Translator

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

29 May 2011

BOBGRAM7 issued 29 May 2011

WEATHERGRAM
YOTREPS
Issued 29 May 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 now starting weakening rapidly in the atmosphere - the
Southern Oscillation Index 30day running mean dropped from 1.21 on 8May
to 0.3 by 22May and has remained that low ever since. We are now in
neutral territory.

TC SONGDA is now re-curving and weakening, and should soon fade as it
sideswipes eastern parts of Japan. So the Northern Hemisphere cyclone
season has started.
USA's tornado season this year has been extremely intense. I'm unsure as
to the contributing factors, one may be the fading La Nina - 2008 was
the last time we had a La Nina fade around May/June and it too was an
intense and early tornado season (but not as intense as 2011).

Galapagos to Marquesas: More of the same. Avoid the Equatorial current
as it is against you this year. Dive south to around 5S 100W and then
head almost due west to 7S 133W and then head for landfall. There may
be some tropical showers (~120W), nothing organised.

South Pacific Convergence Zone SPCZ spent last week in a more relaxed
and spread out fashion. There was a section that linked in with a
mid-latitude trough and fed cloud from New Caledonia to NZ last
Thursday. The main SPCZ now seems to be from Coral Sea to across Vanuatu
then Southeast-wards to south of Fiji/Tonga, and is feeding off a
jetstream as well as converging surface winds. This zone is expected
to travel even further south this week and merge with a trough/low near
the North Island around Sat 4 June and then, next week, it is likely to
travel back northwards as the attached trough travels eastwards.

There is another convergence zone loosely hovering between Northern
Cooks and main Island groups of French Polynesia and this should stay
put without much change.

SUBTROPICAL RIDGE/ NZ/TASMAN SEA
High in the Tasman Sea is maintaining a SW flow over NZ. Around mid
week this system is expected to team-up with a trough forming in the
north Tasman Sea and then both should move east as a couplet onto NZ by
late week. The High is likely to progress faster than the low: so
that, by Sat 4 June, the High may be over Chatham Islands and Low may be
to west of Northland: there will be a squash zone of enhanced
northeasterly winds inbetween, along with frontal rain, over
northeastern North Island. A challenging start for this year's Auckland
to Fiji Yacht race.

That Low is expected to be collected by an incoming trough by late Sat 4
June and this combined system should move from west to east across NZ on
Sun 5 May, preceded by NW winds, accompanied by rain, and followed by
S/SW winds.
Another trough is likely to cross NE from west to east on Tue 7/Wed 8
June followed by colder southerlies. So, the weather over NZ is looking
to be unsettled for and following Queen's Birthday weekend.

WHEN TO DEPART NZ FOR FIJI/TONGA?
There is a squash zone forming near 33S in Tasman Sea by end of Mon
30May and this is likely to spread to 175E by Tuesday, ending the
current sailable pattern.
This squash zone should peak over Northland/Auckland on Fri 3/Sat 4 May.
Avoid. The SW change on Sun 5 June is too brief for a comfortable
voyage, for it may be followed too quickly by a trough on Tue/Wed 7/8
June. At this stage the next comfortable looking weather pattern may be
around 10 June. However, it is too far away to be sure.

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 May 2011

BOBGRAM7 issued 22 May 2011

WEATHERGRAM
YOTREPS
Issued 22 May 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 now starting weakening rapidly in the atmosphere - the
Southern Oscillation Index 30day running mean was 2.55 on 23 April, 1.21
on 8 may and 0.3 today. This is related to a DROP back to normal
pressures around Tahiti.

Now we are two months after the March 21 Equinox and the meteorological
year is building normally... Pre-monsoonal heat is developing in India,
the jetstream have moved off the Himalayas (it is that magic fortnight
called Everest climbing weather). Cyclones are starting in NW Pacific -
SONGDA should develop storm force winds as it re-curves just off the NE
end of Philippines.
And in the Southern Hemisphere the lows of the southern Ocean are
starting to expand their girth, so that a polar outbreak which hit South
Australia in mid May, gave strong winds to NZ from 16 to 18 May, and
brought huge swells to Fiji's South Coast ~20 May.

So, I suppose the Galapagos to Marquesas sailing season is now opening.
It looks OK to go now. Avoid the Equatorial current as it is against
you this year. Dive south to around 5S 100W and then head almost due
west to 6S 133W and then head for landfall. There may be some tropical
showers (~120W), nothing organised.

South Pacific Convergence Zone SPCZ spent last week in active mode from
Solomons to Vanuatu to southern parts of Fiji and then SE to south of
Niue where it mingled in with a passing mid-latitude front. This zone
is now retreating to the north and weakening.

A low is likely to form near 25S 155W by Monday 23 May and then go south
and deepen - may be used as a clockwise helper for vessels on the
NZ/Tahiti route

A new trough should rekindle SPCZ in southern Coral Sea on Mon 23 May.
This should cross New Caledonia on Wed 25 May and Fiji/Minerva area on
Sat 28 May then reach Southern Cooks around Wed 1 June (These features
travel with the upper westerly winds even if the surface winds with them
are modified trade winds). A new Low is likely to form on this trough
around Sat 28 May near 27S 160W and then deepen as it moves South
trough--- another clockwise helper for the NZ to Tahiti crowd.


SUBTROPICAL RIDGE:
Large High is moving east to east of NZ this week. Mainly along 40S.

Next High is expected to cross New South Wales around Fri 27 may and
reach central NZ around Mon /Tue 30/31 May, also along 40S.

These are mediocre anticyclones with average squash zones of enhanced
easterly winds north of them at around 20S. However, for tonight,
there is a squash zone with gale force SE winds near 25S from 165 to
170W between High and Low.


NZ/TASMAN SEA AREA
Between the Highs there is a transitional front and trough. Its clouds
are already active over Victoria and Tasmania tonight, along with some
preliminary troughs that are in mid Tasman. Front is expected to cross
NZ on Tue/Wed 24/25 May, preceded by a strong northerly flow. This
should be followed by the trough on Thursday 26 May--- might be a low
with this over North Island. And then a cold southerly change over NZ
on Fri 27 May - turning into a SE flow over the North Island for Sat 28
May.

WHEN TO DEPART NZ FOR FIJI/TONGA?
The High is now leaving NZ and there is a northerly flow on its western
flank, then the front and trough. SO, it is best to wait now for the
next trough to go by - maybe catch the southerly change on Friday 27 or
go with the SE flow on Sat 28 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

15 May 2011

BOBGRAM7 issued 15 May 2011

WEATHERGRAM
YOTREPS
Issued 15 May 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.
Tonight's edition has been delayed by my attending and giving talks at
local Boat Show past 4 days, then catching up on email this evening..

TROPICS
La NINA is now starting weakening rapidly in the atmosphere -the
Southern Oscillation Index 30day running mean was 2.55 on 23 April, 1.21
last week and 0.49 today. This is related to a DROP from higher than
normal pressures around Tahiti.

South Pacific Convergence Zone SPCZ spent last week in active mode from
Solomons to Vanuatu to a zone between Vanuatu and Fiji, with another
branch from Niue to Southern Cooks area. There is now a convergence
zone over Northern Cooks.

The zone of convergence that has been along 5S between 100 to 140W has
started to fade-Puddle jumpers can now set off from Galapagos to
Marquesas

Low tonight 995hpa near 35S 135w is moving off to the SE.

A small low is likely to form on the SPCZ when it is over southern
Tonga/Niue around Thu 19/Friday 20 May. This low may weaken into a
trough as it wanders off to the SE-it should form a zone of strong winds
and heavy swells near Minerva for Thu/Fri 19/20 May, avoid.

SUBTROPICAL RIDGE:
The Big Fat High that was labelled H1 in last week's (and the week
before) weathergram has finally faded. Another HIGH H2 has been
spending last few days crossing the interior of Australia- this
indicates a radical change of season-Highs before this one found the air
over central Australia too thin and warm, but H2 was preceded by cold
air from the Southern Ocean dropping temperatures to abnormal levels
first in Melbourne and, last week in Sydney. SO H2 has found dense
enough air to take the typical WINTER track across central Australia. It
is expected to cross New South Wales on Tuesday 17 May and then cross
Tasman Sea NZ Area along 37 to 42 S from Wed 18 to Sat 21 May- then off
to the east of NZ next week.

NZ/TASMAN SEA AREA
H1 has been feeding a moist NE flow onto the NZ region since Easter,
energizing the local troughs and producing a frontal graveyard that
spent much of last week neat the dateline.

The collapse of H1 has allowed a REGIME CHANGE over NZ: no more NE
flow, now it's a SW flow. The cold front that crossed NZ on Fri 13/Sat
14 has been followed by an unstable/cold west to southwest outbreak.
This flow is likely to stay in place until it is weakened away on
Wednesday by incoming H2.

WHEN TO DEPART NZ FOR FIJI/TONGA?
Winds around Northland/ Auckland may be around 20 gust 30 knots on
Monday Tuesday
--- but are also likely to be 30 gust 40 knots in passing showers, so
that's one reason to delay until Wednesday.
Also the Tasman Sea is filling up now with large swells, probably
peaking in mid Tasman on Tuesday 17 May at around 8 metres! Heavy
swells are likely as far north as New Caledonia and Fiji from Wed 18 May
to Friday 20 May.

Departing from Northland on Wednesday/Thursday seems to be the best -
avoids the possible squally showers on Monday Tuesday and, if your trip
is into part of the north Tasman Sea, these departure days mostly avoid
the big swells.
Those big swells do not have much impact on a trip to Tonga, but we are
expecting some strong winds and heavy swells around Minerva on Thu/Fri
19/20 May due to a brief trough/low forming near Southern Tonga to Niue.


Departing from Friday 20 May onwards puts you in the easterly winds on
the northern side of H2. These winds may be enhanced into a squash zone
near 20S next week, but at this stage things are looking OK - more about
this 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

08 May 2011

BOBGRAM7 issued 8 May 2011

WEATHERGRAM
YOTREPS
Issued 8 May 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 now starting to weaken in the atmosphere -the Southern
Oscillation Index 30day running mean was 2.55 on 23 April, 1.94 last
week and 1.21 on 8 May .

South Pacific Convergence Zone SPCZ spent last week in active mode from
Solomons to Fiji (some flash flooding in Fiji). There has also been a
rain band hovering around from Niue to Southern Cooks to Gamblers.
And over past week there has been a convergence zone along 5S from 100
to 140W --- this maybe some twinning of the ITCZ, something which
usually happens around the equinox (that was ~6 weeks ago now) and may
be linked to lingering positive SOI. Should fade soon, and puddle
jumpers from Galapagos to Marquesas should wait a few more weeks.

Low is expected to form near Southern Cooks on Tue/Wed UTC with gales,
and then move off to SE, deepening to 998 near 30S 160W by Thursday UTC,
then move further off to S and SSE and slowly weaken. Avoid.

SUBTROPICAL RIDGE:
Intense blocking HIGH (called H1 last week) has been centred east of NZ
and south of Southern Cooks for past week, with pressures rising to over
1038. Fiji Met have had a marine gale warning covering the enhanced
trade winds on north side of this high for past few days 142W to 173W,
20-25S. High has been well supported aloft and has been blocking a
series of lows over NZ area forming abnormal unstable weather. A tragic
tornado over northern suburbs of Auckland last Tuesday faded within 5km
of my place.

This week, this High is finally expected to weaken and more off.

The next High from the west is now NOT expected to be able to get into
the Tasman Sea until Mon 16 May--- so this is expected to be a week when
the subtropical ridge is weak over the South Pacific, allowing
mid-latitude fronts to reach the tropics, and a burst of 'roaring 40'
westerlies to march across NZ and further east.

NZ/TASMAN SEA AREA
After a Monday of light winds and a Tue /Wed of increasing NE flow,
there is likely to be a front Wed night/ Thursday morning (local time)
Then another interlude of light winds, then another front on Friday 13th
(maybe local daytime), and then a burst of strong and squally westerly
winds on Sat 14 May, the wind turning to be more of a southerly on
Sunday 15 May
- this S/SW change gets to the Tasmanian area by Tuesday 10 May and
marks the end of this series of fronts/troughs as it heralds a high
moving into the Tasman Sea.

WHEN TO DEPART NZ FOR FIJI/TONGA?
Again it is not a good week to depart NZ. Either too little wind
between the fronts, or two much NE wind ahead of the approaching fronts.
What we want to look for is the front that is followed by a SW change
that leads in a new High across the Tasman Sea so we can ride these SW
winds to 30S (once they have settled). At this stage, it looks as if
this MAY be the case on Tue 17 May --- and as an added bonus such a
voyage will have some moon-lit nights. More on this, next week.

Heading east from NZ has been hassled by that intense High for past week
or so, but this hassle is now fading.

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 May 2011

BOBGRAM7 issued 2 May 2011

WEATHERGRAM
YOTREPS
Issued 2 May 2011
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas 6are from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Apologies for a delayed edition this week, was too busy travelling on
Sunday.

TROPICS
La NINA is still active in the atmosphere. The Southern Oscillation
Index 30day running mean weakened last week from 2.55 on 23 April to
1.94 on 2 May

South Pacific Convergence Zone SPCZ spent last week in active mode near
20S across the Coral Sea and New Caledonia and to south of Fiji. A low
L1 developed on this zone last Friday, but too far south to be able to
feed off much warm sea... however it has deepened to around 994hPa over
past few days as it moved into central Tasman Sea - and should now
weaken to 1000 hPa as it crosses the South Island on Wed 4 May.

The band that has been appearing along 5S between 100 and 160W started
to break up in the past week. This suggests that reasonable voyages from
Galapagos to Marquesas may start appearing in the next few weeks.

SUBTROPICAL RIDGE:
AN intense 1032+ HIGH H1 was located at 45S 170W today (2 May) and its
squash zone (helped by a few 1005 hPa lows to the north) crossed North
Island mid-last week bringing enhanced rain to Hawke's Bay Coast and
enhanced foehn winds to western NZ between Hamilton and Westport. 'Twas
damaging stuff.

There has been a squash zone between H1 and L1. Not as bad as last
week's but very gusty easterlies on Sunday over northern NZ, leading in
L1's warm front.

The next High, H2, is stuck in the Australian bight for a while ----

NZ/TASMAN SEA AREA

--- and this is likely to allow the cooling southerly winds that it
shovels northwards into the Tasman Sea on Thu 5 May to trigger the
formation of a new Low L2. The models are inconsistent at this stage
with the path for L2 - most likely it will cross central NZ during
Mother's Day weekend, followed by a change to cooler and somewhat
squally W or SW winds, and making this a week of two lows for NZ.

WHEN TO DEPART NZ FOR FIJI/TONGA?
Around the middle of the week there is a gap of lighter winds between
the two lows-maybe OK for motoring north but not for sailing. From Thu
to Sat, the showery NW flow ahead of L2's trough will probably put off
most cruising sailors. There is likely to be some assistance with a SW
to SE wind change after that trough on late Sunday, but this may be
interrupted by a period of light winds in a weak ridge. SO there
appears to be NO great sailing voyages appearing at this stage.

Heading east from NZ is OK in that NW flow, but may be affected by the
NE wind and swell being generated and left behind by H1.

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

Blog Archive