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

25 March 2012

BOBGRAM issued 26 March 2012

WEATHERGRAM
YOTREPS
Issued 26 Mar 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.

Apologies for delayed write-up this week, I was busy on Sunday evening.

The MJO or pulse of enhanced convection that has been moving towards Coral Sea over past few weeks has peaked and is now on its way out. The next pulse will be too late to do much in the way of developing South pacific cyclones, so the next ten days is probably the last chance for something to form to mark the end of the cyclone season.

Pressures in the Coral Sea have been falling and continue to fall—the South Pacific Convergence Zone SPCZ has branches across middle and northern Tasman Sea and computer models are showing that a tropical LOW may form over FIJI on Tuesday and deepen for a few days as it wanders towards Tonga (in mourning with Kings Funeral) and then weaken on weekend31 March/1 April.

Of more interest is the tropical low that may form over eastern end of Solomons around Thurs/Fri 29/30 March and deepen, possibly into a cyclone, around Vanuatu on Sat/Sun/Mon 30/31/1 and then this should move into the Tasman Sea/NZ area as a significantly large storm on Mon/Tue/Wed 2/3/4/5 April – too far away to be sure of its track and development at this stage, but something to consider for any pre-Easter sailing around NZ.

The main branch of the SPCZ stretches from Tuvalu to Southern cooks and terminates in a LOW near 30S 160W. This Low should move off to SE around Fri/Sat/Sun 30/31 March/ 1 April.

Galapagos to Marquesas: Slowly becoming possible. The ITCZ International Convergence Zone did a jump to near 5S in past few weeks between 170W and 90W – this sometime happen around the equinox when the sun is directly overhead there. Well as we move away from equinox the ITCZ returns to normal. At present CZ is directly over Galapagos, so may as well wait another few days for the CZ to drift to its normal position in the north.

SUBTROPICAL RIDGE STR
High stretching from Aussie Bight across Victoria/New South Wales to Mid Tasman is expected to move around south end of NZ on Wed/Thu 28/29 March and Chathams Islands on Fri/Sat 30/31 March, preceded by a southerly change and bringing autumnal weather.

TASMAN SEA/NZ AREA
Weather is dominated by the passing High this week. Keep an eye on the low that is expected to form near Vanuatu by late in the week, as the squash zone between that and the high near NZ is likely to dominate next week's weather over New Zealand as Easter approaches.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

18 March 2012

BOBGRAM issued 18 March 2012

WEATHERGRAM
YOTREPS
Issued 18 Mar 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.

A new pulse of enhanced tropical convection or MJO has been having an impact on Northern Australia during the past week with TC LUA forming off the NW coast and now moving inland, also there is a new tropical low in the Gulf of Carpentaria about to move inland, and clouds are building in the Coral Sea.

All this development has seen the isobars dropping over northern Australia, and as a consequence the Southern Oscillation Index or SOI has been going up: from 0.2 on 11 march to 0.5 on 18 March.

The South Pacific Convergence Zone SPCZ is slowly reforming in the Coral Sea area and has some bursts of activity between New Caledonia an Fiji. The main branch of the SPCZ is from Samoa to Northern Cooks to French Polynesia—somewhat east of normal. The SPCZ is mixing with a mid-latitude trough and a Low near 40S 135W but low is expected to weaken and go off to the southeast, breaking this link.

Galapagos to Marquesas: Too early for this voyage: The ITCZ International Convergence Zone has jumped across the equator and is near 5S from 170W to 90W and this makes the 'puddle jump' too squally to consider just yet. The reason for this rare jump by the ITCZ is because the sun is almost directly over the equator at present --- things should sort themselves out again after the equinox on Wednesday.

SUBTROPICAL RIDGE STR
High1 crossing the South Island tonight is expected to move off to the northeast of Chatham Islands by late Tuesday 20 March--- it may be quicker for the Volvo yachts to get south ASAP to west of Chathams rather than take the great circle that is east of Chathams. Then the High split into two… the northern part should fade away and the southern part should travel east along 45S.
Next large high is expected to enter into the Aussie Bight around Fri 23 March and, at this stage, is expected to cross Tasmania on wed 28 March—taking a track slightly further north than the last few highs.. a sign that the season is changing.

TASMAN SEA/NZ AREA
Two tropical visitors are expected this week. The first is already a low approaching southwestwards from the north so that its rain-band is expected to cross much of the North Island on Monday and Tuesday. Then another low is expected to move onto Northland and Auckland from the north on Wednesday and then go southwestwards and then south across the South Island on Thursday—these somewhat unusual tracks are being directed by a upper low.
NZ should enjoy a brief ridge on Friday 23 March—World Meteorological day- a day where weather forecasters all around the world acknowledge the official commencement of the WMO in 1950.
Then another trough is expected to cross South Island during Saturday and North Island during sat night/sun morning 24/25

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

11 March 2012

BOBGRAM issued 11 March 2012

WEATHERGRAM
YOTREPS
Issued 11 Mar 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.

La NINA continues to relax in the atmosphere. Average atmospheric SOI (30day running mean) is now hovering around zero—was 0.3 on 4 March and 0.2 on 11 March

A new pulse of enhanced convection or MJO cycle is still lurking over the eastern Indian Ocean and Northern Australia, but is taking its time getting to Coral Sea—pressure should fall in that area this week. During the past week this pulse has probably been a factor in the formation of TC KOJI and the Low off NW Australia. During the next week it is likely to help form a deepening Low in the Gulf of Carpentaria around Friday 16/Saturday 17 March—this low is expected to travel inland across central Australia next week.

The South Pacific Convergence zone SPCZ had a quiet week in the Coral Sea and seems to be bunched in the Fiji-Samoa-Tonga area and a small low may form on this near or south of Tonga next weekend and then deepen as it wanders off to the southeast.

An interesting phenomenon is occurring near the equator in the near overhead sun (close to equinox) with a zone of convection along around 8 to 10S near Tokelau right across the equator and into the ITCZ near 3N in the eastern Pacific—some sort of mirror image of the ITCZ in the South Pacific, something that I call equinoctial twinning.

SUBTROPICAL RIDGE STR
High1 over central/southern Tasman Sea on Monday 12 March is likely to spread across/around South Island on Tuesday and then wander east steadily along 40/45S.
High2 is, over Mon-wed, being diverted further south across Aussie bight by a low from Australia's monsoonal trough. It should cross South Tasman Sea on Wed and South Island on Thursday, and then wander east along 45 to 40S.
High3 should have a more standard rip across the Aussie Bight and cross just south of Tasmania on Sun 18 March then central NZ on Tues 20 March.

With lowering pressures in the Coral Sea, there are likely to be squash zones of enhanced easterly winds across the South pacific on the north side of these highs.

TASMAN SEA/NZ AREA
Between the highs there are likely to be passing troughs: one is expected to move off to east of North island on Monday, another should reach Southland by Wed and affect east coast on Thursday, a third should reach Southland on Sat and affect rest of NZ and central Tasman Sea on Sunday 18th March.

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

04 March 2012

BOBGRAM issued 4 March 2012

WEATHERGRAM
YOTREPS
Issued 4 Mar 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.
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I've been using an old backup list for this blog over the last couple of weeks and I think have finally added and removed the email addresses and brought the list up to date. If you would like to drop off the list, just reply to bobmcdavitt@hotmail.com with the word remove in the subject.
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La NINA continues to relax in the atmosphere. Average atmospheric SOI (30day running mean) is now hovering around zero—was -0.3 on 26 Feb and +0.3 on 4 March. This should increase during next week as pressures continue to fall over Darwin.

A new pulse of enhanced convection or MJO cycle is starting up in the Indian Ocean and spreading onto Australia. Under its current timing it should reach Coral Sea between 14 and 24 March. This is likely to trigger a Tropical cyclone in The Indian Ocean around Thu-Fri 8- 9 March which should wander off to the south along 100E (well west of Western Australia).
The Monsoonal trough over central Australia has been producing rain around 100mm/day in places that rarely see such stuff, and this trough connects with a Low that moved over Tasmania tonight and is expected to cross the South Tasman Sea on Monday 5 March and clip the south end of the South Island on Tuesday 6 March –another challenge for the Round NZ race fleet as they approach Stewart Island.

The South Pacific Convergence zone SPCZ has been somewhat disjointed in the past week and is building roughly from Solomon Islands to Tuvalu with another branch from Southern Cooks to French Polynesia- a small Low is expected to form on this branch by Fri 9 March taking cloud and rain with it to south of French Polynesia for a while.

SUBTROPICAL RIDGE STR
High1 over NZ tonight is expected to wander east along 40-45S and fade near 150W by Thu 8 Mar. High2 near 140W and south of French Polynesia is blocked and should stay put this week, causing features upstream to bunch up and stall.

High2 over western Aussie Bight tonight should spread east around the south end of Tasmania on Tuesday 6 Mar and the south end of the South Island on Wed 7 Mar and then make its way on a NE track to 35S when east of NZ.

Pressures are lowering this week in the Coral Sea so there is an enhanced easterly flow across the South pacific on the north side of these highs, and this will also affect the northern North Island on Tues and Wed as High2 bananas around the south end of NZ. East to NE winds to greet the Volvo fleet as they approach Auckland.


TASMAN SEA/NZ AREA
That deepening low mentioned in previous weathergram took a path across central NZ early Sat and indeed deepened rapidly (more that 24hPa in 24hr, so worthy of the description as a meteorological bomb).

Its front is now stalling in the tropics between New Caledonia and Kermadecs, and, what with the enhancing of the surface easterly flow and slackening of the upper westerlies, this is likely to turn into a trough that will spread southwest back onto northern and eastern North Island on wed –Thu 7-8 Mar. This trough should have some spread some slack areas into the otherwise strong E/NE flow, and may challenge the Volvo fleet and catch some of them out.

FRONT crossing southern NZ on Tues 6 March may stall around Southern Alps until Wednesday then fade.

A new LOW should form off Sydney on Thu 8 March and cross southern NZ on Sat 10 – but this one is not expected to deepen much. Of more importance : it may be followed over Southland on Sunday 11 March by a NW to SW gale around a passing deep Southern Ocean Low.

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

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