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

28 July 2013

BOBGRAM issued 28 July 2013

WEATHERGRAM
YOTREPS
Issued 28 July 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
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. During early July it dropped to 0.64 by mid-month and rose to around 0.7 between 21 and 28 July.

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. So far this year NINO3.4 has been on the cool side, but rather weak.

GLOBAL TROPICAL TOPICS
TC FLOSSIE is heading for Hawaii and should make landfall there on Monday UTC.
Over Asia, SW/W Monsoon wind and rain are crossing Indonesia. These equatorial westerlies are likely to help rigger tropical cyclone development around Philippines early in August.
In the past week a large High/Anticyclone moved east along 40S to south of French Polynesia reaching an abnormal central pressure over1040hPa. This produced stronger than normal squash zone over French Polynesia.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ changed its shape last week hugging the north end of the squash zone over French Polynesia. The squash zone is easing now, and the SPCZ is expected to drift slowly south this week, reaching Suwarrow around Thu 1 Aug UTC and Papeete around Sat/Sun 3/4 Aug UTC.

A low is expected to form at 30S to south of Rarotonga on Tuesday UTC and then deepen as it goes SE. This should 'steal' the wind from Southern cooks/French Polynesia making that area light and variable for a while. Another Low is expected to form over Norfolk Island area in the Tasman Sea around Thursday UTC and also deepen as it goes SE across the North island on Fri/Sat 2/3 August. Avoid these Lows.

Sub-tropical Ridge STR
This is getting weaker than last week. The High cell that was over the Tasman Sea this weekend is expected to move steadily east across North island on Monday and then to east of NZ along 40 to 45 South this week with a moderate squash zone on its northern side (not as severe as last week).

Roaring 40s and New Zealand
After the Low crosses northern NZ on Fri/Sat 2/3 August a change of weather pattern is likely. Disturbed westerly winds as far north as 30S are expected to move across the Australian Bight from Thu 1 Aug and reach NZ from Sat 3 Aug then spread to east of NZ. This may allow some weather that may be good enough for sailing away from northern NZ—next week.

Route Briefings
Tahiti to Tonga:
The SPCZ is expected to drift south onto this route from wed 31 Aug, so those who are doing this route at present should be able to finish before then. Others may as well wait for this convergence zone to go away again—maybe around 5 or 6 August

See my yotpak at http://lnk.ie/HL0L/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/HL0M/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/HL0N/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/HL0O/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

21 July 2013

BOBGRAM issued 21 July 2013

WEATHERGRAM
YOTREPS
Issued 21 July 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

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. During early July it dropped to 0.64 by mid-month and rose to 0.77 by 21 July.

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. So far this year NINO3.4 has been on the cool side, but rather weak.

GLOBAL TROPICAL TOPICS
Strangely quiet in the tropics as we enter the dog days of northern summer, the hottest time of the year.

Over Asia, SW/W Monsoon wind and rain are crossing Indonesia and have reached the southern Philippines. In a week or so as these equatorial westerlies stretch to Micronesia, Tropical cyclones are likely to form.


WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ is lingering around the Solomons and has a weak branch in the Coral Sea. It was very active over Vanuatu last week and over just north of Fiji this weekend and expected to be active over Samoa /Tonga next few days along with a branch that stretches along 15S to French Polynesia. This branch is expected to drift south and weaken late in the week.
A low is forming over Minerva/Kermadecs region tonight, between NZ and Tonga, and going quickly south on Tuesday, leaving behind a trough on Wednesday, but the easterly winds should reform in this area late in the week.

Sub-tropical Ridge STR
Large High well east of NZ is expected to expand over next few days with a squash zone of enhanced easterly winds on its northern side from French Polynesia to Southern Cooks. Avoid.
Another HIGH is expected to start moving from South Australia to central Tasman Sea on Wednesday and cross central NZ on Thu/Fri/Sat 25/26/27 with light winds. There may be a squash zone of strong easterlies on the northern side of this High over northern NZ on Sun/Mon 28/29 July as a low forms to NE of NZ near 30S 180.

Roaring 40s and New Zealand
LOW/trough crossing Tasmania tonight is expected to cross the South Island on Tuesday allowing a SW change to spread over NZ on Wed/Thursday.

Route Briefings
Tahiti to Tonga:
Too much wind and waves in the squash zone this week. Just spend the time on other cultural activities.

To/from NZ:
After that Low near the Kermadecs comes on Monday and goes on Tuesday, there is good weather for departure from NZ with the SW winds on Wednesday and Thursday, but get north of 30S before the easterly winds get strong south of there by Saturday.
Coming south, the weather is not looking the best this week and its OK to delay.

Circumnavigation news
Jeanne Socrates made it --- solo nonstop circumnavigation for 70 year old, breaking all sorts of records – reaching Vancouver, Canada on Monday 8 July—I should have mentioned this last week. See http://lnk.ie/HE3U/e=bobmcd1.bobgram@blogger.com/http://synereida.livejournal.com/235637.html . Endeavour such as this should be marked appropriately, and I ask my friends to record their appreciation by dropping off a small donation to Jeanne's charity page for fighting cancer at http://lnk.ie/HE3V/e=bobmcd1.bobgram@blogger.com/http://www.justgiving.com/jeannesocrates

See my yotpak at http://lnk.ie/HE3W/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/HE3X/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/HE3Y/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/HE3Z/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

14 July 2013

Bobgram issued 14 July 2013

WEATHERGRAM
YOTREPS
Issued 14 July 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
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 rose to over plus 1 briefly in April, dropped to minus 0.39 in early May, and has since then bounced back positive, maintaining a value over 1 for most of June. By 14 July the SOI relaxed to 0.64 .

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. So far this year NINO3.4 has been on the cool side, but rather weak.

GLOBAL TROPICAL TOPICS
Remains of TC SOULIK, after clipping north end of Taiwan, are making landfall into China, and it is currently quiet elsewhere.
Over Asia, Monsoon rain has had a wet week over India and now is arriving over western Indonesia.
ITCZ is returning to its normal position and starting to cover the entire Pacific Ocean as a continuous band.

WEATHER ZONES
Lows in the Southern Ocean occasionally deepen and widen out so that they polar blasts northwards, and allow southern ocean SW swells to reach the tropics. A good parameter for monitoring the likelihood of these lows occurring is called the SAM or Southern Annular Mode, as studied at
http://lnk.ie/H8LP/e=bobmcd1.bobgram@blogger.com/http://www.niwa.co.nz/publications/wa/vol14-no2-june-2006/the-southern-annular-mode-and-new-Zealand-climate

A good proxy for SAM is the AAO or Antarctic Oscillation (something like the NAO parameter which explains a lot about European seasonal weather. When SAM or AAO drops negative quickly, as happened around the solstice /June 21st, there is a strong likelihood of a polar blast somewhere in the southern hemisphere—and last month it was New Zealand's turn.

SAM/AAO is currently positive, indicating that the southerly gale over NZ tonight is just a passing feature typical for the time of the year and not really a polar outbreak. It is forecast to be variable for the rest of July with a 50% chance of a polar outbreak (or not) between 20th and 30th July.

South Pacific Convergence Zone SPCZ
The SPCZ is expected by mid this week to split into three branches. One by then is from Papua New Guinea to Fiji, another from Solomons to Wallis/Futuna, and a third between Samoa and Suwarrow. The north end of a transient trough in the roaring 40s/mid-latitude sis expected to move across French Polynesia on Wednesday. By the end of the week the SPCZ is expected to stretch from Wallis/Futuna to Southern Cooks and further SE with a low 1008 forming on it near Borabora by Monday 22 July, and strong winds on its southern and eastern sides.

Sub-tropical Ridge STR
Large High already over central Tasman Sea is a slow-move rand should finally cross central NZ on Thursday/Friday (frosty, but with westerlies on south if STR warming Otago for a change). There is a squash zone of enhanced SE trade winds on the north side of this High. It has been in the Coral Sea for the past week and should travel east with the High this week and get to Fiji Tonga by end of the weak (somewhat weakened as the High weakens).

Roaring 40s and New Zealand
There is a vigorous south/southwest flow over NZ tonight between lows that are north and south of Chatham Island and the High I the Tasman Sea. The High should bring a period of light winds to northern New Zealand. The next trough is expected to breed a series of Lows. It is expected to cross Australian Bight this week, Tasmania on Thursday/ Friday, mid-Tasman Sea on Sat/Sun and NZ on Monday/Tuesday 22/23 July preceded by strong NW winds over NZ on Sat/Sun.

Route Briefings

Tahiti to Tonga:
The challenge on this voyage is to go through or around the SPCZ. The trough crossing French Polynesia on Wednesday is a nuisance as it replaces the trade winds with a period of fluctuating NE then NW winds. SE winds should return during Thursday UTC, but the likelihood of a SPCZ coming and a low forming near Borabora by Mon 22 July makes it a difficult time to go west.
As for going to Suwarrow along the northern route, this is looking OK so long as you get there by Sat 20 July UTC.

To/from NZ:
Departing from northern NZ looks OK on Tuesday and Wednesday – after the SW flow eases and getting north of 30S before the NE/N winds arrive there by Saturday.
Coming south, the weather is looking good for arrival from Thursday to Sunday 21 July. On Monday 22 the front end of the next trough is expected to start arriving.

See my yotpak at http://lnk.ie/H8LQ/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/H8LR/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/H8LS/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/H8LT/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

07 July 2013

BOBGRAM issued 7 July 2013

WEATHERGRAM
YOTREPS
Issued 07 July 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

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 rose to over plus 1 briefly in April, dropped to minus 0.39 in early May, and has since then bounced back positive, maintaining a value over 1 for most of June. By 7 July the SOI relaxed to 0.74 .

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. So far this year NINO3.4 has been on the cool side, but not strongly so, and during June it has relaxed closer to normal.


GLOBAL TROPICAL TOPICS
Off the Mexican west coast, TC DALILA has weakened and TC ERICK has formed and is heading for Baja California and expected to weaken.
Over Asia, Monsoon rain has had an average week after a few abnormally wet weeks.
ITCZ is north of its normal position and has had some active rain recently in Micronesia, after several dry months.

WEATHER ZONES

Blocking anticyclones have been the features which kept Lows over NZ last month producing abnormal wind rain and snow. The blocking index was strong then and is weak now, setting the stage for a good period of disturbed westerlies over southern NZ in recent days.

South Pacific Convergence Zone SPCZ
The SPCZ has retreated north to the Solomons area is the past week, and has second branch at present from Samoa to Suwarrow to Tuamotu. This second branch is expected by Wednesday to shift south from Samoa to Tonga and to weaken around Suwarrow/Tahiti. Around Friday and Saturday 12/13 July part of the SPCZ may wander southeast across Vanuatu and Fiji. Otherwise it is expected to have a quiet week.

Sub-tropical Ridge STR
Next anticyclone to follow the path set by the STR is currently over central Australia and should go east along around 30 to 35S. However, it is likely to be delayed in its progress across the Tasman Sea and finally 'squeeze' its way across central NZ on Sat/Sun 13/14 July.
Between this slow-moving high and the SPCZ, a squash zone of enhanced SE winds is expected to form in the Coral Sea. Avoid until Tuesday 16 July.

Roaring 40s and New Zealand
A series of deep Southern ocean Lows have been travelling east past the south of NZ recently bringing bursts of wind and swell, as is typical during what we call high index weather with the absence of blocking. .The coming week is likely to be different with a swing to what we call low index weather, thanks to the southerly flow on the eastern side of the High approaching from Australia. This southerly has already managed to close the roads out of Queenstown with snow and slips, and is expected to travel north across NZ on Monday and Tuesday, deepening into a slow-moving low over northern NZ by Wednesday that is expected to dance around there until there until Sunday 14 July with strong to gale SE winds on its south side. Avoid.

Tahiti to Tonga:
The challenge on this voyage is to go through or around the SPCZ. This is expected to weaken by over Tahiti by end of Tuesday but some lingering showers and fickle winds may linger in the region on Wednesday 10th, and the SPCZ over Suwarrow should weaken from the 10th. So voyages may be more comfortable for Thursday 11th July (rather than before then).

See my yotpak at http://lnk.ie/H2C7/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/H2C8/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com click the FOLLOW at bottom right to subscribe.
Weathergram text only and translator is http://lnk.ie/H2C9/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/H2CA/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com, Feedback to bob@metbob.com

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