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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

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