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

07 August 2011

BOBGRAM issued 7 Aug 2011

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.

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.

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.

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.

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.
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