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

31 March 2013

BOBGRAM issued 31 March 2013

WEATHERGRAM
YOTREPS
Issued 31 Mar 2013

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.

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number. SOI has been erratic this year and on 31 March it was plus 0.88.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. This region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds around the equator and consequent changes in the latitude zones of weather across the whole Pacific.
At this time of the year, just after the equinox, if there is one definite trend in the SST anomaly in the target area then it usually clicks in and stays for much of the coming year. In those circumstances a swing to El Nino (positive anomaly) or La Nina (negative anomaly) can be picked.
Well, in the past month the SST in this target zone has been warming in the east and cooling in the west-central Pacific. No definite trend to comment on.

Madden Julian Oscillation MJO
The MJO is a cycle of enhanced tropical convection that occasionally moves from Indian Ocean across Australia into the Coral Sea, and it can trigger the formation of Tropical cyclones. In early March an MJO moved into the Coral Sea and helped trigger TC SANDY and TIM. This MJO has faded away now and looking at the cycle, the next one may not arrive until mid-April—so Island- hopping in the South Pacific over next few weeks may be OK.

Panama to Galapagos: The useful NE winds near Las Perlas are expected to fade by 1 April. Then the forecast is for light winds for the next week or more, mainly from south and southwest. No good for sailing.

Galapagos to Marquesas: There is a convergence zone along 5S from 110W to 140W and this extends southwards to 12S around 125W. This week the recommended strategy is to motor in the light winds around Galapagos to near 10s 105w and then going direct with trade winds for sailing. This deviation helps avoids many of the squalls in that convergence zone.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ has weakened a lot in the past week and is now sitting mainly from Papua New Guinea to northeast of Vanuatu to Fiji/Tonga to Niue/Southern Cooks. This is its normal position for this time of the year.

There is a subtropical Low near the Kermadecs about midway between Tonga and NZ this evening. This is expected to stall there for a few days and then fade/move off to the S/SE over Thursday. next few days (similar to last week).

Sub-tropical Ridge STR
Meanwhile the Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is oscillating between 45S (H1 to east of South Island) and 35S (H2 on Aussie South Coast). Not much change since last week.

H1 is moving off to the east, and H2 is expected to cross Tasmania on Thu 4 April and then be redirected around the south end of the South Island on Sun 7 April.

Roaring 40s and New Zealand
The Tasman Sea/NZ area is in-between anticyclones and so that makes for a troughy week (for a change). A trough is expected to fade overt the South Island on Monday. There is a low near Lord Howe Island until Tuesday and this then moves SE and mingles with a trough crossing NZ on Wed and Thu 3 & 4 April, followed by disturbed SW flow on Friday.

See my yotpak at http://lnk.ie/E80Z/e=bobmcd1.weathergram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/E810/e=bobmcd1.weathergram@blogger.com/http://metbob.wordpress.com
Weathergram text only http://lnk.ie/E811/e=bobmcd1.weathergram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/E812/e=bobmcd1.weathergram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

24 March 2013

BOBGRAM issued 24 March 2013

WEATHERGRAM
YOTREPS
Issued 24 Mar 2013
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.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific as one number. It has been behaving erratically so far this year – down and up, then down and now up again, with a 30 day running mean at plus 0.7 on 24 March.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and if this vapour rises then it cools into cloud. This region hosts the warmest sea ion the planet so its abnormalities tend to influence changes in clouds around the equator and consequent changes in the latitude zones of weather across the whole Pacific.

AT this time of the year, just after the equinox, if there is one definite trend in the SST anomaly in the target area then it usually clicks in and stays for much of the coming year. In those circumstances a swing to El Nino (positive anomaly) or La Nina (negative anomaly) can be picked.
Well, in the past month the SST in this target zone has been warming in the east and cooling in the west-central Pacific. No definite trend to comment on.

Madden Julian Oscillation MJO
The MJO is a cycle of enhanced tropical convection that occasionally moves from Indian Ocean across Australia into the Coral Sea, and it can trigger the formation of Tropical cyclones. In the past few weeks an MJO has moved into the Coral Sea and tropical cyclones SANDY and TIM have formed. This MJO has faded away now and looking at the cycle, the next one may not arrive until mid-April—so Islands hopping in the South Pacific over next few weeks may be OK.
Note that tropical cyclones CAN form even if there is no MJO around. Some computers models are picking that a disturbance may form on SPCZ by Tuesday in Timor Sea and then go south making landfall near Darwin later in the week and the into the North Australia interior by the weekend

Panama to Galapagos: Light winds. Next patch of useful NE winds is expected for a few days from 28 to 30 March.

Galapagos to Marquesas: Around the equinox for a few weeks there is the strange "twinning " of the International tropical Convergence zone ITCZ- another convergence zone may form along 5South between 90 and 150W . This can produce too many squalls for those planning to take off to the southwest of Galapagos and then go direct when they encounter trade winds near 5 or 6S. For the early birds that want to go soon, the alternative route to just north of the equator and a ride on the west-going equatorial current is recommended.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ is sitting mainly between 10S and 5S at present, and is shrinking after its more active time last week.
There is a Low L in the sub-tropics to south of Tonga and it is expected to fade off to the southeast.

Sub-tropical Ridge STR
Meanwhile the Sub-tropical ridge-STR, the zone dividing the SE trace winds from the roaring 40s, is oscillating between 45S (H1 to east of South Island) and 35S (H2 on Aussie South Coast).
H1 and H2 are both expected to travel east, with H2 over central NZ on Tues-Thurs 26 to 28 March.
Over the next few weeks the STR id expected to drift north into the interior of Australia- since the nights are now longer than the days the deserts are moving into the cool half of the year and that helps make the air denser and isobars higher.

Roaring 40s and New Zealand
One consequence of post-equinox weather is that the troughs in the roaring 40s can spread further to the north. The trough that is expected to cross the South Tasman Sea on Sat/Sun 30/31 March (Easter weekend) is likely to be preceded by N/NW winds over all NZ on Friday and bring a cold southerly change to all NZ on Easter Mon/Tues 1/2 April.

This trough is likely to induce light winds in the north Tasman Sea during the Easter weekend. Also it may help a low to form between Tonga and New Zealand, and that Low may be pushed southwards so that it side swipes the eastern North Island during the annual Auckland to Tauranga Yacht race.

See my yotpak at http://lnk.ie/E21P/e=bobmcd1.weathergram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/E21Q/e=bobmcd1.weathergram@blogger.com/http://metbob.wordpress.com
Weathergram text only http://lnk.ie/E21R/e=bobmcd1.weathergram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/E21S/e=bobmcd1.weathergram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

17 March 2013

BOBGRAM issued 17 March 2013

WEATHERGRAM
YOTREPS
Issued 17 Mar 2013
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.
The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific as one number. It was almost minus 1 during December, relaxed to near zero during January, fell to minus 0.9 during mid-February and has jumped up to plus 1.09 on 17 March. So it is down and up like the proverbial ---.


The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. When the sea temperature is abnormal we get a change in clouds around the equator and consequent changes in the latitude zones of weather across the whole Pacific.
Well, in the past month the SST in this target zone has been slightly below normal, but relaxing toward normal, indicating a tendency towards neutrality.

Interestingly there is a zone of slightly cooler than normal SST in the north Tasman and around New Caledonia, probably partly thanks to the stirring done by last week's TC SANDRA. This renders the region slightly less likely to support the growth of any topical system. There is a similar cyclone cooler around Samoa to Southern Cooks.



WEATHER ZONES
Panama to Galapagos: Some useful NE winds for departure until Tuesday. Follow the surface current clockwise around Malpelo Island and carry enough fuel to motor in the light winds from there to Galapagos.
Galapagos to Marquesas: Probably still too early to get good weather for this trip due to light winds around Galapagos. Motor to just north of the equator and hitch a ride of the west-going equatorial current to around 110W and then take the most direct route.

Coral Sea
Ex-TC Tim in the central Coral Sea was circling clockwise and keeping its distance from Willis Island. It is a weakening system and has been downgraded to depression status and is expected to travel towards the Townsville area later this week.
Another tropical low might form in the Coral Sea later this week and travel west this weekend 23/24 March.


South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ1 is sitting mainly along 10S at present, with a second branch SPCZ2 across Vanuatu, Fiji and Tonga and currently connected to a Low near 32S 162W (L1).
L1 is expected to wander off to the southeast leaving SPCZ2 behind.

Sub-tropical Ridge STR
Meanwhile the Sub-tropical ridge-STR, the zone dividing the SE trace winds from the roaring 40s, has been finally knocked to north of NZ by a trough that combined in the Tasman Sea with the remnant cloud from last week's TC SANDRA – This trough was crossing NZ tonight, and has brought some welcome rain. The STR had been sitting over NZ since early February—and six dry weeks is abnormal for NZ.
The STR is still well south of normal in other longitudes. This is a feature of La Nina episodes but that cannot really be used as an explanation for the current abnormality.

A large anticyclone tonight in the Australian Bight, H1, is expected to track across Bas Strait on Monday and then across Tasman Sea on Tuesday and over central NZ on Wed and Thursday, then off to the east of NZ along about 40/45S. A trough following H1 is expected to move across New South Wales on Sat 23 March and then NZ on Sun/Mon 24/25 March.

See my yotpak at http://lnk.ie/DT2W/e=bobmcd1.weathergram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/DT2X/e=bobmcd1.weathergram@blogger.com/http://metbob.wordpress.com
Weathergram text only http://lnk.ie/DT2Y/e=bobmcd1.weathergram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/DT2Z/e=bobmcd1.weathergram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

10 March 2013

BOBGRAM issued 10 March 2013

WEATHERGRAM
YOTREPS
Issued 10 Mar 2013
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.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific as one number. It was almost minus 1 during December, relaxed to near zero during January, fell to minus 0.9 during mid-February and has swung to plus 0.77 on 10 March. So it is unsteady.

The Ocean: Sea surface temperatures across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. When they are different from normal we get a change in clouds around the equator and in the latitude zones of weather across the whole Pacific.
Well, in the past month the SST in this target zone has been slightly below normal, but relaxing toward normal, indicating a tendency towards neutrality.

WEATHER ZONES

Around the South Pacific at present there is a zone of slightly above normal warmth from Coral Sea to Samoa. This is also where the South Pacific Convergence Zone SPCZ is sitting proudly at present.

Meanwhile the Sub-tropical ridge- the zone dividing the SE trace winds from the roaring 40s, is over NZ and then extends northeast to around 20S to south of Tahiti.

A Madden Julian Oscillation of enhanced convection is reaching its peak this week in the Coral Sea area. Already TC SANDRA is moving slowly south-southeast from the mid Coral Sea. It should affect New Caledonia from now until Wednesday. Another tropical feature is expected to form in the Coral Sea this weekend 16/17 March and there may also be a tropical low over the Rotuma/Wallis /Futuna area. It's going to be a busy few weeks in the tropics.

SANDRA is expected to peak at category 4 around now (Sunday UTC) and spread out and weaken as it goes south—may be located in-between Lord Howe Island and Norfolk Island by Sat 16 and then cross central NZ on Sunday /Saint Patrick's day.

The rain from this feature may be welcomed by NZ. There hasn't been much here since early February--- some sprinkles about southeast end of North Island and around northern Hawke's Bay is the past few days have brought some green tinges to the Fire Authority's Fire weather index map.

After the trough of SANDRA crosses central this weekend another HIGH pressure area is expected to spread across Tasmania on Mon 18 March and then across NZ mid next week.

See my yotpak at http://lnk.ie/DKVO/e=bobmcd1.weathergram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/DKVP/e=bobmcd1.weathergram@blogger.com/http://metbob.wordpress.com
Weathergram text only http://lnk.ie/DKVQ/e=bobmcd1.weathergram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/DKVR/e=bobmcd1.weathergram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

03 March 2013

BOBGRAM issued 3 March 2013

WEATHERGRAM

YOTREPS

Issued 3 Mar 2013

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.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific in one number. It was almost minus 1 during December, relaxed to near zero during January, fell to minus 0.9 during mid-February and has recovered to minus 0.38 by 3 March. So it is unsteady.


The Ocean: Sea surface temperatures across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. When they are different from normal we get a change in clouds around the equator and in the latitude zones of weather across the whole Pacific.

Well, in the past month the SST in this target zone has been slightly below normal, but relaxing toward normal, indicating a tendency towards neutrality.

WEATHER ZONES

The weather is getting busy in the South Pacific over the next few weeks.

A Madden Julian Oscillation of enhanced convection is moving from the Indian Ocean onto northern Australia and towards the Coral Sea, and this is likely to increase the risk of tropical cyclone formation during the next few weeks.

At present there are no tropical cyclones around, but there is a good chance that one may form in the Coral Sea by Tuesday 5 /Thursday 7 March and deepen there and go onshore on northern Queensland around Monday/Tuesday 11-12 March. There is likely to be a squash zone of strong SE winds between this low and the High in the Tasman Sea.

The South Pacific Convergence Zone SPCZ has strengthened during the last week and at present extends over Coral Sea, Vanuatu, Fiji /Samoa, Southern Cooks and French Polynesia. Tropical Lows are likely to form in the Fiji to Niue area by Tuesday 5 march and move off to the SE /S – to the area east of NZ, accompanied by squally showers with winds sometimes gale force. Avoid.

The Sub tropical ridge STR extends from Aussie Bight across the Tasman Sea / NZ area and then eastwards along 45 to 40S. The High in the Tasman Sea tonight is a slow-mover, well supported aloft and somewhat blocked, keeping rain bearing features away from NZ for a while. It is expected to divide into two centres straddling NZ by Thursday 7 March.

The high centre over eastern NZ is expected to move away on Fri 8 March and that allows a brief southerly front to make its way northwards along NZ's east coast. Straddling Highs are then expected to reform over NZ for a while.

See my yotpak at http://lnk.ie/DDPP/e=bobmcd1.weathergram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is http://lnk.ie/DDPQ/e=bobmcd1.weathergram@blogger.com/http://metbob.wordpress.com
Weathergram text only http://lnk.ie/DDPR/e=bobmcd1.weathergram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/DDPS/e=bobmcd1.weathergram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

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