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

25 July 2009

BOBGRAM7 issued 26 July 2009

WEATHERGRAM
YOTREPS
Issued 26 July 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 is gradually reassembling itself
after another quiet week last week. Convection has been active about
northern Papua New Guinea and over the Solomons... and occasional
convection is appearing around Tuvalu and Tokelau, and around Southern
Cooks. Fiji had a dry week last week.

This week, the SPCZ is expected to start re-building itself in the
northern Coral Sea and extend towards New Caledonia /Vanuatu.

Much of the South west Pacific will have fairly steady trade winds this
week, but there will be some variations.

The tropical extension of a mid-latitude trough is affecting Southern
Cooks over next few days (27 to 29 July UTC), and this is expected to
affect southern Island of French Polynesia from 30 July to 1 Aug.

As for those sailing from Galapagos to Marquesas: the next week or two
is looking OK, with east to southeast winds maintained OK as Highs
travels east along 30S from 140W to 100W... These winds may weaken for a
few days around 2 Aug as that trough (between Highs) travels east along
30S.

SUBTROPICS
The subtropics east of 180 is currently dominated by a large low that
formed near Kermadecs after last few days. This low is forecast to move
east along 35S sucking away some of the force in the trade winds
further north as it wings along this path - south of Niue on 26-27 Jul,
and south of Cooks on 27-29 July.

Following this Low a new HIGH is forecast to form east of Chathams by 29
July (from the ridge that is moving east across NZ this weekend). This
HIGH should build and travel NE to 30S 140W by 2 Aug... forming a squash
zone of enhanced trade winds over French Polynesia

TASMAN /NZ AREA
Trough is crossing the Tasman Sea and is forecast to cross NZ on
Wednesday, followed by a disturbed westerly flow... that will be SW on
Thursday, a front over the South Island on Friday, westerly on Saturday,
and northwesterly by Sunday.

This disturbed westerly flow is expected to bring heavy rain to the
Southern Alps, and dry mild weather to the eastern South Island. A new
subtropical High cell may form between Lord Howe and Norfolk Island on
1-2 August, bringing another settled weekend (like this one has been) to
North Island.

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 2009

BOBGRAM7 issued 19 July 2009

WEATHERGRAM
YOTREPS
Issued 19 July 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 is gradually reassembling itself
after a quiet week last week. Convection has been active about
northern Papua New Guinea and over the Solomons... and also in an
unusual band between 2 and 6S from 140 to 160W - this could be extra
convection triggered off by warm seas near the equator. There is
currently an amazing blob of convection near EQ q65 to 170E, with
squally gales and westerly winds on its northern side/easterly winds on
its southern side---a sign of El Nino kicking in - and is more likely to
go north rather than south.

El Nino is pretty much here now as far as the equatorial Pacific ocean
temperatures are concerned, but, so far, just a few signs in the
atmosphere. One of these is a distinct dropping off in wind speed in
the trade winds between 100to 120W fro those sailing from Galapagos to
Marquesas to think about.

This week, the SPCZ is expected to start re-building itself, and extend
from Solomons to Tuvalu and Tokelau maybe to Rotuma and Futuna.

There is a weak trough moving east across Fiji on Monday and Tonga on
Tuesday --- the tropical extension of a front that crossed Northland on
Friday and Gisborne on Saturday. This trough is forecast to develop a
Low near 25S on Tuesday, as it crosses Niue. That Low should be taken by
a jetstream quickly S=southeastwards and may well bomb on Wednesday near
30S 160E as its trough crosses Southern Cooks. Standby for some squalls
from this passing trough.


SUBTROPICS
A subtropical HIGH around 1024 hPa has formed in west Tasman Sea and is
about to go east along to 25 to 30S zone. There will be a bunching of
isobars on its northern side as it travels--- a squash zone ----for
Coral Sea and Vanuatu on Monday and Tuesday, for Fiji from Tuesday to
Thursday, for Samoa from Wednesday to Saturday, and for Southern Coos
from Thursday to Saturday. These are good days for windsurfing, not so
good for fishing.

TASMAN /NZ AREA
That High travelling east along 25/30S will bring a touch of spring-like
weather to NZ for the start of this week, but more cold air from the SW
will likely remind us of winter again from mid to late week.

For Monday and Tuesday, west to NW winds - shower sin the west and
milder dryness in the east. Ah, I can see daisies in the lawn at last.

On Wednesday a southerly is forecast to rip across Tasmania and cold air
entering the south Tasman should mould a low. This Low is forecast to
move northeastwards across North Island on Thursday and Friday, and then
(over the weekend as it moves off to east of NZ) it may deepen and
re-curve to the E then ESE as it goes past 180. It is followed by a
cold SE wind change that will linger near Gisborne on Saturday and
Sunday.

The following HIGH is NOT at 25S - it is more likely to be at 45S (so
breaking up the spring-like pattern for a while). It should bring a
period of light winds to Tasmania on Thursday and Friday, to South
Island on Saturday and Sunday, and to North Island on Monday 27th.

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 2009

BOBGRAM7 issued 12 July 2009

WEATHERGRAM
YOTREPS
Issued 12 July 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
Rather than a South Pacific Convergence Zone this week, there is, at
present, a straightforward tropical trough almost-north-to-south and
moving east across the South Pacific---it passed over New Caledonia last
Thursday/Friday and is over Fiji today - and should be over Tonga on
Monday, Niue on Tuesday, Southern Cooks on Wednesday and, in a weaker
and more drawn-out state, may cross the southern Islands of French
Polynesia FP on Thursday and Friday UTC.

This trough is forecast to be followed by a HIGH, zipping east along 30S
this week - the High should be south of New Caledonia on Monday, reach
180 on Tuesday, and be south of FP on Saturday.

As the trough approaches a place, winds turn northerly and become fitful
and the weather becomes squally; and following the trough are southerly
winds and clearing weather.

This trough is only moving along places south of 15S---further north are
healthy trade winds over the entire Southwest pacific. But these trade
winds weaken when the trough travels by to the south, and then
strengthen when the following high zips by to the south.

Another tropical trough is expected to form over the Coral
Sea/Vanuatu/New Caledonia area on Thursday and maybe reach Fiji on
Saturday and Tonga on Sunday 19 July. There seems to be a rhythm here,
but the following high is likely to behave differently for this week's
high, so don't get too settled into this pattern.

TASMAN /NZ AREA
Not much rhythm in the Tasman.

This weekend's low was formed by interaction of a jet stream and a
tropical trough - it brought damaging wind/rain to Northland last night,
but a dense surface-hugging layer downwind of the Coromandels stopped
the southeast gales from reaching sea-level at Auckland harbour.

Next front should roll in from the south, reaching South Island late of
Tuesday and eastern North Island on Wednesday.

Then a trough should linger over the North Island on Thursday. On
Friday, this is likely to join forces with that tropical trough already
mentioned over New Caledonia/Vanuatu and form a new LOW to north of NZ.
This new LOW should swing ESE past northern NZ on Late Friday/Saturday,
deepening as it goes... Avoid.

New High is likely to cross south of Tasmania on Thursday/Friday and
move into Tasman Sea on 18/19 July weekend, strengthening the trade
winds in the Coral Sea. Avoid the squash zone between this high and the
low over northern 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

04 July 2009

BOBGRAM7 issued 5 July 2009

WEATHERGRAM
YOTREPS
Issued 05 July 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 active at the northern
extend of its range between 10 and 15S especially between Papua New
Guinea and Solomons, between Tuvalu and Tokelau, and between Tuamotu and
Marquesas.

Having the SPCZ slightly further north than normal is a sign consistent
with EL NINO, and indeed the sea surface and sub-surface temperatures
near Galapagos have been on a warming trend for the past month or so and
if this continues the tropical pacific ocean will be into an El Nino
pattern by August. The Atmosphere is lagging behind the ocean, but
maybe this northern SPCZ is a sign of things to come. The Monsoon is
about a week late over inland India and delivering around 75% of its
normal rain - these sort of events are usually associated with an El
Nino, but it is not possible to jump to conclusions yet. There has been
a noticeable DROP in speed in the easterly winds between Galapagos and
Marquesas over the last week or so, and this trend is expected to
continue.

Anyway the SPCZ is, this coming week, expected to drift SOUTH in the
northeastern Coral Sea, and spread into the region between Vanuatu and
Fiji.
According to the EC model a trough or LOW may form around New Caledonia
on Friday 10 July and then wander southeastwards across the Kermadecs
area on the weekend of 11-12 July. Keep a watch on this.


SUBTROPICAL RIDGE
Weak ridge near 20S in the French Polynesian longitudes is expected to
fade away by Wednesday.

The westerly flow that dominated between Lord Howe and Kermadecs last
week is starting to ease now - this zone with have a subtropical ridge
on 7 July and then the trade winds will shift southwards.

The HIGH in the Australia Bight is expected to finally cross the Tasman
Sea on Wednesday 8 July and NZ on Thu/Fri 9/10 July. Avoid the squash
zone that will form sandwiched between the northern side of this HIGH
and that trough forming over New Caledonia (8-13 July).

TASMAN /NZ AREA
It has been unsettled over NZ lately, with occluded conditions bringing
snow to the south and cold pools bringing thundery showers to the north.

Forecast maps have one more trough/Low to cross the northern areas on or
around early Wednesday followed by a final burst of southerly showers,
and then that should be the end of this period of unsettled weather, and
the start of a period of settled weather lasting into the first weekend
of the School holidays (ENJOY) but there may be lingering southerly cold
rain/snow on the Gisborne mountains on Thursday/Friday.

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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