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

24 April 2011

BOBGRAM7 issued 24 April

WEATHERGRAM
YOTREPS
Issued 24 April 2011
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
La NINA is still active in the atmosphere.
The Southern Oscillation Index 30day running mean has been on a roll
upwards : from a low of 1.85 on 25 March to a high of 2.55 on 23 April.

That recent pulse of active convection, known as a MJO period, which
brought unsettled weather to North Australia earlier this month, faded
away upon reaching the Coral Sea last week. This was contrary to our
thinking a week ago, showing that the atmosphere is always capable of
nice surprises.

This suggests then that South Pacific is going through a relatively
settled period for the coming week.

The International Tropical Convergence Zone ITCZ lives near 3 to 5
degrees North across the Pacific, and there are signs of a "mirror" to
this near 5S, especially along 100 to 180W. This tends to happen
around the equinox, but that was a month ago so the mirror zone should
break up over next few weeks and then it should be ok to puddle-jump
travel from Galapagos to Maquesas. No rush :)

South Pacific Convergence Zone SPCZ is near the southern edge of its
realm, across the Coral Sea where it is entwined with trough in the
Tasman Sea. Another branch of SPCZ stretches from Vanuatu across Fiji
and Tonga to Southern Cooks, coming and going.

SUBTROPICAL RIDGE STR:
The STR has been knocked south of normal as well.
One High starts this week near 45S 150W, perfectly placed to develop a
LOW on its left shoulder near 30S 160W - close enough to bring a
westerly flow to Southern Cooks and Australs for a few days. There is
an intense squash zone between Low and High to avoid, and the Low is
expected to go SSE and the High goes east, so that the winds in this
squash zone are likely to start out as being from the east and end up as
being from the north.

Of more interest in the NZ area is another HIGH that starts the week at
43S to east of Tasmania. This HIGH is stuck by troughs trafficking over
NZ and finally should be able to swing around southern NZ on Thu 28
April, and then expand over Chatham Islands area on Fri 29 April and
wander further east next week.

NZ/TASMAN SEA AREA
With low pressures to the north and high pressures to the south, an
easterly flow is expected to linger over northern NZ this week. One
rather shallow low is likely to cross Northland on Monday and the marine
part of Bay of Plenty on Tuesday causing an intense squash zone of
southeast winds over central NZ. This low should fade on Wednesday and
Thursday, but the squash zone is likely to build a heavy swell on
Gisborne/Hawke's Bay Coast by Thursday 28 April. Avoid.
On Thursday and Friday another low should deepen-this time in Tasman Sea
near Lord Howe Island --- this Low should move SSE and may cross
southern NZ around Sun 1 May, but there is some uncertainty at this
stage on this track so please update.

These troughs are expected to bring unsettled weather between Northland
and the tropics this week and much of next week, so it is NOT a good
time to plan a voyage north just yet. Patience will be rewarded in May.


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

17 April 2011

BOBGRAM7 issued 17 April 2011

WEATHERGRAM
YOTREPS
Issued 17 April 2011
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
La NINA is still active in the atmosphere. The Southern Oscillation
Index 30day running mean has been on a roller coaster ride: from a low
of 1.85 on 25 March to a high of 2.51 on 9 April, and, by 15 April,
relaxed 2.40.

Recent pulse of active convection, known as a MJO period, has been
lowering the isobars in North Australia and has helped form TC ERROL far
to west northwest of Darwin; this system is expected to stay offshore
and go west.

In our part of the world the South Pacific Convergence Zone has been
stretching from North Australia across the Coral Sea and Vanuatu to
south of Fiji and then off to the southeast. Over the weekend a
Tropical depression has formed about and just south of New Caledonia,
taking with it a lot of the activity that was in the Coral Sea.

Fiji Met has a gale warning on the W /NW winds associated with this
system at present. It is already showing signs of fading as it goes SE
--- some of its upper vorticity is expected to be consumed by the trough
that is now getting east of NZ, and help deepen a large LOW to BELOW 980
near 45S 170W on Mon to wed 18 to 20 April. Avoid.

Not much else happening in tropics this week--- but low pressure in
Coral Sea and high pressure in Tasman Sea are likely to keep fresh trade
winds along 20S from New Caledonia to Queensland.

SUBTROPICAL RIDGE:
New autumnal HIGH is crossing Tasman Sea along 37S on Mon 18 April and
then slowly across North Island from Tuesday to Thursday 19 to 21 April.


Next HIGH is expected to form at 45S in South Tasman Sea on Sun 24 April
and then to swing around South Island on Mon 25 (ANZAC Day) and build
over Chatham Islands on Tue 26.

On the northern side of both these Highs, there are likely to be squash
zones of enhanced easterly winds worth avoiding.

NZ AREA
A cold southerly flow dominates eastern areas tonight and Monday, with a
marine gale + heavy swells. Avoid

For this year's Auckland to Tauranga Yacht race starting Thursday
morning a passing High is likely to give light winds--- as that High
moves off on Good Friday, a northerly breeze may help.

Then the next trough should cross NZ on Sat/Sun 23/24 April as a weak
transition southerly change moving from south to north.

It should be Ok for anyone thinking of sailing north from Northland - to
do so Tue to Fri, so long as they route through the subtropical squash
zones properly. Over Easter and for much of the last week of April, NE
winds speak against a smooth voyage.

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 April 2011

BOBGRAM7 issued 10 April 2011

WEATHERGRAM
YOTREPS
Issued 10 April 2011
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.

Now that the equinox has faded away, the next thing to watch in this new
'meteorological' year is the jetstream over Tibet. The subtropical jet
in the northern hemisphere is now near the Himalayas so no one will be
climbing Everest yet- but, by mid May, this jet should get further
north and a brief Everest climbing season should occur before the Asian
monsoon kicks in.

TROPICS
La NINA is making a rebound as isobars lower over North Australia. The
Southern Oscillation Index 30day running mean moved from a low of 1.85
on 25 March to 2.51 on 9 April.

In our part of the world the South Pacific Convergence Zone is taking on
a position towards the south end of its range and is defined in a long
stretch from northern Australia across the Coral Sea and Southern
Vanuatu to south of Fiji and then off to the southeast. There is a
tropical depression within this system - tonight it is east of Tanna and
west of Fiji. This system is expected to go SE and deepen to below
1000hPa by Wed 13 April when it is well off to SE of Raoul Island.
Avoid.

A Madden-Julian-Oscillation MJO has been activating convection over
northern Australia recently and is wandering into the west pacific. It
seems to be having more impact in the Northern hemisphere rather than
the Coral Sea.

SUBTROPICAL RIDGE:
A typical autumnal anticyclone is crossing central New Zealand and
should move off to east from Wed 13 April.
Another High is expected to wander over Tasmania on Fri/Sat 15/16 April
and then cross central NZ on Sun/Mon 17/18.

NZ AREA
Between the highs are troughs and lows. Low should deepen just northeast
of Tasmania on Tue 12 April and then weaken and stretch out into a
trough that is likely to move from SW to NE across NZ on Wed 13 to Fri
15 April, followed by a southerly change.

Anyone planning an early get-away from Northland to the tropics may
takeoff over next few days OK; so long they avoid that low from Tanna.
NE to NW flow and a passing trough make for bad days from Wed 13 to Fri
15 April. The SW to SE flow on Sat/Sun 16/17 April make for a few good
days, but these are likely to be quickly followed by an E/NE flow on 18
April onwards, so may as well wait for next window and forget taking off
this 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

02 April 2011

BOBGRAM7 issued 3 April 2011

WEATHERGRAM
YOTREPS
Issued 3 April 2011
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.

We live in interesting times with the weather patterns in transition. NZ
switch from Daylight (summer) time to Standard time today, and the weeks
after the March equinox seem to mark the beginning of a new
meteorological year to go along with the new financial year.

TROPICS
La NINA is making a rebound as isobars lower over North Australia. The
Southern Oscillation Index 30day running mean moved from 1.95 on 20
March to 1.85 on 25 March and back to 2.05 on 3 April.

The eyewall of TC Bune brought torrential rain and gale winds to Raoul
Island, see http://blog.metservice.com/2011/03/the-eye-of-the-storm/.
Then the system unwound and expanded as it rolled away to the southeast
over open sea.

The Intertropical convergence zone ITCZ is showing signs of doing an
equinoctial twining, due to the overhead sun--- one line along 4 to 5N
as normal and another line along 4 to 10S (not normal) all the way from
dateline to Galapagos. Anyone contemplating "puddle jumping" should
wait until at least May for this pattern to fade.

A Madden-Julian-Oscillation MJO has been activating convection over
northern Australia in the past week and is expected to wander into the
Coral Sea this week. This MJO has triggered a tropical depression about
the coast of NW Australia... and this system is expected to be able to
get back to sea next few days and feed off the warmth to possibly grow
into a Tropical cyclone that may make landfall west of Port Hedland on
Thu/Fri 7-8 April. Avoid.

South Pacific Convergence Zone SPCZ stretches from northern Australia
across Coral Sea, New Caledonia, and then to south of Fiji. A jetstream
extends high cloud to south of SPCZ as far as 25 to 30S. SPCZ
(north-edge) is expected to become activated by this MJO during the
coming week, so watch for tropical depressions to develop in Coral Sea
or about New Caledonia. Avoid the Coral Sea this week.

SUBTROPICAL RIDGE:
Two typical autumnal anticyclones to talk about:
HIGH 1 is 1030 tonight near 48S 170W and should wander east along this
latitude to 155W then be forced to take a more southern track by troughs
near 40S.

HIGH 2 is expected to cross Tasmania as 1030 on Tue 5 April then move
very slowly along 40S across the Tasman Sea, expanding to encompass
central NZ from Sun 10 April to mid-next week.

NZ AREA
Between the highs are troughs and lows. A double-barreled trough is
crossing NZ tonight/ Mon 4 April followed by a low deepening off eastern
North Island on Tue 5 April-this should produce southerlies over NZ that
may be cold and wet enough to bring a dusting of snow to Southern Alps.
The disturbed SW flow that follows all this should last until
Friday/Saturday as HIGH 2 approaches.

So we have a week with both a tropical cyclone and the word snow
mentioned - such is autumn.

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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