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

29 July 2012

BOBGRAM issued 29 Jul 2012

WEATHERGRAM
YOTREPS
Issued 29 July 2012
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 Southern Oscillation Index or SOI continues to relax. Its 30-day running mean started July at a year-to-date low of -1.21, and had reverted t0 +0.27 by 28 July.

Monsoon continues to rage over India /Malaysia and Singapore. There are also some deluges in northern China.

In the South Pacific, the Convergence Zone SPCZ has had a fairly quiet week, as a slab of dry air (possibly dropping down from the stratosphere, downstream of the monsoon) has been making its was slowly eastwards across the South Pacific.
There have been squalls in a convergence zone across Solomons but these have been rather weak over Vanuatu and to south of Fiji. A trough on this zone is likely to deepen into a low between Vanuatu and Fiji by Thu 2 Aug and then deepen as it moves SE past Minerva on Fri 3 Aug. Avoid.

There has been another line of convergence from Tuvalu to central Cooks and the trough associated with this is expected to cross Southern Cooks on Mon/Tue and weaken over French Polynesia on Wednesday/Thursday 1/2 Aug in association with a deepening low moving from 25 to 30S.

SUBTROPICAL RIDGE STR
High east of NZ near 170W tonight is expected to reform a cell of high pressure between 20 and 30S at 160W on Monday. This new cell should go east and fade away on Tuesday and Wednesday.

High is southern Bight is having a difficult time to get east. Its normal path into the Tasman Sea is blocked by a Low and so it is likely to extend a tongue southwards across Campbell island on Thursday and then off to the east of NZ along 45 to 50S.
This High is expected to move onto central Australia by Thursday and another tongue of High pressure from this high is likely to extend along 20 to 25S across New Caledonia on the weekend 4/5 Aug and early next week.
This all helps maintain strong SE winds in the Coral Sea this week.

NZ/Tasman Sea
That low which is tonight and on Monday pushing a frontal trough across northern NZ is also deepening and likely to get its central pressure below 980 to west of NZ. By then the Low will have expanded vertically and the upper part of it will trap the lower portion so the system is likely to look around the Tasman for the remainder of the week, finally crossing the North Island around Mon 6 August when the upper low weakens.

The original frontal zone does have a link with the subtropics, but that slab of dry air mentioned above is interfering with the supply of moisture so it isn't as wet as the frontal trough which brought flooding a few weeks back. However the low itself is a breeding ground of instability, so there may well be some severe thunderstorms especially over the North Island during the week.

Traveling towards New Zealand this week:
Lots of squally westerly winds to contend with around the northern side of that low, so take care.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

22 July 2012

BOBGRAM issued 22 July

WEATHERGRAM
YOTREPS
Issued 22 July 2012
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 Southern Oscillation Index or SOI continues to relax. Its 30-day running mean fell during June to a low of -1.21 by 7 July, and relaxed back to -0.34 by 21 July.

Monsoon is now moving onto Malaysia and Singapore.

In the South Pacific, the Convergence Zone SPCZ has been active about and east of Solomons, returning some welcome rain to Kiribati. One branch stretches across Vanuatu and Fiji. Another rather broken branch affects parts of Tokelau and Northern Cooks.
During the next week, trade winds are expected to weaken and westerly winds of the mid-latitudes are expected to creep northwards.
On Mon/Tue 23/24 July a combo of the SPCZ and a mid-latitude trough is expected to move from west to east across French Polynesia. This trough is expected to deepen into a low near 30S 125W on Thu 26 Jul, and this low may deepen to 990 and drift south. Avoid.

SUBTROPICAL RIDGE STR
High east of NZ near 170W tonight is expected to continue to go east-wards between 30 and 35S this week.
High over New South Wales is stalled at present, and rather weak. It is expected to cross the central Tasman Sea on Thursday 26 July and northern NZ on Fri /Sat 27/28 July… then it fades so that there is not much left of the STR by early Aug. This is a sign that the SOI will be dropping again shortly.

NZ/Tasman Sea troughs
Slow-moving Low is expected to drag itself across the North Island on Monday and Tuesday, propelled by the slowly rising pressures in the southern Tasman Sea.
New Low is expected to deepen quickly off Sydney on Fri 27 July and cross NZ of Sun/Mon 29/30 July.

Traveling between NZ and the tropics:
Wednesday 25 July is best day for departure between Lows.

Traveling between New Caledonia and Australia
Going west; well the Tasman Lows over challenges, but Monday to Thursday look OK. Going east; maybe think about departing on or after Friday 27 July rather than before.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

14 July 2012

BOBGRAM issued 15 July 2012

WEATHERGRAM
YOTREPS
Issued 15 July 2012
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 Southern Oscillation Index or SOI has stopped dropping in the past week. Its 30-day running mean feel during June to a low of -1.21 on 7 July, and recovered to -0.71 by 14 July.

Indian Monsoon covered the entire sub-continent and surrounding seas by 11 July, and is now racing across Malaysia and Singapore. It looks normal at present—there was some flooding last week in South China but unsure if that was related to Monsoon.

In the South Pacific, the Convergence Zone SPCZ continues to be active between Solomons and Tokelau. There was also some activity across Samoa and the Cooks last week and this seems to have been absorbed by the low that is today near 24S165W or south of the Cooks. This low is expected to wander off to the SE and its associated SPCZ linked-trough should cross French Polynesia early next week Mon/Tue 23/24 July.

A convergence zone or trough is likely to move south across Vanuatu and Fiji from Fri 20 July to Monday 23 July- this trough should cross the Tonga area on Tuesday 24 to Thursday 26 July.

Apart from erratic winds and tropical squalls near convergence zones described above, the remainder of the tropics should just have average trade winds this week, good for Island-jumping westwards.

SUBTROPICAL RIDGE STR
The next STR high is moving off Australia and into the central Tasman Sea on Monday/Tuesday 16/17 July and southeast-wards across central NZ on Wednesday 18 July. It is then expected to move east away from NZ dipping to 45S and then heading for 40S.

NZ/Tasman Sea troughs
Trough that is crossing North Island on Monday should be followed by clearing weather on Tuesday. A new Low is expected to deepen and stall in the central Tasman from Thursday 19 to Sunday 22 July. Avoid. This maintains a northerly flow over NZ until the front arrives on Mon 23 July.

Traveling between NZ and the tropics:
Tuesday 17 July is the best day for departure from northern NZ this week.
Wednesday is second best and the other days are worse.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

08 July 2012

BOBGRAM issued 7 July 2012

WEATHERGRAM
YOTREPS
Issued 8 July 2012
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 Southern Oscillation Index or SOI continues on its downward trend. Its 30-day running mean has fallen from -0.14 in late May to -1.21 on 7 July. If this remains below -1 for the remainder of July then we are in El Nino territory.

Indian Monsoon has re-surged so that it is now showing above normal activity as it bursts into NW India. However an El Nino episode is usually associated with a weak monsoon, so in the next few weeks its activity over SE and central Asia will be worth watching

In the South Pacific, the Convergence Zone SPCZ continues to rebuild in the in the Coral Sea region. Activity is also slowly stretching out along 10 to 15S from Tuvalu to Tokelau – and so by Friday 13 a trough is expected to deepen between Samoa and Southern cooks. This area has only had dry air for the past few weeks so this trough may bring rain that could be a shock to their systems. A low should form south of the Cooks on Sat 14, compacting isobars around its centre and opening the gradient out over the Cooks. This Low is then expected to deepen further as it wanders off to the SW and then the south. Avoid.

SUBTROPICAL RIDGE STR
This is expected to dominate proceedings in the South West Pacific this week: The High which is crossing central NZ on Monday to Wednesday is expected to continue migrating eastwards along around 40S from Thursday to Sunday, and then it should get pushed to the SE by the low developing south of the Cooks. For a while it should be a banana-cushion south of that low.
North of this high there is likely to be a squash zone of enhanced trade winds mainly over the Coral Sea, but also stretching from Vanuatu to Fiji on Tuesday and Wednesday and from Tonga to Southern Cooks on Thursday and Friday. This squash zone should then be caught between the Low and its high cushion.

The next STR high is expected to move across central Australia on Friday and Saturday and then weaken in the Tasman Sea on Sunday and Monday.

NZ/Tasman Sea troughs
A large Low is expected between those two highs. It should move across New South Wales on Thursday, deepen rapidly east of Tasmania on Fri 13 July and then start weakening as it travels on a SE path to south of NZ on Sat.
A secondary low is likely over central NZ on Sunday and then stall over northern NZ.

Traveling between NZ and the tropics:
Best days to depart from northern NZ this week are Monday to Wednesday.
A Thursday departure is likely to get caught by the secondary low over the weekend.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

01 July 2012

BOBGRAM issued 1 July 2012

WEATHERGRAM
YOTREPS
Issued 1 July 2012
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 Southern Oscillation Index or SOI has been steadily dropping into the negative for the past month. It has fallen from -0.14 on 27 May to -1.03 on 1 July.

Indian Monsoon is not progressing well, and is now about 2 weeks late over NW parts of India.

In the South Pacific, the Convergence Zone SPCZ is rebuilding well in the in the Coral Sea region. A bunch of clouds were able to spin around each other on Friday producing 91P, but this was near to an area of strong wind shear so the system lost its top before it could get any traction.

The SPCZ should gradually build and move further south in the Coral Sea this week. The branch of SPCZ that is showing intermittent activity along 10 to 15S across Tuvalu, Tokelau and Northern Cooks should persist this week.

SUBTROPICAL RIDGE STR
High that has come from the south and is building east of the South Island tonight at 45S is expected to migrate slowly eastwards shunting away that large low on its eastern side Island.
Next large High is currently in the Australian Bight and should wander east across South Australia and New South Wales from Wed 4 to Sat 7 July and the cross the Tasman Sea and NZ next week.

NZ /Tasman Sea troughs
The warming prefrontal NE flow and warm front is spreading over NZ tonight and during Monday. The multi-centred trough that follows is expected to linger until Sat 7 July, followed by a southerly flow from 55S.

Traveling between NZ and the tropics:
Best days to depart from northern NZ this week are when the SW flow arrives and settles down around Sun 7 July.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

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