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

25 December 2011

BOBGRAM issued 26 Dec 2011

Issued 26 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 continues to increase in the atmosphere. The Southern Oscillation Index SOI (30day running mean) was 2.02 on 19 Dec and 2.26 on 25 Dec. It is likely to persist for the next few months, displacing South Pacific Convergence Zone SPCZ and subtropical ridge slightly pole-wards.

TC FINA briefly formed in the Coral Sea, but encountered strong winds aloft and lost its top… the low centre is still hovering mid way between Queensland and New Caledonia and drifting south or southeast… it is expected to wander and fade to west of or over North Island on Thu /Fri 29/30 Dec.

TC GRANT deepened east of Darwin on Christmas Day, and is expected to weaken as it wanders further east and reaches the Coral Sea around Thu 29 Dec and then may deepen again and move towards New Caledonia by Sat 31 Dec/Sun 1 Jan. Avoid.

Cyclone risk is winding up this week… an MJO cycle of enhanced convection is expected to arrive there this week also some equatorial westerly winds have reached the Solomons, so all is set for good cyclone formation especially in the Coral Sea.

The South Pacific Convergence zone, SPCZ, has been active over Coral Zone, New Caledonia, and Fiji to Niue and to the southeast--(where SE winds are converging into northerly winds) to Fiji, and Samoa to Niue. This part of the SPCZ may drift north and east across Southern Cooks and allow some tropical lows to develop there on Thu/Fri 29/30 Dec. Avoid.

There is a low that has come form the tropics near 34S 160W today and this should over to the southeast and fade by Wednesday.
Another branch of SPCZ has been lurking between Tahiti and Marquesas over past few days, but I think this will fade away soon.

Big fat high stalled over New Zealand during the past week and should now wander to east along 40S reaching 150W by Sun 1 Jan. There is a squash zone of enhanced easterly wind between it and the SPCZ.

Next high is likely to nose across Tasmania on Thu 29 Dec and into South Tasman Sea from Friday 30 Dec, brining clearing weather.

A transitional trough is expected to take from Wed 28 Dec to maybe as long as wed 4 Jan to cross NZ. During Wednesday and Thursday an increasing NE flow is likely to spread a rain band southwards over the North islands. At about the same time a southerly change is expected over the South Island. These may converge together and stall the rain band for a while over central NZ or the northern parts of the South Island. Location, timing and intensity of peak rain are still changing.

As for Northland—winds should become light and variable there for a time on Sun 1 Jan, then another period of easterly winds may occur next week.

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