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

28 August 2011

WEATHERGRAM
YOTREPS
Issued 28 August 2011
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.

TROPICS
Southern Oscillation Index SOI is hovering slightly above zero:
its 30day running mean was 0.47 on 20 August and 0.38 on 28 Aug.

Cyclones in the Northern hemisphere… NANMADOL intensity 85kt already responsible for around 10 fatalities in Philippines, heading for Taiwan on Monday 30 Aug, and likely to make landfall in China. And there is one called TALOS waiting in the wings and likely to make landfall on southern Japan later this week. The remains of IRENE (latest reported intensity 70kt) is moving north along US east coast...its dangerous right front quadrant is forecast to mostly miss New York and hit Boston.

South Pacific Convergence Zone SPCZ split into three during past week, one from Solomons and across Coral Sea to New Caledonia has triggered a small low near Lord Howe Island. Another zone lies along 15S from just N of Fiji to Northern Tonga to Southern Cooks--- this zone is likely to help trigger a Low to deepen near 25/30S 150W on Mon/Tue 29/30 Aug that will then move off to South, weakening the winds over southern French Polynesia for a few days. The third convergence zones lies roughly along 10S mainly about Tuvalu and Tokelau, and west of Marquesas. By the end of this week these are likely to all merge into one zone roughly from Solomons to Southern Cooks.

SUBTROPICAL RIDGE STR
STR had upper support over New Zealand area last week (long wave ridge), but there is likely to be a phase shift towards a long wave trough in this region this week.
The high that starts this week moving off to east of NZ along around 45S should split into weaker zones at 30S and 50S by Tuesday 30 Aug and these should move off to the east at different rates.

Of more intensity, a new high last of over 1030 hPa may move into south of Tasmania on Thu 01 sep and then northeastwards across the Tasman Sea to reach northern North Island by wed 7 Sep.

TASMAN SEA/NZ AREA
Remains of the rain /low pressure system that is today near Lord Howe Island should cross NZ area on Mon/Tues29/30 Aug. This should be followed by a westerly flow of increasing intensity on wed/Thu 31 Aug/01 Sep (wet in west/warm in east), and then a trough and southern change on Friday 02 /Sat 03 Sept. The GFS model currently has a deepening low east of the South Island along with this Fri/Sat southerly, making it something of a polar outbreak--- but EC model at bit.ly/ecoz, simply has it as a transition trough leading in the new high. As the day draws near these models may start to get more pieces of the puzzle and resolve the scenario more closely—if this affects you then keep checking. No point in hyping one idea yet when models still disagree.


SAILING TO/FROM NORTHERN NZ.
Avoid departure /arrival on Mon/Tue and reconsider Friday/Saturday, otherwise things are looking good.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

21 August 2011

BOBGRAM issued 21 Aug 2011

WEATHERGRAM
YOTREPS
Issued 21 August 2011
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.

TROPICS
Southern Oscillation Index SOI is in the positive and relaxing this week: its 30day running mean was 0.87 on 12 August and 0.47 on 20 August. .

South Pacific Convergence Zone SPCZ is steady in the zone from Coral Sea to New Caledonia. This zone has been caught by a jetstream and is being wrapped into a small surface low that started off near the east Australia coast a few days ago. This system has a few more days to run, mainly between Lord Howe and Norfolk Island and then should move west towards the Queensland coast and fade. Avoid.

There is another convergence zone – strong to east of northern Vanuatu then weak and lying across Wallis and Futuna then southeast towards French Polynesia. This system is expected to remain slow-moving this week, and then is likely to activate early next week with a LOW forming between Southern Cooks and French Polynesia.

SUBTROPICAL RIDGE STR
STR had a few weak weeks in the South pacific and is about to go through a counter-balancing strong period. The BFH Big fat High in Tasman Sea is a slow-mover and should finally reach Northland on Sat/Sun 27/28 Aug and then next week move along 40S to east of NZ. There are enhanced trade winds in a squash zone in the tropics on north side of this high.

TASMAN SEA/NZ AREA
Westerly winds are expected to return to South island by Wed 24 Aug, warming up places east of the divide. A strong gusty NW flow should arrive over much of NZ during the Sat/Sun 27/28 weekend, and an active trough then may cross NZ on Mon/Tue 29/30 Aug.

SAILING TO/FROM NORTHERN NZ.
The squash zone in the tropics is only recommended for spirited sailors and is expected to start easing by end of the week. It is OK until Friday to depart from Northland to north or east, and Sat/Sun 27/28 Aug are looking good as target arrival dates for voyages to Northland.

Congratulations to Bruce Arms for wiping almost 3 days off the record for a solo circumnavigation of Australia. Way to go , bro!.

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

14 August 2011

BOBGRAM issued 14 Aug 2011

WEATHERGRAM
YOTREPS
Issued 14 August 2011
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.

TROPICS
Southern Oscillation Index SOI continues to hover in the positive: its 30day running mean was 0.79 on 5 August and 0.87 on 12 August.

South Pacific Convergence Zone SPCZ is steady in the zone from Solomons to Fiji. There is another convergence zone lying from west to east from Tokelau to Northern Cooks sort of along 10S.
That portion of the SPCZ which is over Fiji at present is expected to travel east (the upper winds are westerly), and reach Tonga on Wed 17 Aug UTC and Southern Cooks on Fri 19 Aug UTC and then fade over southern French Polynesia FP this weekend.
The portion of SPCZ near Solomons remains in place and should be able to spread south into Coral Sea by Sat 20 Aug and onto Vanuatu/New Caledonia this weekend.
Maybe some strong SE winds in the western Coral Sea this week.

SUBTROPICAL RIDGE STR
STR is weak over Southwest Pacific at present….a fading high south of FP extends a weak ridge westwards mainly along 25S to south of New Caledonia and then southwards across the Tasman Sea to a high over 1032hPa in the south Tasman sea
--- this Tasman High is centred near 50S at present, shovelling a cold southerly flow onto NZ. It should just slowly wander along 50S to be south of NZ by Thu 18 Aug and then move NE along eastern NZ this weekend and set off to east of North Island along 40S early next week, intensifying to 1038 for a while on Monday 22 August.

TASMAN SEA/NZ AREA
There is a polar blast over NZ today, shepherded by large areas of low pressure to east of the country and that 1032+ high in the south Tasman High. There may be a small low caught in these southerlies and sideswiping the east North Island Coast on Tuesday 16 August UTC intensifying the winds. The polar blast is making for large seas that may peak on Tuesday 16 Aug at around 5m in the eastern Tasman and around 9m between Canterbury and Wairarapa coasts. Avoid.

Southerly winds should turn SE then E and ease, first in the south Wednesday 17 Aug, reaching North Island by Sat 20 Aug.

A new LOW is expected to move across Australia and deepen off its east coast near Sydney on Thu/Fri 18/19 August and then wander to the NE near Lord Howe on Sun 21 Aug and Norfolk around Tue 23 Aug, preceded by an area of strong NE winds. Avoid.

SAILING TO/FROM NORTHERN NZ.
Polar blast winds and sea should ease enough for OK sailing from Northland on Thursday/Friday…after that the NE winds of the next Low may interfere.

It isn't really the right weather for coming south but if you intend to, then Friday/Saturday are looking OK weather for arrival days.

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

07 August 2011

BOBGRAM issued 7 Aug 2011

WEATHERGRAM
YOTREPS
Issued 7 August 2011
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.

TROPICS
Southern Oscillation Index SOI continues to hover in the positive: its 30day running mean was 0.76 on 29th July and 0.79 on 5 August.

Galapagos to Marquesas: If you are still looking at this voyage, it seems that there is a slight advantage with the wind to first head off to 3dge 30min S at 100W and then to 6S at 133W and then to Marquesas. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ spent most of last week from Papua New Guinea PNG to Fiji/Tonga and has now shifted to be from PNG more to Tuvalu/Samoa/Southern Cooks. That part of the zone which is east of the dateline is likely to wander north and weaken over next few days. The portion west of the dateline will probably stay put.

SUBTROPICAL RIDGE STR
At present there is a dominant and extended example of the Subtropical Ridge STR along 30S from just north of NZ all to way to south of Southern Cooks.

In a weird way the 'Roaring 40s' westerly pattern has gone awry in the Australian Bight so that the STR there has been deflected SOUTHWARDS to near 50S. This part of the STR has one high cell within it that is likely to fade south of Tasmania on Fri 12 Aug and then another cell should wander along 50S to 45S in the South Tasman Sea by Monday 15 Aug.

TASMAN SEA/NZ AREA
NZ starts off with a strong disturbed southwest flow, easing away on Monday/Tuesday 8 Aug.

There is a weak tongue of high pressure across the Tasman Sea linking those two branches of the STR together. This weak link is expected to cross Tasman on Monday 8 Aug and NZ of late Tuesday 9 Aug/Wed 10 Aug. It will bring a period of light winds and clear skies good for frosts and fogs.

The LOW that is over South Australia/New South Wales at present is 'out of position' as far as the normal pattern is concerned. It started in the Southern Ocean and got knocked into the normal STR latitudes by some upper winds--- brought a shock of cold air to Melbourne. Anyway, it should cross the Tasman on Mon-Tue 8-10 Aug and then cross NZ on late wed/Thu–Fri 10.11.12 Aug, proceeded by some warmer NE winds.

This paves the way for a polar blast over NZ on Sat 13th/Sun 14th Aug, shepherded by low pressures to east of the country and an incoming High in the South Tasman Sea. Avoid.

SAILING TO/FROM NORTHERN NZ.
SW flow on Monday is still good for going north. NE on late Tue/Wed is good for coming south. Fronts on Thursday/Friday are probably no good for anyone. Strong SW/southerly wind changes Sat /Sun 13/14Aug worth avoiding. It may be good again for going north early next week.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com

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