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

29 September 2013

BOBGRAM issued 29 Sep 2013

WEATHERGRAM
YOTREPS
Issued 29 Sep 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.
Background influences
SOI: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has dropped to just below zero in early September and bounced back to 0.4 by 29 Sep.

TROPICAL TOPICS
TC WUTIP is heading westwards across the NW Pacific heading to make landfall in Vietnam.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ is active between Solomons and Tuvalu and is expected to extend towards Niue by Wednesday UTC.
There is another convergence zone which is very active mainly along 10S from Tuvalu to Northern Cooks. This zone is a mirror of the ITCZ that tends to form for a few weeks around the equinox and is expected to remain active this week, occasionally spreading as far south to Samoa and Suwarrow.

Sub-tropical Ridge STR
The STR is still weak around the NZ longitudes and this is allowing lows to spread over the North Island. One is expected to cross this Auckland early on Monday and continue ENE, and then turn SE on Wednesday.
Over Australia and east of 180 the STR is strong along 30S as is normal for this time of the year.

Roaring 40s and New Zealand
A disturbed westerly flow, typical of spring, covers the Aussie Bight and southern NZ.

Route Briefings
Tahiti to Tonga:
Weak trough is expected to affect Niue on or around Wed/Thu UTC and another may affect Tonga from Sat to Sun 5/6 Oct. Try and time your voyage to avoid these troughs.

Between NZ and Fiji/Tonga
There is a good opportunity to travel south from Mon 30/Tue Sep/Oct as this should go nicely around the back end of a High.

See my yotpak at http://lnk.ie/J9SR/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/J9SS/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/J9ST/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/J9SU/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

22 September 2013

BOBGRAM issued 22 Sep 2013

WEATHERGRAM
YOTREPS
Issued 22 Sep 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.

Background influences
EQUINOX: It is logical to divide the seasons on earth use the four 'corners' of our orbit around the sun as a guide. At the equinox the earth's equatorial plane lines up with the sun's, or the sun's sub-solar point (overhead sun) crosses the earth's equator. This happens at 22:20:44 UTC, bringing spring into the southern hemisphere.
Note that day length as measured by sunrise and sunset tables is slightly MORE than 12 hours on the day of the equinox--- Because sunrise is times when the top limb of the sun clears the horizon, rather than the mid-point of the sun, same for sunset,-- and that adds 7 to 10 minutes to the day.

SOI: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has dropped to just below zero in early September and bounced back to 0.52 on 22 September.

TROPICAL TOPICS
Mexico was hit by the combo of Cyclones MANUEL in the eastern Pacific and INGRID in the Atlantic, last week, but America is free of cyclones at present. In the NW Pacific TC USAGI is about to make landfall just north of Hong Kong and the large catherine-wheel like structure of TC PABUK is posed south of Japan and expected to go then northeast skirting the east of Japan.

Looking at the anomaly map of last week's average wind we can see the monsoon has got across Philippines into NW Pacific. Another feature that shows up is the extreme anticyclone in the South Pacific - between NZ and South America intensified to over 1040 at one stage , creating squash zones of enhanced wind on its northern and southern fringes, explaining the strong winds over Tahiti.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has broken into several small branches and these are likely to remain separate during the coming week. One is positioned from Solomons to near Tuvalu and another weak one occasionally extends west east from around Tokelau to Northern Cooks. A more intense Convergence Zone CZ over Vanuatu/Fiji is expected to go south during Monday and form a rapidly deepening Low that is likely to affect New Zealand's North Island on Tuesday and Wednesday. Avoid.
Another CZ is expected to linger over the Tahiti area this week, bring some squally showers and variable winds that come and go.

Sub-tropical Ridge STR
The STR has a weakness in the New Zealand area at present and this is expected to allow a low to deepen in the area between NZ and Fiji on Monday. This low, of tropical origin, is expected to completely disrupt the STR—and go south during Tuesday and Wednesday affecting North Island, and then peel off to the southeast.
The old STR should get pushed to the south by this low and weaken, A new STR is expected to start forming over New Caledonia by Tuesday and the slowly expand south and east into the Tasman Sea North island area by the weekend of 28/29 September.

Roaring 40s and New Zealand
A disturbed westerly flow, typical of spring, is expected to spread from the Aussie Bight across the South Tasman Sea and onto the South Island from Wednesday.

Route Briefings
Tahiti to Tonga:
There are likely to be some squally showers near Tahiti this week, but the weather pattern is looking OK on the way to Tonga.

Between NZ and Fiji/Tonga
Will need to wait for the Low to clear away, then may be OK to travel either from N to S or from S to N--- voyage will encounter some light winds from the new STR. At this stage don't have enough data for the entire voyage and so need to get another update.

See my yotpak at http://lnk.ie/J2AP/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/J2AQ/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/J2AR/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/J2AS/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

15 September 2013

BOBGRAM 15 Sep 2013

WEATHERGRAM
YOTREPS
Issued 15 Sep 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.

Background influences
Drought: Areas that miss out on rain for an extended time tend to have the ability to perpetuate themselves by diverting rain-bearing fronts around them. In our part of the world this has been the case for many months in northern New South Wales. However this week a rain-bearing low is expected to reach this region.

SOI: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has relaxed to around -0.07 in early September. This week is has risen to +0.11 (15 September).

GLOBAL TROPICAL TOPICS
Mexico has Cyclone MANUEL in the eastern Pacific and INGRID in the Atlantic. And there is also another Low heading west across the north Atlantic. In the NW Pacific MAN-YI is about to make landfall over Japan.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has a main branch from Papua New Guinea to approximately Samoa. There is a smaller side branch along about 10S to north of Suwarrow. The intensity in these zones is expected to remain weak the week. The main zone is expected to drift across Samoa at times, most likely on Tuesday and Wednesday UTC. The side branch may drift south across Suwarrow on Saturday UTC.
A trough is expected to develop over Vanuatu on Sun UTC and deepen over Fiji early next week.

Sub-tropical Ridge STR
High that is tonight in the South Tasman Sea is expected to spread NE across NZ on Tuesday and Wednesday and then expand as it goes east of NZ along 35 to 33S from Thursday to Monday, intensifying the trade winds on its northern side.

Roaring 40s and New Zealand
Jetstreams have gone north to around 30S over Australia and this has bred one Low that is tonight between Lord Howe and Norfolk Islands and is expected to roll past northern NZ on Tuesday.

Another Low is now moving from Victoria to New South Wales, and is expected to deepen off Sydney on Tuesday. It has the warmth and moisture potential to bring heavy rain to New South Wales, the first decent soaking since March. This Low is then expected to track off to the south-southeast, and another low may form in Tasman Sea on Friday and cross the South Island by Sunday.

Next week the outlook is for a disturbed westerly flow to spread from Australian Bight to Tasman Sea/New Zealand.

Route Briefings
Tahiti to Tonga:
This is likely to be a week with consistent trade winds. There is an increased risk of tropical squalls near Suwarrow on Sat 21 Sep UTC (local Friday).

Between NZ and Fiji/Tonga
Take care near northern NZ as that Low flies by on Tuesday, otherwise the week offers a mix of light winds and NE/NW winds, good for travelling southwards.

See my yotpak at http://lnk.ie/IV34/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/IV35/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/IV36/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/IV37/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

08 September 2013

BOBGRAM issued 8 Sep 2013

WEATHERGRAM
YOTREPS
Issued 08 Sep 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.

Background influences
Jet streams
This week, and for the next few weeks Lows and Highs in the NZ area are expected to be more extreme than they have been lately, assisted by jet-streams which visit the NZ area at this time of the year.

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has relaxed to around -0.07 (1 to 8 Sep).

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet.
This parameter has been near average so far this year, slightly on the cool side.

GLOBAL TROPICAL TOPICS
There are three possible areas of formation in the North Atlantic at present, and the low now over the Azores is likely to develop into a cyclone by Wednesday and then peel off into the mid-Atlantic.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has increased in intensity over the Solomons, and extends as a trough which during the weekend has shifted from Fiji/Tonga to Samoa/Southern Cooks. This trough and the westerly wind that follows it is likely to skirt the Austral group on Monday UTC, and the main part of the SPCZ is expected to weaken.
There is a weak part of the SPCZ along 10S between 140 and 170W and this is expected to intensify during the coming week but should stay north of Suwarrow – this week anyway.

Sub-tropical Ridge STR
High in north Tasman Sea tonight is expected to travel eastwards along 30S past 180 on Monday and then E/SE along 30/35S for the rest of the week. This High is expected to intensify to over 1032hPa, creating a squash zone of enhanced SE winds between Tahiti and Samoa from Wed to Fri UTC (Tues to Thu local). This can bring some excitingly fast sailing but the roughened seas are likely to be uncomfortable.

Roaring 40s and New Zealand
It is to be a troughy week over NZ, with jet streams intensifying the surface winds, NW on Monday to Wednesday, and cold southerly on Friday 13th and Saturday.

Route Briefings
Tahiti to Tonga:
Squash zone over the route from local Tuesday to local Thursday with some enhanced trade winds. These conditions should ease from local Friday onwards.

Between NZ and Fiji/Tonga
Squally trough is expected to cover the area between northern NZ and 30S on Fri 13 Sep local and then a broader trough is expected on Sat 14 Sep.

See my yotpak at http://lnk.ie/INNO/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/INNP/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/INNQ/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/INNR/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

01 September 2013

BOBGRAM issued 1 Sept 2013

WEATHERGRAM
YOTREPS
Issued 01 Sep 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.

1st September=start of southern Spring? Not necessarily so: the timing of the seasons is done by various conventions – there is no "official" definition.
Some people, such as climatologists who like to follow monthly means, use 1 Sep as the start of spring—very practical. Others use the four 'corners' of the earth's orbit to mark that start of the seasons, and wait for the equinox on 22 September—very logical. Others again use signs from the garden such as daffodils and can thus compare one year with another ---very interesting. I tend to be one of the logical ones.

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has relaxed to around -0.07 (1 Sep).

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet.
This parameter has been near average so far this year, slightly on the cool side.

GLOBAL TROPICAL TOPICS
The Baja California coast remains busy with cyclones, as KIKO reaches Hurricane force off there and travels northwards. The NW Pacific is quietening down after a few busy weeks. The Atlantic has got through half its cyclone season with NO HURRICANE yet (first such year since 2001), but there are two possible areas of formation there at present.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ currently extends from Solomons to Fiji/Tonga and has increased in intensity during the last week. It is expected to stay in much the same position during this week, but to weaken late in the week. A tropical trough is expected to move southeast along the convergence zone and cross Fiji on Tuesday and Tonga on Wednesday. This trough is expected to weaken in the tropics and continue eastwards, getting to Tahiti by Sunday, and is likely to be followed by a W/SW flow to south of 20S. There are also a few minor convergence zones near Suwarrow and Northern Cooks areas .

Sub-tropical Ridge STR
High over central NZ is expected to fade away on Monday, and High in Tasman Sea is then expected to expand so that the squash zone of enhanced SE trade wind on the north side of this High over Coral Sea and New Caledonia is expected to reach its peak by Wednesday. Also on Wednesday a High centre is expected to break free from Antarctica in the area south of NZ, this should allow the High in the Tasman Sea to extend south, and these two highs may combine to shovel a chilled southerly flow onto NZ. That's a MID WEEK POLAR OUTBREAK.
Later in the week the Tasman High should weaken and shift to the north Tasman Sea.

Roaring 40s and New Zealand
By Thursday, the trough that accompanies the mid-week polar outbreak over NZ is expected to combine with that tropical trough crossing Tonga on Wednesday so that they form a deepening complex are of low pressure east of the North Island on Thursday. The gale SW winds around this low are likely to produce high seas between NZ and Fiji/Tonga on Thursday and Friday. Avoid.
This complex area of low pressure is expected to move quickly off to the east, followed by a disturbed westerly flow over NZ on Sat/Sun 7/8 September.

Route Briefings
Tahiti to Tonga:
Avoid the squalls that are likely to accompany the trough that is expected to cross Tonga on Wednesday (afternoon local) and Southern Cooks by local Friday. This trough is also likely to be followed by a period of W or SW winds, mainly south of 20S, and these are counter progressive.

Between NZ and Fiji/Tonga
Best to wait until after the expected gale SW and high seas between NZ and the tropics on Thursday and Friday 5/6 Sept.

See my yotpak at http://lnk.ie/IFX6/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/IFX7/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/IFX8/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/IFX9/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

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