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

10 December 2011

BOBGRAM issued 11 Dec 2011

Issued 11 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.

La Nina is increasing in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 1.32 on 3 Dec and 1.61 on 10 Dec. This La Nina 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 in our area.

There was a MJO – an enhanced pulse of tropical convection – over Western Australia last week but it seems to be fading away before it gets to the Coral Sea, so I now think the cyclone threat in the South Pacific remains low for the next few weeks.

SPCZ weakened and split into two last week, one zone from Solomons to Northern Vanuatu, and other zones near Tokelau, Southern Cooks and Southern French Polynesia, only weakly kinked together. The weather engine in the South Pacific is turning over more slowly than normal. A LOW may form out of the zone near Southern Cooks; its passage to the south may be blocked by a HIGH for a few days but should be able to get going from Thu/Fri 15/16 Dec. leaving behind a zone of rising pressure and light variable winds over Southern Cooks /Niue. OK for diving, no good for sailing.

The long wave pattern seems to still be dominating the weather in our part of the South pacific, with a long wave ridge near the date line and a long wave trough to south of Australia.

The surface HIGH that is east of NZ has been acting like a road block to troughs upstream over recent days, and should continue like this until Tue 13 Dec, then weaken and move east along 45S

Next high should push across Tasmania into the South Tasman Sea on Sat 17 Dec. This is expected to take a pat along 50S, crossing southern NZ around Mon 19 Dec and then blossom when it encounters upper support from then long wave ridge and expand to 1030+ to Southeast of South Island on Tue 20 Dec.

NZ may be next to a long wave ridge, but will spend the next week under attack from some short wave transitory troughs.
The first produced a small area of intense rain between Norfolk and Raoul yesterday, and its low is fading as it moves south along 160E tonight. Remaining rain band is visiting northern NZ next few days: may be useful over Northland Sunday night / Monday and briefly heavy over Taranaki/Nelson high ground on Monday.

This trough may act as the entree and help condition NZ's atmosphere to enhance the impact of the main course: a new LOW should form in the Tasman Sea on Tues 13 Dec and take its time, finally crossing North Island on Sunday 18 – some models have this feature going faster than this. It is preceded by a frontal zone that is expected to cross NZ on Thu 15 Dec and followed by an upper trough good for scattered outbreaks of thunderstorms and finally some cooler south to southwest winds. If a southerly crosses Canterbury on Thursday afternoon it may be preceded by hot air and the cold shock may trigger hail.

For those still planning to sail into Northland:
Northeast winds over Northland this week, these are likely to be strong on Thursday ahead of the front (avoid Thu) then northwest and showery until Sunday. Next west to southwest/southerly winds for Northland are likely on Sun /Mon 18/19 Dec.

I will be travelling next Sunday, so next edition of Weathergrams is postponed to Mon 19 Dec.

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