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

30 July 2011

BOBGRAM issued 31 July 2011

WEATHERGRAM
YOTREPS
Issued 31 July 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
Southern Oscillation Index SOI continues to slowly rise; its 30day running mean was 0.5 on 16 July and 0.6 on 22 July and 0.76 on 29th July.

Galapagos to Marquesas: If you are still looking at this voyage, it seems that there is a slight advantage with the wind to first head off to 3dge 30min S 100W and then to 6S 133W and then to Marquesas. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ is slowly returning after a weak period. The main band is affecting Solomons Vanuatu and Fiji. There are weak convergence zones along 3 to 5S, and near 15S between Samoa and French Polynesia.

Not much change with the SPCZ during the coming week, except for a low budding off to south of Fiji on Mon-Tue 1-2 Aug then moving off to SE then E, possibly re-intensifying near 30S 150W on Sun 7 Aug, with its trough affecting French Polynesia FP on Sun-Mon-Tue 7-8-9 Aug UTC.

Another trough MAY bud off near New Caledonia on Wed-Thu-Fri 10-11-12 Aug— but unsure at this stage, tune in again next week for an update on that.

SUBTROPICAL RIDGE STR
Subtropical ridge is expected to be strong this week, getting support from aloft. The high that is crossing NZ may stall for most of this week in North Tasman Sea. There should be some enhanced trade winds north of this high cell—affecting the Coral Sea on Mon 1 Aug and this affect spreading east to reach the Fiji area from Fri 5 to Tue 9 Aug – this little squash zone should be OK for sailing but not so good on divers, and should continue east onto Samoa, etc., next week.

Next High cell should cross Australian Bight Thu-Sat 4-6 Aug and eastern Australia Sun-Tue 7-9 Aug.

TASMAN SEA/NZ AREA
High hanging around northern and central areas until Thursday should bring frosts and fogs. There should be enough westerly flow over southern areas to just bring showers in the west, but a Roaring 40s trough on Thursday may bring a cold southerly change to all of NZ on Fri-Sat 5,6 August .

SAILING TO/FROM NORTHERN NZ.
Not a good week for sailing southwards.
OK for sailing northwards from Tues to Friday 2-5 Aug --- depending on how next week's trough near New Caledonia develops.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

24 July 2011

BOBGRAM7 issued 24 July 2011

WEATHERGRAM
YOTREPS
Issued 24 July 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
Southern Oscillation Index SOI continues to hover slightly above zero; its 30day running mean was 0.5 on 16 July and 0.6 on 22 July.

Galapagos to Marquesas: The sea around Galapagos now seem to be reverting to normal temperatures. If you are still looking at this voyage, it seems that the direct path is as good as any other. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ is having a weak period. There is one band that is active over Solomons and then weakens as it extends SE to east of Vanuatu and about or to Southwest of Fiji. There is also a weak convergence zone in the French Polynesia FP region.

Not much change with the SPCZ during the coming week, except that a trough may form on it around or to south of Fiji/ New Caledonia on 31 July.

SUBTROPICAL RIDGE STR
The high that is east of Tasmania tonight 24 July is shoveling a polar chilled southerly flow onto eastern NZ. It is showery stuff but with enough snow and ice to impede traffic. This High is expected to travel NE across Tasman Sea on Mon and then, when it gets to 30S on Tuesday, to extend itself east along 30S from 160E to 150W, intensifying the trade winds in the tropics and the westerly flow on its south side.
The SE trade winds in the tropics are likely to intensify over the whole South Pacific and reach a peak around Wed/Thu 27/28 July due to this rebuilding of the STR.
The STR may be reinforced by another high cell making its way across central Australia and budding off at 30S around Thu /Fri 28/29 July.

This is a typical weather pattern.

Next weekend: as this STR goes east and when air-pressures start to fall between Fiji/ New Caledonia and NZ: a trough is likely to form – GFS has this by 31 July, and EC has it, at this stage, around 1 Aug. For the EC – see http://bit.ly/ecoz .

'Roaring 40s and a deep low' is the expected pattern for Aussie Bight next week —no sign of the next High until end of next week.

TASMAN SEA/NZ AREA
Polar chilled southerlies should turn to be from the SW and weaken on Monday as that high migrates across the Tasman Sea.

A disturbed westerly flow is then likely for Tue 26 to Sat 30 July, with an embedded front on Wed and Friday.

SAILING TO/FROM NORTHERN NZ.
Useful winds on Tuesday to Saturday--- but check again on the possible trough to north of NZ next weekend, and maybe avoid the fronts on Wednesday and Friday.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

17 July 2011

BOBGRAM7 issued 17 July 2011

WEATHERGRAM
YOTREPS
Issued 17 July 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
Southern Oscillation Index SOI has jumped up a little in past week; its 30day running mean was 0.03 on 8 July and 0.5 on 16 July.

Galapagos to Marquesas: The seas around Galapagos are slightly warmer than normal. Even so, at present there seems to be little extra advantage is straying from the rhumb line/great circle.

South Pacific Convergence Zone SPCZ is active from northern Coral Sea (near Solomons) in bands --- one to Vanuatu, another to Rotuma area then Tonga, and a third to Tuvalu. That's about it at present and there isn't much shifting about either.

SUBTROPICAL RIDGE STR
After a long wave trough over the Tasman Sea / NZ area for the past 2 weeks, a subtropical ridge has finally managed to make into the Tasman Sea this weekend. This High should move east across NZ on Mon/Tues at 40S and then stretch northeastwards and weaken during remainder of the weak, so not much of a squash zone in the trade winds on its northern side.

Next High is starting to form in Australian Bight, and consists of some polar air – so is a frosty one, being modified as it encounters warmer water (brewing unstable showers). Should cross Tasmania on Sun 24 July and then move NE across Tasman Sea so that it passes by to north of Northland around Thu 28 July… a normal winter path, allowing a disturbed westerly flow over Tasman NZ by late next week.


TASMAN SEA/NZ AREA
A return to the pattern we had in June, with a moist mild northerly flow over NZ as a Low forms in the mid Tasman Sea. This Low may be complex and multi-centred, and there may be strong winds near some of its compact centres… especially one near Lord Howe on Tue 19 July. Its main frontal rain band should stall over the North Island on Thursday/Friday 21/22 July and an easterly flow should stay over central NZ as the Low weakens and wanders east across area north of NZ on the weekend 23/24 July. This should set the scene for a southerly blast over all NZ on Mon/Tue 25/26 July between the low (then moving off the SE) and the incoming High.

SAILING TO/FROM NORTHERN NZ.
Departing from Northland – There are challenges. Avoid that incoming front especially on Wednesday/Thursday, 20/21 July. There may be a window on Friday/Saturday 22/23 July, going clockwise around a passing weakening low. Then take care to double-check the strength of the southerly outbreak that is likely on Mon/Tues 25/26 July.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

10 July 2011

BOBGRAM7 issued 10 July 2011

WEATHERGRAM
YOTREPS
Issued 10 July 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
Southern Oscillation Index SOI remains in neutral territory: 0.08 on 3
July and 0.03 on 8 July.

Galapagos to Marquesas: The seas around Galapagos are slightly warmer
than normal and the winds there are now southerly 5 to 15knots. So the
optimum route is to head due west across these southerlies to 95W and
then head for a waypoint near 6S 134W.

South Pacific Convergence Zone SPCZ is active from northern Coral Sea
(near Solomons) to Vanuatu and then weakly active to the Fiji Area. The
part near the Solomons is expected to wander southwards into Coral Sea
this week.
There are strong SE winds over northern Queensland - associated with the
SPCZ and the high pressures stuck this week over central Australia.
There is a minor branch of the SPCZ coming and going from Tuvalu to
French Polynesia. This zone extends a trough to the southeast into the
mid-latitudes and a LOW is expected to form within this trough between
30S and 35S near 135W on Monday UTC.

SUBTROPICAL RIDGE STR
The High that is southeast of French Polynesia today, 10 July, has a
zone of enhanced trade winds on its northern side, but is now moving off
to the east and fading away.
There is a very weak Sub-tropical ridge along 25S across the South
pacific this week- typical of mid winter.
The next high is expected to stay over central Australia until Fri 15
July and cross the Tasman Sea on the 16/17 weekend and then hang around
central NZ during next week.

TASMAN SEA/NZ AREA
The disturbed westerly flow which arrived here last week is set to stay
for next few days. Between the 1020 isobar of the subtropical ridge and
the 960 isobar near 50S there are 80hPa and westerly gradient across the
area-so the embedded squall lines continue to be thundery - and snowy in
the SW mountains.
There is a SW storm near Tasmania tonight 10 July, and this should move
onto southern NZ on Wed 13 and remainder of NZ on Thu/Fri. There may
then be that new High in the Tasman Sea and a deepening low near (or
east of) Chatham Islands, with an eggbeater of a S or SW flow inbetween
harassing eastern coasts of NZ.
Heavy SW swell is likely in Tasman Sea until 14 July - avoid.

SAILING TO/FROM NORTHERN NZ.
Departing from Northland - wait a while: westerlies may be strong at
times until Thursday 14 July. Conditions should turn SW on Friday 15
July with barometer rising above 1012, and that may be OK for a
departure... but check again nearer that time as there may be a problem
low developing between Northland and Tonga next week.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

02 July 2011

BOBGRAM7 issued 3 July 2011

WEATHERGRAM
YOTREPS
Issued 3 July 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
Southern Oscillation Index SOI remains in neutral territory, dropping from 0.29 on 26 June to 0.08 on 3 July.

Galapagos to Marquesas: The seas around Galapagos are slightly warmer than normal and the SSE winds there are slightly less than normal. It is making more sense to get west as soon as possible and seek the better winds west of 95W. So just one way point may be all that's needed now…somewhere near 6S 134W and then head for Marquesas. The current along this route is meant to be mostly west-going, but one yacht has reported an east-going head current near Marquesas.

South Pacific Convergence Zone SPCZ is active from northern Coral Sea (near Solomons) to the Fiji Area and then to the southeast where it merges with jet streams and the mid-latitude troughs. The troughs are forming a chain of lows between Northland and Tonga tonight, with associated gales and heavy rain, and these are expected to move southeast, fading over eastern North Island on Tuesday.

SUBTROPICAL RIDGE STR
The High that moved across NZ this weekend is expected to peel off to the east along 40S, reaching 120E by next sat 9 July with a zone of enhanced trade winds on its northern side as it travels.
The next high is expected to stay over central Australia until 12 July and then cross Tasman Sea maybe 15 to 20 July. SO this is a week of weak STR in the SW Pacific- typical of winter.


TASMAN SEA/NZ AREA
So what we have this week for Tasman Sea/NZ area is a disturbed westerly flow, strong to gale at times. This will set itself up after the Tuesday Low moves off eastern North Island, and may reach a peak over NZ in wind and cold (and mountain snow) on Thu 7 July, and then again on Sat 9 July, and Mon 11 July.
Heavy SW swell is likely in Tasman Sea from 6 to 12 July – avoid.

SAILING TO/FROM NORTHERN NZ.
Departing from Northland – wait until Tuesday for the winds to turn offshore, then brace for those swells.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

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