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

26 March 2011

BOBGRAM7 issued 27 March 2011

WEATHERGRAM
YOTREPS
Issued 27 March 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 weakening. The Southern Oscillation Index 30day running mean
moved from 1.95 on 20 March to 1.85 on 25 March.

Hmmmm, developments over the past week in Southwest Pacific came from
the east rather than the west (which was where I was looking). A
rejuvenation of the South Pacific Convergence Zone SPCZ spread westwards
along 15/20S from Tahiti to Fiji, and there it stalled and ganged
together on Thursday to form TC BUNE (in Fijian this is pronounced
MBU-NAY).

BUNE has since then been creeping southwards and feeding well on
tropically warm seas, ending slightly west of 180 (the anti-meridian).

This week the main SPCZ is likely to reform from Solomons to Tuvalu to
Tokelau to Northern Cooks, and it might move slowly south.

There is a lot of convection over northern Australia, and strengthening
trade winds over Queensland... There may be the development of a
tropical depression near NW Australia.

SUBTROPICAL RIDGE:
HIGH 1 is a typical La Nina high - wondering along 43S - on Sunday 27
March it is at 140W and moving off to the east.

HIGH 2 is expected to taking a middle route - crossing the Tasman Sea at
40S on Monday 28 March and then being diverted across South island and
along 45S as it works itself east around BUNE on Tuesday 29 March. An
interesting dance.

HIGH 3 is expected to be slightly further north, crossing Australian
Bight and peaking at 1030 on Wed/Thu 30/31 March, then New South Wales,
northern Tasman Sea/northern North Island on Friday/Sat 1/2 March at
1025 or less.

NZ AREA

A trough crossed NZ over the weekend a, leading in a SW flow fresh from
the southern ocean, along with some heavy swell, easing on Monday as
HIGH 2 approaches.

BUNE is now moving out of the tropics into cooler seas so can not
produce high enough clouds to keep its central ring going, but will take
a few days to unravel. That SW flow is expected to reach BUNE on
Tuesday and this may help expand its area of gales and rain. On Mon and
Tue 28/29 March it will dance with a passing anticyclone-they may waltz
or they may tango, we will not really know until it happens. One
scenario is for this to be a tango, moving the wind and rain close to
eastern North Island on Wed/Thu 30/31 March. There are many other
possible outcomes that keep the wind and rain offshore the mainland, so
if this affects you then get updates.

Easterly Swell from BUNE is likely to start reaching North Island
eastern coasts from Tuesday and peak on Wednesday.

Cold front is expected to reach South Island on Thu 31 March and North
Island on Friday 1 April; the NW flow ahead of this system should kick
remnants of BUNE away. The westerlies of the southern ocean are now
creeping steadily north.

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

20 March 2011

BOBGRAM7 issued 21 March 2011

WEATHERGRAM

YOTREPS

Issued 21 March 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.

 

I didn't get time to issue a regular weathergram yesterday == I was driving back to Auckland after attending Central District Fieldays over the past week. So here it is a day later than normal….

 

TROPICS

La NINA is still strong, and weakening. The Southern Oscillation Index 30day running mean moved from 2.17 on 11 March to 1.95 on 20 March.

 

There have been some signs of extra convection off the eastern sea-board if Australia, as we expect when a MJO moves into the Coral Sea --- Sydney had a downpour, for example.  However, nothing organised has appeared so far, and current thinking is that nothing is likely to become organised in the Coral Sea over the next week or so we seem to be in a holding pattern at this stage.

 

South Pacific Convergence Zone SPCZ is loosely draped from Solomons to Fiji and around Southern Cooks and French Polynesia.

 

SUBTROPICAL RIDGE:

HIGH 1 is typical of a La Nina summer high – wondering off to east of South Island along 45S and then heading NE to around 30S east of 160W.

 

HIGH 2 is actually one from the sub-polar regions,  today 21 March/Equinox it is at 50 to 55S to south of Tasmania.  It should wander to east of South Island on Wed 23 March, (World Meteorological day), preceded by a cold southerly, and then go east along 45S from Thu 24 to Sun 27 March, followed by a broad northerly flow.

 

HIGH 3 is expected to take a more northern route – crossing New South Wales on Mon 28 March and North Island on 1-2 April.  A sign that we are now in the equinoctial time of the year, and the roaring 40s are generally heading north.

 

There are useful trade winds on the northern side of all these Highs.

 

NZ Area

 

A sub-tropical low formed on the western shoulder of HIGH 1 and tonight / Tuesday is moving SE and sideswiping the NE of North Island.  It may deepen to below 1000 when east of southern Hawke’s Bay, bringing them wind and rain after a long dry spell - a good chance, too, for Wairarapa and maybe for Marlborough to have an autumn flush, a last chance for some grass growth.

 

A large sub polar low is expected to follow HIGH 2, with a minor front crossing NZ over the weekend - preceded by a strong N to NW flow on Friday/Sat 25/26 March and followed by disturbed windy and showery W/SW flow on Sunday 27 to Wed 30 March – something more like winter is on its way.

 

SO it may be an idea to avoid North Island/ Tasman Sea this week

           - Weather pattern here should settle by mid 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

12 March 2011

BOBGRAM7 issued 12 March 2011

WEATHERGRAM
YOTREPS
Issued 13 March 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.

This weekend my thoughts and feelings are with those displaced by the
Japan quake and Tsunami. Kia Kaha.

TROPICS
La NINA is still strong, and steady. The Southern Oscillation Index
30day running mean moved from 2.19 on 5 March to 2.17 on 11 March.

There are low pressures and persistent convective rain over much of
northern and northeastern Australia, consistent with a new Madden Julian
Oscillation MJO that is now moving into the Coral Sea. It seems to be
weakening as it moves east, and there have been good trade winds in the
Coral Sea for the past week, but the coming week is likely to see
changes--- weakening winds in Coral Sea and increasing convection.

South Pacific Convergence Zone SPCZ is loosely draped over Coral Sea and
Vanuatu, coming and going over Fiji and Tonga and at present to south of
Southern cooks. Another zone is establishing itself along 10S from
Tuvalu to east of Tokelau--- this branch is likely to act as a twin of
the ITCZ around the weeks of the equinox.

SUBTROPICAL RIDGE:
HIGH1 is above 1030 and at 40S to east of NZ today Sunday 13th and
should go east. On Tue 15 March it is expected to fade at 40S but
another cell is likely to form and take over at 45S- and this cell
should head NE to get to 40S 130E by Sun 20 March. As this system
wanders east we can expect enhanced trade winds on its northern side.

A couple of sub-tropical lows are likely to form on the western shoulder
of this High-one between Norfolk and Raoul Island on Wed 16 March and
another to southeast of Rapa on Fri 18 March- these lows are likely to
be shoved off to the south-southeast.


HIGH2 from then Australian Bight should cross Tasmania on Fri 18-Sat 19
March and The South Island on Sunday 20 /Monday 21 March. There should
be an increasing squash zone of east to northeast winds between HIGH2
and lowering pressures in the Coral Sea. Avoid.


NZ Area
A front is poised to move onto southern NZ on Monday 14 March and should
weaken and stall for a few days over South Island, finally fading out
over central NZ on or around Friday 18 March.

On Sat 19 March, another front from Southern Ocean is expected to bring
a wet and chilly wind change to eastern South Island. By then a broad
trough is likely to have developed in the central and north Tasman Sea
and should sideswipe northern NZ.

On Sun 20 that trough should move off the far north as the South
Island's southerly change moves onto eastern North Island (ahead of
HIGH2).


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

05 March 2011

BOBGRAM7 issued 6 March 2011

WEATHERGRAM
YOTREPS
Issued 6 March 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.

People in Christchurch have moved from rescue to recovery mode, remember
them at christchurchearthquakeappeal.govt.nz

TROPICS
La NINA is still strong, and steady. Its 30day running mean moved from
2.14 on 26th Feb to 2.19 on 5 March .

There are falling pressures and persistent convective rain over much of
northern Australia, consistent with a new MJO pulse that is expected to
move into the Coral Sea from late this week. Standby for another period
of active weather in SW Pacific next week and until after the Equinox.

South Pacific Convergence Zone SPCZ as returned to its normal position
and is basically sitting over the Coral sea and Northern Vanuatu, coming
and going over Fiji and Tonga and more consistent over Niue and to south
of Southern cooks.
Some BUT NOT ALL models are picking that a series of tropical
depressions may form on SPCZ especially around Vanuatu, Coral Sea and
Gulf of Carpentaria this week. Keep an eye out but do not get diverted
by model extrapolation just yet.
If you have internet keep an eye on http://bit.ly/ecoz for comparative
purposes.


SUBTROPICAL RIDGE:
We seem to be moving into the magical equinoctials already. With the
sun now close to being directly over the equator, the subtropical ridge
is doing a good job in the South Pacific of mixing cool air northwards
and warm air southwards-what we call more 'meridonal' rather than
'Zonal' patterns. Energy is starting to move from the planetary waves
(1 and 2) into the long waves, especially Wave 4. This does mean more
weather in terms of wind and rain between some of the individual high
cells of the STR.

This week in our area HIGH1 is tonight in the South Tasman Sea east of
Tasmanian. There is a squash zone of enhanced trade winds on the north
side of HIGH1 and south side of SPCZ--and Fiji Met have a gale warning
out on this zone for tonight/Monday. HIGH1 should wander east along
around 42 south and fade east of the south Island by Thu 10 March, and a
sub-cell is likely to cross the North Island on Fri 11 March and then
fade away.

HIGH2 is expected to cross Tasmania around Thu 10 March and then rotate
around south end of South Island by Sat 12 March and then get east of NZ
early next week - the Northerly flow on its "backside" is likely to open
a pathway to NZ for whatever may then be forming in the Coral Sea then.
More on this in the next weathergram.


NZ Area
We have had a trough cross NZ over the past few days-it has been
meridonal enough to bring 'southerly rain' to eastern North Island, the
first in about a month to do this, pleasing pastoral farmers and
displeasing apple-pickers.

A small low centre is expected to form within that trough tonight,
making for a day or so of southerly gale on the Hawke's Bay/Gisborne
Coast-good that the restart from Napier for last leg of Round North
Island Race will take off on Monday after this gale.

For remainder of this week there may be a southeast then easterly flow
over northern NZ (this may strengthen over the 12/13 March weekend) and
a disturbed westerly flow over southern NZ. Inbetween places may have
morning mist areas. A weak trough between HIGH1 and HIGH2 is expected
to fade over the South Island 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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