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

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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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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