Issued 10 Mar 2013
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.
The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific as one number. It was almost minus 1 during December, relaxed to near zero during January, fell to minus 0.9 during mid-February and has swung to plus 0.77 on 10 March. So it is unsteady.
The Ocean: Sea surface temperatures across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. When they are different from normal we get a change in clouds around the equator and in the latitude zones of weather across the whole Pacific.
Well, in the past month the SST in this target zone has been slightly below normal, but relaxing toward normal, indicating a tendency towards neutrality.
Around the South Pacific at present there is a zone of slightly above normal warmth from Coral Sea to Samoa. This is also where the South Pacific Convergence Zone SPCZ is sitting proudly at present.
Meanwhile the Sub-tropical ridge- the zone dividing the SE trace winds from the roaring 40s, is over NZ and then extends northeast to around 20S to south of Tahiti.
A Madden Julian Oscillation of enhanced convection is reaching its peak this week in the Coral Sea area. Already TC SANDRA is moving slowly south-southeast from the mid Coral Sea. It should affect New Caledonia from now until Wednesday. Another tropical feature is expected to form in the Coral Sea this weekend 16/17 March and there may also be a tropical low over the Rotuma/Wallis /Futuna area. It's going to be a busy few weeks in the tropics.
SANDRA is expected to peak at category 4 around now (Sunday UTC) and spread out and weaken as it goes south—may be located in-between Lord Howe Island and Norfolk Island by Sat 16 and then cross central NZ on Sunday /Saint Patrick's day.
The rain from this feature may be welcomed by NZ. There hasn't been much here since early February--- some sprinkles about southeast end of North Island and around northern Hawke's Bay is the past few days have brought some green tinges to the Fire Authority's Fire weather index map.
After the trough of SANDRA crosses central this weekend another HIGH pressure area is expected to spread across Tasmania on Mon 18 March and then across NZ mid next week.
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Bob McDavitt's ideas for sailing weather around the South pacific
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