades This High should then move NE and reach 40S to south
of French Polynesia around Fri 9 July, with a squash zone of enhanced
trade winds on its northern side.
Next high upstream is stalled over South Australia but should be able to
send a cell across Tasmania on 8 July, South Tasman Sea on 9 July, and
South Island on 10-11 July.

TASMAN/NZ
This week the NZ region will be dominated by a large slow-moving low.
It isn't expected to ingest any cold polar air until 9 -10 July and by
then should be mostly off to east of NZ. The centre is expected to
track east along 33S to 180 and then southeast, so that most of its wind
and rain should stay out to sea, but Gisborne and Hawke's bay are in for
steady rain from Monday to Wednesday. The SE winds associated with this
low should cover northern NI tonight/Monday and central/southern North
island on Tuesday and Wednesday 6-7 July.

The low should be far enough away to allow reasonable sailing to the
north again by Thu 8 July. There is likely to be a left-over trough
between NZ and Fiji and this may activate early next week, so updates
will be needed.

As for sailing from Tonga to NZ, well : there may be a reasonable voyage
afte the low has gone, starting around 9 July, but you will need to be
prepared to sail through a trough.

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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