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

27 September 2008

BOBGRAM7 issued 28 Sep 2008

WEATHERGRAM
YOTREPS
Issued 28 Sep 2008 NZDT
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 below are given in UTC unless otherwise stated.

NIWA in NZ have issued their annual idea of the coming cyclone season in
the South West Pacific. It is likely to remain in neutral territory -
neither a La Nina nor an El Nino. Around the South Pacific previous
neutral cyclone seasons have given an average number of cyclones -
around 8 to 10. The coming season is likely to bring a REDUCED RISK of
cyclones around Northern cooks and French Polynesia. For NZ there is an
INCREASED RISK - by the time such systems reach NZ they are no longer
Tropical Cyclones but some still pack a punch. Memorable cyclones in NZ
such as FERGUS and DREANA have occurred in neutral years.

No sign of any tropical cyclones in the South pacific during the coming
week. There are some near equatorial westerly winds to north of Papua
New Guinea, and these may be heading for Micronesia, so take care there.


TROPICS The South pacific Convergence Zone, SPCZ, is having a brief
break over the Coral Sea but should reform there this week. There is
one branch over Fiji/Tonga at present and it is being propelled
eastwards by the upper westerly winds. It should fade on Tue/Wed UTC
from Samoa to Southern Cooks, but will likely trigger a LOW for form
southeast of the southern Cooks , near 30S 150W , on Thu/Fri/Sat.

A mid-latitude Low crossing NZ on Monday is likely to link into the
tropics on Tue forming a trough that crosses Fiji and Tonga on Wed, Niue
on Thursday and Samoa to Southern Cooks on Friday.


SUBTROPICS
A migratory HIGH is expected to arrive in the Tasman Sea on Wednesday
and then move east along 30S finally reaching 150W, south of French
Polynesia, the following Tuesday. These HIGHS enhance the SE trade
winds on their northern side as the pass by. In Fiji they are called
Bogi Walu (eight nights) because the enhanced trade winds give a week of
wet nights to Suva. Have fun riding these winds, going westwards.


TASMAN/NZ
Front crossing NZ on Monday should be followed by a strong SW flow on
Tuesday, and then a typical spring westerly flow until Friday. Next
front, preceded by strong NW winds, should cross the South Island on Sat
and ease over the North Island 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

21 September 2008

BOBGRAM7 issued 21 Sep 2008

WEATHERGRAM
YOTREPS
Issued 21 Sep 2008 NZST
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 below are given in UTC unless otherwise stated.

The overhead sun crosses the equator at 1544Utc on Monday 22 Sep (around
4am Tuesday in NZ) - its about a day after the usual 21st sep because of
the leap day we had this year. Around the equinox we sometimes notice a
twinning of the ITCZ with the South Pacific Convergence Zone SPCZ, and a
branch of the SPCZ intensifies along 10South from about Tuvalu /Tokelau
to Northern Cooks and sometimes to Marquesas-no sign of this yet but it
may appear.

TROPICS Easterly winds to north and southeast winds to south of the
SPCZ. The winds are strong within the SPCZ over northern parts of Coral
Sea (avoid). And the SPCZ is active from there to North of Vanuatu to
Rotuma/Wallis/Futuna to Tonga to Niue. It is expected to travel south
this week, reaching Vanuatu/Fiji on Mon/Tue then fading away on Wed/Thu.
In this southern excursion a small LOW may form in Coral Sea on Fri/Sat
then head south into Tasman Sea early next week bringing wet NE winds to
New Caledonia on Fri/Sat. There will be a squash zone on the south side
of this LOW so take care if travelling between Vanuatu/New Caledonia and
Australia.

SUBTROPICS
For those of you from the Northern Hemisphere-welcome to the South
pacific with its migratory HIGHS.

HIGH between Norfolk and Raoul on Sunday is moving slowly along 33S and
should fade east of 180 by Thu.

HIGH on Aussie Coast on Sun should travel along 35S reaching Tasman Sea
wed/Thu and Northland on Fri/Sat and tongue out a ridge along 33S to
south of French Polynesia early next week.

On the northern side of these HIGHs there will be an enhancement of the
trade winds--- mainly between 25 and 20S.


TASMAN/NZ
Between the HIGHS are troughs and fronts, but no LOWs this week. So it
is a typical spring pattern.

First front is fading over North Island Sun/Mon.

Second front should cross South Island on Tue, preceded by strong NW
flow (especially for eastern areas) and accompanied by heavy rain in
western areas. Then fade over North Island on Wed.

Third front should reach South Island on Friday and hang about the North
Island on Saturday and Sunday.

Try and plan your Northland arrival/departure dates to be on Tuesday
/Thursday -Friday; in-between days rather than frontal days.


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

13 September 2008

BOBGRAM7 issued Sunday 14 Sep 2008

WEATHERGRAM
YOTREPS
Issued 14 Sep 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 We are now approaching the equinox -it will be on 23 Sep and
then the days in the southern Hemisphere will be longer than the nights
and soon after that I will start to give out my ideas for the coming
South pacific cyclone season. Things are still in the "non-cyclone"
season mode with trade wind easterlies over the whole area and no sign
yet of any equatorial westerlies.

South Pacific Convergence Zone SPCZ is patchy in coverage... It is
strongest about Papua New Guinea and Solomons... Another burst of
activity at times between Tokelau and Samoa, and occasionally over north
Tonga, then it sort of stretches from about Niue to Southern Cooks and
southern parts of French Polynesia. Not much change is expected in
these areas during this week.

The main thing to consider, if Island-Hopping this week, is the
subtropical trough currently crossing Fiji. Its rain band should reach
Tonga on Monday UTC and Niue on Tuesday and then fade over Southern
Cooks by Thursday. It goes eastwards even in the trade wind belt north
of 25S by virtue of the westerly winds it has about and above its
clouds. The surface easterly winds turn NE and ease as the trough
approaches , there are some squally tropical showers as the trough
crosses and then the winds fill in again as steady SE winds as the
trough moves off to the east. If you are watching this trough's
footprint by means of Grib winds do NOT be led astray but the light
winds it shows with this trough - The computers cannot resolve the
tropical squalls and each will be able to produce 30 knots in 30 minute
bursts. SO if you are intending to sail across this trough, then go on
squall watch.


SUBTROPICS /TONGA and FIJI
A LOW is developing in this trough near Kermadecs today / Sun 14 Sep /
and should wander off to the south. SW to Southerly winds on the
western side of this LOW are likely to affect Fiji and Tonga until Wed
17 Sep so best to wait for these to swing back to SE winds before
sailing west in this area.


TASMAN/NZ
Spring grass growth should flush in NZ this week.

A trough and front crossing Tasmania today, 14 Sep, should cross South
Tasman Sea on Mon and Tue, proceeded by a NW flow that will be wet for
Southern Alps and dry and warm further east over the South Island. The
front should cross NZ on Wed followed by a SW flow on Thurs and Friday.
Avoid this front.

A HIGH moving east across Australia is expected to follow that front and
cross the Tasman Sea on Wed/Thu /Fri along 35S and the cross central NZ
on Sat Sun 20/21 Sep. This HIGH should be followed by another NW flow
then another front early next week - a typical spring pattern


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

06 September 2008

BOBGRAM7 issued 7 Sep 2008

WEATHERGRAM
YOTREPS
Issued 7 Sep 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 AND SUBTROPICS
South Pacific Convergence Zone SPCZ is steady in strength and patchy in
coverage... It is strongest about Papua New Guinea and Solomons...
Another burst of activity at times between Tokelau and Samoa, and
occasionally over north Tonga, then it sort of stretches from about
Suvarov to midway between Southern Cooks and French Polynesia. There
has been a squash zone between the SPCZ and a Big fat high recently-this
squash zone still affects Southern Cooks to Niue with strong trade winds
and rough seas.
Highlight of the coming week to look for is for a build up in activity
near Suvarov on wed 10 Sep UTC moving South onto Southern Cooks by sat
13 UTC, There will enhance that squash zone even further, with tropical
squalls on its north side and steep big swells on its south side, so I'd
say it IS NOT the week to sail from French Polynesia to Tonga, unless
you like riding a bucking bull.

When this system moves off to the south early next week, it may pave the
way for a Low to possibly form about or south of Niue then SW towards
Niue early next week. After that we may have a good weather for sailing
westward ho!

Rest of the South Pacific between Tonga and Australia is looking OK this
week-a gentle subtropical ridge along 30S and average trade winds mainly
along 20S. Computers are picking some kind a drawn out surface trough to
form south of New Caledonia and extend towards Kermadecs by Friday---
probably caused by an upper Jet Stream and - this trough is forecast to
cross Fiji and Tonga on 14 and 15 Sep utc, but not much in it until it
possibly forms that Low south of Niue early next week.


NZ/TASMAN AREA
In a reversal of pattern back to winter NZ had a ridge over the weekend
(frosty in places) and is following up with a LOW from the Tasman Sea
this week. The frontal zone is likely to cross the North Island on
Monday and may linger across the south end until Thursday and the Low
itself slowly works its way in pieces across central NZ. On the north
side of this LOW, over the North Island, will likely be squally
westerlies from Tuesday til Thursday. A Southwest flow should clear the
weather over NZ on Friday and then the outlook is for another ridge of
light winds for next weekend.

Conditions should be OK for sailing from NZ to Fiji/Tonga after the
Monday front, but wait til Friday if sailing NZ/New Caledonia.
Best day this week to sail Tonga to NZ would be, depending on your
cruising speed, around Mon/tue utc -so that you encounter that trough at
around 30S on Fri 12th, and time you arrival in NZ with a SE flow
maintained by that weekend ridge.

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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