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

25 September 2010

BOBGRAM 7 issued 26 Sep 2010

WEATHERGRAM
YOTREPS
Issued 26 September 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.

LA NINA
SOI (90 day) is now 2.02, up 9% from last week, now over 2 standard
deviations from its mean so La Nina is just getting stronger.
During October I'll explain what this means for coming migration/
cyclone season.

TROPICS
South Pacific Convergence zone is showing signs of hugging 10S... This
seems to happen around any equinox and so is likely to stay this way a
while. However there are signs of it spreading southwards in the Coral
Sea.

A pulse of extra convection is making its way eastwards from Indonesia /
North Australia into the North Tasman Sea, assisted by a subtropical jet
Stream --- this is producing cloud at high and mid levels and these
should thicken into a surface trough that is likely to cross the North
Tasman Sea on Wed/Thu 29/30 Sep and fade over Kermadecs around Sat/Sun
2/3 Oct.

A similar trough may come and go around Southern Cooks during the week.

SUBTROPICAL RIDGE: STR.
High 1026+ between Northland and Kermadecs at present (26 Sep) is
expected to move slowly SE to 38S and get south of French Polynesia by
the 2/3 Oct weekend and intensify to 1038+. This will strengthen the
trade winds on its northern side into a yucky squash zone. So if you
intend puddle jumping from Tahiti westwards this week, set off early
before the squash zone arrives.

Next High is crossing Australia at present and should move across the
Tasman Sea just as a weak 1025 ridge along 34S from Thu 30 Sep to Sun 2
Oct, and then be reinforced and get to a 1032 Cell and wander along 40S
from 2 to 3 Oct and cross central NZ 3-4 Oct. A welcome change from
past few weeks. Should be good for anyone wanting to come to NZ so long
as you arrange arrival before the next trough strengthens in the Tasman
Sea, and that, at this stage, is looking to be around 7 Oct. Also,
there is likely to be a squash zone building over Tonga/Kermadecs from 3
Oct.


NZ AREA
Monday 27 Sep : NW strengthening
Tuesday 28 Sep : Front embedded in NW flow.
Wednesday 29 Sep : Front stalls over North Island, Low whizzes past
Southland.
Thursday 30 Sep : Front lingers North Island, SW flow establishes
elsewhere.
Friday 1 Oct : Southwest flow ahead of a High.
Saturday 2 Oct : Dying southwest flow ahead of a High.
Sunday 3 Oct: Lighter variable winds around High in central/E
Tasman.
So, at this stage looking OK for the annual CANANZ Kowhai cruise in
Hauraki Gulf.


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

19 September 2010

BOBGRAM7 issued 19 sep 2010

WEATHERGRAM
YOTREPS
Issued 19 September 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.

Tonight's write up is a bit later than normal as I'm just now back in Auckland after attending NZ Coastguard national Conference... they are a great bunch indeed!

LA NINA
SOI (90 day) is now 1.85, up 0.11 from last week, so La Nina is just getting stronger.

TROPICS
South Pacific Convergence zone is beginning to return to normal now, showing up and bubbly convection over Papua New Guinea and Coral Sea, and also in an extending line from NW of Samoa across to French Polynesia. It is expected to hold this position this week.

There is a conveyer belt/upper trough over inland Queenstown and this is expected to move out to the Tasman Sea on Mon 20 Sep. It should then fade, but its jetstream is likely to feed some more convection in the Coral Sea, worth avoiding.

SUBTROPICAL RIDGE: STR/ NZ.
It is interesting to note that the parameter I'm using to measure the strength of the Polar Vortex went negative on Thu 16 Sep-That's when the large High I mentioned last week came into play and intensified over the entire Australian Bight. This High also had a Southern Ocean Low to help form an eggbeater onto Tasmania and into the Tasman Sea and onto NZ.

Tasmania took the brunt last Thursday /Friday, then, during the weekend, squally showers brought power cuts to Wellington and Auckland, snow that collapsed roofs in Invercargill, and slips /flooding that caused clusters of road closures. As is the nature of the beast with squalls, not everywhere got hit, and the damaged areas were rather random. Swells in mid Tasman got to an estimated 8 metres last Thursday and peaked on west and south coasts of NZ on Friday night

It's not over yet, there's another couple of shovel-full of cold air for NZ, especially on Tuesday 21 and Wednesday 22 Sep. Another 8 metre burst should roll across southern NZ seas on wed 20 to 22 Sep and reach Chathams around Fri 24 Sep, but the swells in the Tasman Sea are expected to ease to more normal levels by Wed 22 Sep.

That high in the Aussie Bight has not ridged all the way to Antarctic ice-shelf so this isn't a classic polar blast, really it's a wintry entrée to the equinox. Around the equinox, the air along the Antarctic fringe is just starting to get some sunlight after 6 dark months--- it is at the coldest it ever gets , and the sun tends to stir this cold air so that, when encouraged by surrounding pressure systems such as a passing Southern Ocean Low, it gets dislodged northwards. In this case, the shovelling is continuing to be done by that Aussie Bight High.

However, that Aussie Bight high is too far south for the STR at this time of the year--- even in a La Nina year (which twigs the STR to the south), the correct latitide for the STR over Australia around the equinox is about 30-35S , sort of along the South Coast. I think this high got knocked south by that upper trough/conveyer belt that has been over North and Central Aussie over the last few days. SO I agree with the models, and they all take this Aussie Bight high and, slowly, shift it north to 30/35S by Thu 23/Fri 24 Sep so that some high cells ooze across the North Tasman Sea from Thu 23 Sep and over North Island on the 25/26 weekend.

Naturally the strong and sometimes squally westerly winds of the roaring 40s and equinoctial gales will continue unabated to south of this STR line, and reasonably useful trade winds are continuing to north of the STR line.

Taking into account the expecting easing of well in the North Tasman sea by Wed and then the high that is expected to cross the North Tasman and Northland area next weekend, it looks to be a reasonable time to sail to NZ this week.

An added advantage during the next week or so is the full moon. This peaks on Thu 23 sep - and that's the date of this years Sep equinox.


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 September 2010

BOBGRAM7 ISSUED 12 Sep 2010

WEATHERGRAM
YOTREPS
Issued 12 September 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.

LA NINA
I've been asked to supply some details about the strength of this La Nina... The Oceanic Nino Index ONI, which measures the difference from normal of Sea temperatures in the target area of the eastern equatorial Pacific (Steep) now has a 90 day funning mean of -0.6 == roughly the sea is on average that much cooler than its norm. The Atmospheric Southern Oscillation index SOI, taken from the pressure difference between Tahiti and Darwin, has a 90 day running mean of +1.64 (standard deviations). Both indicate moderate to strong La Nina conditions and the ocean and atmosphere are coupled in this.

TROPICS
South Pacific Convergence zone is beginning to return to normal now , showing up as bubbly convection over Papua New Guinea and Solomons, and as a shear zone of vertical convergence (a layer of trade winds near the surface topped by some vigorous W to SW winds) around Tokelau/Samoa to Tonga/Niue to Southern cooks/ Tuamotu.

The upper trough the marks the boundary of this shear zone is expected to drop to the surface around the Southern cooks/ Tahiti on Tue/Wed 14/15 Sep and then to move south-maybe-Southwest across the Southern Cooks Thu 16 to Sat 18 Sep. Anyone planning the west jump from Tahiti should take this into account - maybe delay a while.

The northern extension of a mid-latitude front that moved off North Island of NZ today is affecting New Caledonia tonight Sun 12 Sep, and should fade away over Kermadecs around Tue/Wed 14/15 Sep.

SUBTROPICAL RIDGE: STR
A BFH big fat high which is well supported aloft continues to dominate proceedings. Its central pressure is around 1038 tonight Sun 12 Sep near 36S 155W and should slowly wander east and ease to be around 1028 at 31S 133W this time next week 19 Sep. So the squash zone of enhanced easterly winds on its northern side is bringing vigorous easterly winds and rough seas between Tahiti and Niue - at least until that trough drops down around Tue 14 Sep, and even after that it will take a while for things to settle down.

Not the best week then for venturing westwards from Tahiti.

Next high cell in the subtropical ridge doesn't have much vertical support - it is budding off eastern Aussie now and will cross the northern Tasman Sea on Mon 13/Tue 14 Sept and fade over Kermadecs on wed 15/Thu 16 Sep.

The high after that is expected to wander east across the Aussie bight along 40/45S on Sat/Sun 18/19 Sep.. shovelling cold air from 55S to Tasmania .. not quite a polar blast but it'll be a wintery entrée to the equinox for SE Aussie.

TASMAN/NZ
A brief break on Monday and then another front on Tuesday 14 Sep UTC, with some heavy rain for western districts, and followed by a cold SW in southern districts.

After another brief break late Wednesday and then some roaring westerlies of spring, enhanced by a deep low in the southern ocean -- these should reach NZ for Thursday 16 and much of Friday 17 Sep, followed by cold SW flow on Sat/Sun 18/19 Sep.

So a voyage from Tonga to NZ departing early this week may encounter a weakening trough/ridge couplet around 30S and then some strong west to SW winds over Northland around 20 Sep, reasonably adventurous.


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 September 2010

BOBGRAM7 issued 5 sep 2010

WEATHERGRAM
YOTREPS
Issued 05 September 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
There is a strong Walker/Hadley cell combo across the Pacific equator,
with rising air strongest between Malaysia and Taiwan and sinking air
strongest over Fiji to French Polynesia.

The South Pacific Convergence zone SPCZ is looking normal along 10S
across Papua New Guinea/ Solomons, but fades off over Tuvalu and to
north of Samoa. It is expected to extend east at times this week as far
as Suwarrow.

Passing troughs in the mid-latitudes have been extending sufficiently
northwards to affect the trade winds as far as 15S. One of these
brought some welcome rain to Fiji mid last week (not enough to have much
impact on their dominating dry spell). This has faded somewhat is should
wander east across Southern Cooks on Mon 6 Sep and southern parts of
French Polynesia on Tue 7 and Wed 8 Sep. It is followed by a period of
strong trade winds with a bumpy south to southwest cross-swell. The
next such trough is likely to cross the North Tasman Sea and reach New
Caledonia on Sat 11 Sep.

SUBTROPICAL RIDGE: STR
A big fat high with central pressure over 1030 is moving east along 30S.
It has a squash zone of enhanced trade winds on its northern side over
the next few days. Tonight, Sun 5 sep it is over the dateline.
On Thu 9 Sep it should have wandered east as far as 160W, to south of
Southern Cooks, where it will be reinforced by another High wandering
along 35/40South and the combo high should get to way south of French
Polynesia on the Sat/Sun 11/12 Sep weekend.
Yes, this high is enjoying some of that curtain of sinking air referred
to above in the combo Walker/Hadley cell combo.

TASMAN/NZ
The Tasman Sea/NZ area is slowing moving into a long wave ridge area
after being under the influence of an upper trough for weeks. Even so
there is one more low to come ... This is the one that flooded Victoria
late last week and on Saturday (some places getting heaviest rain in 50
years). Its Northwester and front is crossing most of NZ tonight and on
Mon 6 Sep - heavy rain for Southern Alps and gusty east of the divide.

The low itself should follow and is expected to split into two across
central NZ on Tue 7 Sep--- that will allow a cold southerly to possibly
bring rain/sleet/snow to southern NZ on Monday night to Tuesday night,
and may be some heavy rain to Bay of Plenty during Tuesday.

Thanks to the new long wave ridge, these lows are likely to fade on Wed
8 Sep as a new HIGH crosses southern NZ, but this does bring onshore
flow and rain to eastern North Island.

On Thu/Fri 9/10 Sep, as that new High moves off, a northerly flow is
likely to develop over NZ, but there may be a trough over the North
Island. Then the forecast is for a front on Sat 11 Sep followed by a
westerly flow on Sun 12 Sep. The outlook next week 13-18 Sep is for
West to southwest winds over Northland, so those days may not be good
days for sailing towards 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

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