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

28 November 2009

BOBGRAM6 issued 29 Nov 2009

WEATHERGRAM
YOTREPS
Issued 29 November 2009
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 are in UTC unless otherwise stated.

Tropical cyclone NIDA spend last week wandering off to the north of Yap
Island.

There is an equatorial westerly burst today 29 Nov and this is likely to
make another tropical low over Micronesia area during the week-but is
NOT expected to reach Nauru/Kiribati. The Low should deepen as it moves
north and may reach Guam area by around 9 Dec.

Another equatorial westerly burst in expected just north of Papua New
Guinea/Solomons from 9 to 12 Dec.

The South Pacific Convergence zone stretches from the Northern Coral
Sea/ northern Vanuatu area to around Tokelau/ Samoa and then
southeastwards to between southern Cooks and French Polynesia. The
section around Southern cooks is likely to get more active from Thu 3
Dec UTC. Vaguely related to the SPCZ, a LOW is expected to form near
25S 130W around Thu 3 Dec UTC ... something for Pitcairn's to be aware
about.

SUBTROPICS
A good sized high is dominating the subtropical ridge along 30S from 180
to 140W at present and this has enhanced trade winds on its northern
side. This high is expected to fade by Thu 3 Dec, allowing a trough to
spread east. Following this trough there is expected to be a new high
spreading east of NZ along 40S, thus taking the subtropical ridge
southwards this week.

TASMAN SEA / NZ AREA
The subtropical ridge is shifting south this week making a pattern
change in NZ away from the El Nino southwest outbreaks of the past
month.

So this week is different: a trough with several little lows is
expected to cross central NZ on Monday, Tuesday, and Wednesday 30 Nov-2
Dec. The key to the pattern change is the High that is expected to zip
across the south Tasman Sea on Tuesday and Wednesday 1-2 Dec and then
expand east of NZ along 40S on Thursday Friday 3-4 Dec. As this High
bubs east the remains of that moist trough are expected to fade over the
North Island - so that by the weekend 5-6 Dec a moist humid northerly
flow should encompass NZ for a few days. This is a good way to start the
calendar months of summer.

A brief SW change is expected over Northland around Mon 7 Dec, but
generally this new pattern enables some good voyaging form Tonga and
Fiji to NZ for anyone still looking.

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

22 November 2009

BOBGRAM7 issued 22 Nov

WEATHERGRAM
YOTREPS
Issued 22 November 2009
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 are in UTC unless otherwise stated.

Tropical cyclone ANJA faded east of Mauritius during the past week.
The South Pacific Convergence zone stretches from the Northern Coral
Sea/ northern Vanuatu area to around Tuvalu to Tokelau and then
southeastwards between northern and southern Cooks and then across
southern islands of French Polynesia FP.

A small low is expected to form in the SPCZ around southern FP on Mon 23
Nov UTC and wander off to the south - some clockwise winds with this low
and it is likely to be followed by a wind flow from the north or
northeast over FP.

SPCZ is expected to get active this week west of the dateline, in
response to the travel of a Madden Julian Oscillation MJO from Indonesia
to the Papua New Guinea/Solomons area. SPCZ may take a trip south
towards Fiji and Tonga on the 27/28 /29 Nov weekend, according to
today's run of the NOGAPS model-bringing them some now-welcome rain.
Other models say no.

The trough which crossed NZ this weekend (dropping temperature in Timaru
from 28 to 15 C at Saturday brunch in typical Canterbury spring fashion)
is expected to help trigger a low south of southern Cooks near 30S 160W
on Mon/Tue 23/24 Nov. This Low will "steal the wind" between the Cooks
and Niue as the low itself moves off to the south from Tue 24 to
Thursday 26 Nov.

TASMAN SEA / NZ AREA
The upper atmosphere has been encouraging highs over South Australia and
Tasmania and lows around Chatham Islands. These highs have built
heat-wave conditions over South Australia. The combination of highs to
the west and lows to the east has encouraged cold southwest outbreaks
over NZ - a typical El Nino spring scenario.

This coming week is offering more of this pattern, but with a
slow-moving high that may well touch Northland.

The high in the north Tasman Sea today 22 Nov is expected to fade there
by Tuesday 24 Nov, as a front and SW flow spreads over NZ on 25 Nov,
severe in the south.

A new High is expected to squeeze along 35S from Australian Bight to
Tasman Sea on Tues 24 Nov, and then noodle northwards across the Tasman
Se on Tue to Thu 24 to 26 Nov so that it stretches west to east along
30S between NZ and Fiji from Thu 26 Nov to Mon 30 Nov. There is likely
to be a squash zone of enhanced trade winds to north of this high - in
Coral sea from 24 to 26 Nov and extending from Fiji to French Polynesia
from 26 to 30 Nov.

On Fri 27 Nov a low at 50S in the Southern Ocean may deepen as it passes
by, making for another vigorous SW blast over southern NZ.

Lows are also expected to deepen in west Tasman Sea on Fri 27 Nov and
Sat 28 Nov and cross central NZ on Sat/Sun/Mon/Tue 28/29 Nov and 1 and 2
Dec. Avoid these lows. Also these lows may trigger a change in the "SW
pattern" that has been affecting NZ over last month or so.

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

14 November 2009

BOBGRAM7 issued 15 Nov 2009

WEATHERGRAM
YOTREPS
Issued 15 November 2009
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 are in UTC unless otherwise stated.

Tropical cyclone ANJA in the South Indian Ocean has started the Southern
Hemisphere cyclone off. It was perhaps helped by a pulse of extra
convection called a Madden Julian Oscillation, and this pulse is
expected to reach the equatorial western pacific around 20 to 30 Nov...
so the next few days marks the end of a relatively quiet period for the
South Pacific Convergence Zone SPCZ and next week may mark the beginning
of some extra convection in our part of the world.

Those cruising yachts still looking for a trouble-free trip to NZ or
Australia had better set off this week.

SPCZ has recently been extending from Solomons/Northern Coral Sea to
northern Vanuatu and from Tuvalu to Samoa to French Polynesia FP.

The section over FP is likely to develop a small tropical low or two
on Mon -Tue 16-17 Nov, UTC, and this will then peel off to the
southeast, leaving behind typical convergence zone conditions of light
variable winds and a few tropical squalls.

A branch of the SPCZ in the Coral Sea is likely to spread across Vanuatu
and New Caledonia from now to Tuesday 17 Nov and then fade. This zone
is embedded in a easterly flow ,around 10 knots on the north side of the
zone, around 20 knots on south end of zone and squally within the SPCZ.

The branch from Tuvalu across Samoa to FP should stay in place this
week, with ENE winds 10 knots on its northern side. SE winds 15 to 20
knots on its southern side and squally within a 5 to 7 degree wide zone.


Less wind in the Coral Sea than there has been over past few weeks.

TASMAN SEA/NZ
HIGH fills the subtropics along 28S from 160E to 160W today is expected
to turn into a wind flow from the east in the area west of the dateline
by Tuesday 17 Nov, and in the areas east of the dateline by Friday 20
Nov. This makes sailing south to NZ or west to Australia a reasonable
prospect this week. Go for it before the MJO turns up J

LOW is expected to cross Tasmania on Monday 16 Nov and then peak in the
central Tasman on Tue 17 Nov and cross NZ on wed 18 Nov and then fade
over Chatham Islands on Thu 19 Nov. The NW flow ahead of its front may
be useful for yachts making landfall in Northland early on Tue 17 Nov.
However the SW winds around Northland on Wed-Thu 18-19 Nov will be in
the face of incoming boats.

Next HIGH is expected to take a path further south than the last-may
bring some warmth with it. Should cross Bass Strait on Tue 17 Nov,
central Tasman Sea on wed 18 to Fri 20 Nov, and then have centres west
and east of North Island on Sat-Sun 21-22 Nov. OK for sailing south-may
have to motor near Northland on Fri-Sat 20-21 Nov.

South of this High, a passing front should bring strong NW winds to
southern NZ on Fri-Sat 20-21 Nov along 35S


I will be at Swordfish Club in Russell around 5:30pm this sat evening 21
Nov, talking LOCAL WEATHER.

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

07 November 2009

BOBGRAM7 issued 8 Nov 2009

WEATHERGRAM
YOTREPS
Issued 8 November 2009
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 are in UTC unless otherwise stated.

There were 22 yachts at north Minerva last Tuesday as this spring's
yacht-migration hits peak. The wind flow around Northland has been
generally from the southwest over the past week - occasionally from the
south or southeast following a few fronts, and there has been a large
zone of calm along the subtropical ridge neat 30S, so weather has been
suitably challenging and produced some stints of analysis paralysis at
the Minerva marina.

Northern Hemisphere is still producing tropical cyclones: IDA started
in central America and now expected to build briefly over Gulf of
Mexico, and a TC called TWENTYFIVE is wandering northeast away from
Micronesia.

On our side of the equator, our South Pacific Convergence Zone SPCZ is
now entering a quiet period. During the past week it extended from
Solomons to Wallis/Futuna to northern Tonga to Northern Cooks. During
9-12 Nov UTC a piece of this zone is expected to go southeast to south
of French Polynesia FP and briefly become a low system, stealing the
wind from FP for a few days.

Another piece is likely to drift southwards towards Fiji and Tonga on
10-13 Nov. Otherwise I think this is a week where the main SPCZ will be
hugging 10S.

Because of HIGH pressures in the Tasman Sea, strong winds are expected
to continue all week across the Coral Sea. This squash zone covers
central parts of Vanuatu and extends to Fiji at times.

TASMAN SEA/NZ
During a brief gap between Highs a few days ago, the Queensland heat
trough was able to ingest some moist air from the Tasman Sea and dump it
onto Coffs harbour area on Friday: their fifth flood this year.

Highs are continuing to linger and fade in the Tasman Sea. It is as if
the New Zealand's southern door has been left open allowing
fronts/troughs to roll onto NZ from the southern ocean: one on Monday 9,
one on Wednesday 11 and one of Friday/Saturday 13-14, each slightly
cooler than the one before. There seems to be some Antarctic melt water
around Macquarie Island at present- a large iceberg was seen passing by
there late last week.

Watch the wind: There is likely to be a deep low passing by in the
southern ocean on Sat 14, and that should make for very strong winds in
central and southern NZ areas this weekend.

As for the HIGHS, today's Tasman High got over 1036 but should move into
the north Tasman Sea during the week and fade away by Thursday 12 Nov.
In the gap-between-the-highs the forecast, at present, is that we don't
get a repeat of last week's Coffs harbour flood. This time a low is
likely to form off the southern New South Wales Coast on Thursday 12
Nov, then deepen a little between Lord Howe and Norfolk Islands on Fri
13 Nov, then fade as it goes northeast.

This low should be followed by another High crossing Bass Strait on Fri
13 Nov, then heading northeastwards.

Next week, from around 16 Nov, this high should be east of Northland and
a NW flow is finally likely over Northland, making landfall slightly
easier for a few days.

Note that I'll be attending to the MetService display at Royal Show in
Christchurch this week, so may not be able to respond easily to emails.


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

01 November 2009

BOBGRAM7 issued 1 NOV 2009

WEATHERGRAM
YOTREPS
Issued 1 November 2009
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 are in UTC unless otherwise stated.

As the calendar ticks into the month that is regarded as the start of
the cyclone season, and into the traditional time that yacht weather
damage is not covered by insurance in the tropics of the Southern
Hemisphere, yachts tend to migrate from the South Pacific Islands to New
Zealand or Australia.

There have been four damaging cyclones over the Philippines in the past
month-one a week-so the Northern Hemisphere cyclone season is still
having its last hurrah.

The South Pacific Convergence Zone SPCZ was more active than normal in
the past few weeks-and had some northwest winds on its northern side
making for troughing conditions-briefly a LOW formed on it in the Coral
Sea, but it appears to be easing now. It is draped from Coral Sea
across northern Vanuatu to Wallis and Futuna to northern Tong and then
southeastwards to be sort of mid-way between Southern Cooks and French
Polynesia. A small low might form on the SPCZ to south of southern
Cooks around Fri 6 November, but if it does it will just peel off to the
southeast and fade.

It looks as though the SPCZ may move a little south this week, perhaps
reaching Fiji on Tuesday 3 Nov and southern Tonga on 4 to 5 Nov, but it
also appears to be fading.

So the cyclone season starts with a low risk of any disturbances this
week.


CROSSING THE SUBTROPICS and HEADING FOR NZ The subtropical HIGHS
dominate proceedings this week, basically killing the wind along 30S.
Heading for NZ this week from Tonga or Fiji is good for motor vessels,
but yachts looking for sailing winds all the way should wait until next
week, maybe longer.

One HIGH is already in evidence on the central Tasman Sea and should
slowly get into the north Tasman Sea on Monday Tuesday 2-3 Nov and then
slide east along 30S as it fades on Wednesday/Thursday 4-5 November.
There will be a small but noticeable increase in the trade winds over
Fiji and Tonga on Wed and Thursday 4-5 Nov as this High glides by - a
small squash zone.

On Friday to Monday 6 to 9 Nov, and maybe longer, a new HIGH is expected
to build in the Tasman Sea, enhancing the trade winds between New
Caledonia and Queensland.


Between these highs, on 4 and 5 Nov, a heat trough is likely to over
inland Queensland, turning winds along its coast to come in from the
northeast.

Through all this, all week, New Zealand should remain in a disturbed
southwest flow, with one front crossing on Mon-Tues 2-3 Nov, another on
Wed-Thu 4-5 Nov, and a third brushing past the South Island on Friday 6
Nov. Boats heading for Northland should find Wednesday or Friday and
Saturday will be the best days to make landfall.

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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