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

31 October 2010

BOBGRAM7 issued 31 Oct 2010

WEATHERGRAM
YOTREPS
Issued 31 October 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.

Welcome to the official start of the South pacific Cyclone Season.
Nothing expected this week. The South Pacific Convergence Zone SPCZ has
been going through an active period recently. It has cleared off Coral
Sea and New Caledonia but is still being strengthened by an upper trough
near Vanuatu and lies almost west-east between 12 and 15S from Rotuma
across Samoa and to Suwarrow, it then extends SE across Southern Cooks.
The upper trough is expected to cross Fiji on Mon 1 / Tue 2 Nov where it
may form a surface low for a time, and then it should weak and it moves
over Tong on Wed 3 Nov. This will likely encourage the SPCZ to come
southwards this week.

SUBTROPICAL RIDGE: STR.
We did indeed get a High well over 1044 east of NZ this weekend. It is
now weakening and migrating NE to east of NZ and that area should become
more zonal and high index by the end of this week with just a ridge left
over along 30S and a disturbed westerly flow further south.

The next High is wandering along 45S across the Australian Bight. Its
nose end is shovelling cold air north, but the showers are expected to
mostly clear in time for the Melbourne Cup on Tuesday. Anyway, this
High should edge NE wards, crossing Tasmania / 40s on Fri 5 Nov, and
tonguing itself onto North island /35S on the 6-7Nov Weekend, then
dividing into two parts on 8-9 Nov - one part moving off to east of NZ
along 40S, and the other stalled in central Tasman Sea at 35S. There
are squash zones of enhanced easterly winds on the north side of this
high.

NZ AREA
Between those highs, there is a replacement trough. This is currently
over Tasmania and, at present, includes a rapidly intensifying low
centre that is rushing off to the South. It's the same trough which
deluged the Melbourne derby yesterday. Anyway it is expected to cross
NZ on Thu 4 Nov, preceded by a strong NW flow over central and southern
areas, and followed by a crisp cool SW with showers that will last into
the start of the fireworks on Fri 5 Nov.

TASMAN SEA
As indicated in my previous weathergram, on 5 Nov a LOW is expected to
form off Queensland coast, it should move SE and deepen, reaching peak
intensity SW of Norfolk on 7 Nov, and then it should slow down and
wobble near Northland on 8-9 Nov - then wander to the east to north of
NZ by around 10 Nov (uncertain). Strong squash zone of enhanced winds
between this LOW and that HIGH to the South especially from 9 to 12 Nov.

SAILING TO NZ???
The High east of NZ will maintain an easterly flow until 4 Nov.
That trough crossing Northland on 5 Nov is followed by a period of SW
winds, but for yachts coming from Fiji or Tonga these winds will likely
only briefly get to 30S and so are just a minor irritation (they will
also bring SW swell - reducing the comfort factor).

Of more interest is that LOW from the Queensland Coast. The computer
models are not yet is agreement as to what it may do, so updates will be
needed. Avoid this LOW neat Norfolk especially on 6-7 Nov as that's
likely to be when it peaks.

As for the Northland coast; most likely (not yet certain) idea is that
LOW will bring a period of enhanced ENE on Mon Tue 8-9 Nov and then
enhanced SE to S winds on 10-11-12 Nov. This complicates all voyages
that may be planning to arrive in NZ in the period from 9 to 12 NOV.
The impact of this Low should clear away so that a new weather window
may start opening for good voyage from New Caledonia /Fiji/ Tonga to NZ
after 13th Nov.

A further complication is that I will be UNAVAILABLE during 9-12 Nov, as
I'll be attending to a MetService display at the Canterbury Show. So,
if you want an update for your arrival, maybe it's a good idea to avoid
arriving on those 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

24 October 2010

BOBGRAM7 issued 24 Oct 2010

WEATHERGRAM
YOTREPS
Issued 24 October 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.

When the moon fades into its last quarter there may be a rising of
Pololo (or, in Fijian, Balolo). These coral worms choose this time of
the year to spawn. Ask the locals about them, may be fun to watch and
perhaps catch and cook.

LA NINA and the coming cyclone season
First, a recap on our weather zones: The weather engine starts with
energy from the sun. The warmest seas are near the equator and sun on
them causes evaporation which rises to form the Intertropical
convergence zone. Air rises as far as the tropopause and then travels
pole-wards. In the southern hemisphere a lot of this air sinks around
30S and returns along the surface as trade winds back to the equator---
this is the Hadley cell. The zone of sinking air is called the
subtropical ridge. Further south are the westerly winds of the roaring
40s. These weather zones move about, causing seasons. By the time we
get to the longest day, around 22 Dec, the subtropical ridge is usually
"following the sun" southwards and gets to around 40S. This is all part
of the annual cycle.

The second strongest cycle for seasonal weather is the ENSO = El Nino
Southern Oscillation (there are others). When the seas along the
equatorial Pacific Ocean are cooler than normal, as the are now, we call
it a La Nina episode. The weather engine turns over more slowly. The
peak upward motions in the Pacific occur over the Australian side rather
than the Peru side. This has the impact of, in the Southern Hemisphere,
encouraging the weather zones to go further south than normal. Already
the "anticyclones of summer" are reaching northern NZ, and there are
signs of an early start to the wet season in northern Australia. This
also tugs the South Pacific Convergence zone SPCZ to the south and west.

When we look at the sea surface temps, the Oceanic Nino Index ONI for
Sep was -1.0 and when we look at the weather maps the SOI (Tahiti to
Darwin normalised pressure difference) was +2.5. Both these point to a
La Nina that is already moderate to strong. The sub-surface temps in
the central Pacific are cool as well, so the consensus is that this La
Nina will continue through the coming South Pacific Cyclone centre.

NIWA has been able to come up with 8 similar cyclone seasons: Nov 1970
to April 1971 or 70/71, also 71/72, 73/74,75/76, 88/89, 98/99, 99/00,
and 07/08. Using these years as a guide, the forecast for the Nov 2010
to April 2011 season is for 9 to 12 named storms (9 is average). 3 are
forecast to reach at least category 3, and 1 is forecast to reach at
least category 4 (average winds near centre of 64 knots or more). The
Coral Sea and surrounding places west of 180 have an elevated risk of
Cyclone impact.

Although there appears to be a reduced risk for places east of 180, all
communities should remain alert and prepared. In the 8 similar seasons
chosen there have been some cyclone impact in southwest parts of French
Polynesia and in the Southern Cooks. During previous moderate to strong
La Nina's, cyclones have been able to leave the tropics and cross the
Tasman Sea onto southern NZ.

TROPICS (this week)
South Pacific Convergence zone SPCZ is rather spread out, active, and
wide, from the Solomons to Fiji to Northern Tonga and then stretches
southeast across Southern Cooks. There is an active upper trough just
NW of New Caledonia tonight Sun 24 oct. This should cross New Caledonia
on Mon 25 Oct, southern Vanuatu on Tue 26 oct, and then Fiji and Tonga
on Tue/Wed 26 and 27 Oct. Another upper trough is being picked by the
GFS model to cross New Caledonia on Sat/Sun 30/31 Oct. Avoid.

SUBTROPICAL RIDGE: STR.
On Mon 25 Oct a new, replacement, HIGH is expected to develop over NZ as
the older one wanders off to NE and fades away. This new High then
should migrate east along 40 to 45S--- that's further south than normal,
but there will still be an zone of enhanced trade winds on its northern
side - a weak squash zone-- mainly between Fiji/Tonga and NZ from around
27 Oct to 1 Nov. This will provide spirited and bumpy reaching
conditions.

NZ AREA
Another replacement trough is expected to travel NE across NZ on
wed-Fri, 27-28-29 Oct , stalling awhile over central North Island ...
light winds and showery. This should be followed by an intense high
migrating east along 50S, making for a broad zone of easterly flow
between tropics and NZ, that then rotates into a northerly flow in the
Tasman Sea. This is called low-index*.

SAILING TO NZ
Conditions are OK this week, if you are happy with the enhanced trade
winds on the way and if you dodge the squalls of the SPCZ and its
passing upper troughs. Too early to say much about next week yet,
but the weather pattern does seem to be becoming so much of a "low
index" that the next likely step may well be a deepening low in area
south of New Caledonia in first week of November. So it may be better
to leave early rather than linger.

* 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 October 2010

BOBGRAM7 issued 18 oct 2010

WEATHERGRAM
YOTREPS
Issued 18 October 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.
I'm back in the office now from attending to a display for MetService at
the Waikato Boat Show over the weekend, but had to wait until tonight to
get any spare time to write this.

LA NINA
SOI (90 day) is now 2.2 and reasonably steady. This is a strong La
Nina, more than two standard deviations from its mean at present. More
on this in next few weathergrams.

TROPICS
South Pacific Convergence zone SPCZ is now in its normal October
position from Papua New Guinea to northern Vanuatu, then, weak at
present but hovering between Rotuma and Fiji, then move active over
northern Tonga and Niue/Southern Cooks.

An upper trough over South Coral Sea and New Caledonia tonight 18 Oct is
embedded in the upper westerlies rather than the surface trade winds, so
it migrating eastwards... it should activate the SPCZ over Fiji and
Tonga on Tue wed 19-20 Oct. On late wed 20 and during Thu 21 Oct, a
cold southerly / SW wind (from NZ) is likely to reach as far north as
Kermadecs, just as this upper trough reaches the Niue area and this
combination may deepen the pressure around Kermadecs, thus strengthen
the cold winds between NZ and Fiji Tonga for a day or so. Wait for this
to move on before sailing south.

SUBTROPICAL RIDGE: STR.
Remains of last week's BFH are 1028+ at 40S to south of French Polynesia
FP moving off to the east, maintaining good trade winds over FP, much
like last week.

High 1028 is expected to move east along 35S across New South Wales on
19 Oct and then slowly across the Tasman Sea to cross the Auckland area
on Sat 23 Oct, just in time for the NZ Labour Day holiday weekend. Also
in time to impact on this year's Coastal Classic, so that there should
be a good SW wind in Auckland for a spinnaker start on Friday and then a
fading left-turning breeze over night to challenge the heavier boats.
And as that High continue its eastward migration early next week, a
northerly flow is likely on Monday for the boats then returning to
Auckland.

This slow moving high is a good marker for those planning voyages from
Fiji or Tonga to NZ. Wait for that upper trough to pass by, and so
arrange to depart from Fiji on 21 or 22 Oct (or over weekend if you can
get Customs clearance) and arrange to depart from Tonga on 22-23-24-25
Oct period. The High is expected to be over 1030 and east of NZ from
24 Oct and will then have a squash zone of enhanced easterly winds on
its northern side, ending our departure window. With these voyages as
we sail south the high moves off to the east, so we need to time things
OK and arrange to reach Northland before the following trough and its SW
wind change. In this case that trough is likely to be delayed until the
end of the month, say around 30-31 Oct... but it's so far away this will
need to be updated both at the start and during the voyage.

Good voyages from New Caledonia to NZ or Australia are also on the menu
after that upper trough has got east of Noumea, say, around Wed 20
Oct--- there will be some squash zone impact on the north side of the
Tasman High but nothing major.

NZ AREA
There is a strong upper trough and associated thundery cold front
crossing South island on Tuesday 19 Oct and North Island to the
Kermadecs on Wednesday 20 Oct. On Thu 21 Oct, SW winds still likely to
still be strong and showery in the west, but should be easing. From
then on it's the Tasman High that commands proceedings.

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

09 October 2010

BOBGRAM7 issued 10 Oct 2010

WEATHERGRAM
YOTREPS
Issued 10 October 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 2.19 and reasonably steady. This is a strong La
Nina, more than two standard deviations from its mean at present. We are
currently investigating analogue years - this investigation is still
being done with NIWA. A classic La Nina shifts the South Pacific
Convergence Zone to south and west of its normal Summer position---
this encourages cyclones in the Coral Sea area. Sea surface
temperatures in there are already warmer than normal, so the season may
start earlier than normal. More on this in next few weathergrams.

TROPICS
South Pacific Convergence Zone SPCZ is now in its normal October
position from Papua New Guinea to Rotuma to Samoa to Southern Cooks.
That surfacing upper trough (SUT) near Niue referred to last week,
managed to work westwards and activate some squalls about Fiji Tonga as
well as Niue over past few days--- First real rain for Fiji for a long
time and I think there may have been some jetstream assistance as well.
Anyway -it hailed in Sigatoka, and that's unusual. Maybe we can call
this an early start to the Fijian wet Season.

Another trough is on the map == this time it's a surface trough that
started off from the mid -latitudes and has been stretched out by a
jetstream between Australia and New Caledonia. It should cross Vanuatu
on Mon Tue 11-12 Oct and then Fiji/Tonga on Wed Thu 13 14 Oct and then
fade.

SUBTROPICAL RIDGE: STR.
Remains of last week's BFH Big Fat High is a centre near 1028+ at 40S to
south of French Polynesia FP moving off to the east, maintaining good
trade winds over FP.
High 1038 in south Tasman sea today near 42 S is shovelling some cold
air onto NZ, and has a squash zone with a little trough on top of it
near Brisbane. High is expected to split into two crossing the South
Island on Tue Wed 12/13 Oct - frosty! The western part should then head
NE for Norfolk and Kermadecs for the 16/17 Oct weekend, and the eastern
part should follow the path of last week's BFH and keep the trade winds
well supported over FP.

NZ AREA
The leading edge of the cold air being shovelled over NZ should cross
North Island tonight (sun 10 Oct). During Mon 11 and Tue 12 Oct the
trough associated with this air is expected to sharpen and form a low
between Northland and Kermadecs. AVOID. This low should then move SE on
Wed Thu 13 14 Oct. There's a ridge over central NZ mid-week and then
over the North island for the weekend. For South and central NZ, a
moist NW flow is expected for the 16 17 weekend.

Heading for NZ? Don't venture south of 25/30S until that LOW had gone
away - say, around Thu 14 Oct. Then try and time arrival to fit in
with a northerly flow over Northland early next week.

NOTE: I'll be unavailable 14 to 17 Oct, busy attending to MetService
display at Waikato Boat Show. So next weathergram may not be until Mon
18 Oct-or later.

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

03 October 2010

BOBGRAM7 issued 3 Oct 2010

WEATHERGRAM
YOTREPS
Issued 3 October 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 2.15, up 6% from last week, so La Nina is just
getting stronger. It now has values not seen since 1988. NZ's NIWA
Institute will be meeting to consider what this means for the coming
cyclone season, and I shall hold fire with my comments until after that.


TROPICS
South Pacific Convergence zone is mostly found at around 10S, but did
take an excursion across the Coral Sea during last week-touching New
Caledonia briefly.

There is an upper trough over the Suwarrow to Niue area slowly surfacing
to south and east of its axis. It's thundery. Should develop a LOW to
east of the Kermadecs on Mon 4 Oct that will move off to the SE then
stall around Thu 7 Oct.. The tropical extension of the trough will
likely kill the winds over the Cooks this week, and may even turn the
surface winds to a weak westerly, along with cloud and rain. SO it
isn't a good week to "puddle jump" westwards from Tahiti.

SUBTROPICAL RIDGE: STR.
Big Fat High BFH 1038 in mid Tasman Sea tonight 3 Oct is expected to
wander slowly along 40S this week easing to 1030 by the time it gets to
around 170W around Thu 7 Oct, and then slide around the south side of
the LOW that will be blocking its way. Yes there is a squash zone of
enhanced easterly trade winds on the north side of this High. It is, at
present, Sun 3 Oct in the North Tasman sea between 25 and 30S - and
should wander east with the high. It isn't a major, but has been near
gale force about the Whitsundays over past few days.

Next High is not expected to be as intense of the BFH but is likely to
wander east along 35S. It should bud off Australia into the Tasman Sea
around Sat 9 Oct and slide across the north of North island by Wed 13
Oct.

NZ AREA
Between the aforementioned Highs there is to be a passing trough, it
should cross the NZ area on Thu 7 Oct to Sun 10 Oct, preceded by NW
flow, accompanied by a series of fronts and followed by come cool SW
winds. This front is expected to have only minor impact at 30S so it
you are in Fiji or Tonga and wish to sail to NZ and can arrange a voyage
around the back end of the outgoing High, and rendezvous with the front
on 9 Oct near 30 to 35S, then that should be a reasonable voyage to 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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