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

03 December 2011

BOBGRAM issued 4 Dec 2011

Issued 4 December 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.

Welcome to the months of the southern summer. For practical purposes many climatologists take the months of December to February to be summer, but for logical reasons I wait until the Solstice for my summer to start.

Either way, the southern hemisphere cyclone season seems set soon to start, with what looks to be a developing tropical depression in the Indian Ocean neat 13S 90E today, possibly set to take a path to the southeast but to fade before making land fall between Shark Bay and Perth in WA around Sun 11 Dec. It is riding on the wave of a zone of enhanced tropical convection, called a Madden Julian Oscillation or MJO that seems set to bring the Monsoon from Indonesia to Northern Australia over the next few weeks.

Australia had a very wet November and is still a place favoring upper troughs, whilst the upper ridge partners have been favuored about and east of the date line.

La Nina is increasing in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 1.0 on 26th Nov and 1.32 on 3 Dec. It is likely to continue through our cyclone season, displacing the South Pacific Convergence Zone SPCZ somewhat southwest of its normal position and thus helping promote a quieter-than-normal cyclone season is our area. The MJO is likely to reach the Coral Sea around mid-December, so that is likely to be the first period of increased threat.

SPCZ weakened west of the date line last week, from Solomons to northern Vanuatu to north of Fiji, and strengthened east of the dateline from Samoa to Southern Cooks. That little low near 25S 165W today is attached to a branch of the SPCZ which is active over the Southern Cooks. The Low is expected to wander to SE and the activity should ease by wed 7 Dec.

Large High at 35 to 40S east of NZ and south of Southern Cooks managed to reach over 1040 hPa last week … It is now slowly weakening and moving east mainly along 30/35S.

New High is expected to cross Tasmania on Mon 5 Dec and then build in the mid Tasman Sea and cross southern and central NZ on Fri 9 to Sat 10 Dec. This has the makings of a fine arrangement for pre-Christmas outdoor functions in NZ next weekend.

Between the Highs there are troughs. The one over Tasman /NZ today is in two parts… first frontal rain band is over much of the northern areas of both Island tonight and should clear off on Monday. Second part is expected to deepen into a Low in mid-Tasman during Monday and then cross northern and central NZ on Wednesday along with some heavy downpours. After that second rain band moves off, the incoming high is expected to bring dry and clearing weather to NZ, good for a few days of haymaking.

Avoid the Tasman from Mon 5 to Wed 7 Dec.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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