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

19 December 2011

BOBGRAM 19 Dec 2011

Issued 19 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 2.02 on 19 Dec. It is likely to continue through our cyclone season, displacing South Pacific Convergence Zone SPCZ and subtropical ridge slightly pole-wards.

TC Washi, in the Philippines, turned out to be more severe than its satellite imagery was indicating, and death toll in the landslides is now exceeding 650. As for last Wednesday/Thursday's rain in Nelson, MetService had issued warnings of 500mm+ for the hills from a stalled trough, and this amount was delivered to a wide region also producing landslides.

In the equatorial regions things are changing …. Equatorial westerlies have arrived in Timor Sea and Norherlies have appeared over Coral Sea, just as a MJO cyclone of increased tropical convection arrives in the Coral Sea. Already small tropical lows are appearing—tonight one is near 5S 155E.

GFS and EC models are both now picking formation of a tropical depression near 10S 130E on Tue 20 Dec, deepening into a more intense feature as it makes land fall near Darwin around Christmas Day --- this can still change so please check BOM for updates.

The South Pacific Convergence zone, SPCZ, is active from Coral Zone (where SE winds are converging into northerly winds) to Fiji, and Samoa to Niue. A tropical depression should develop to west of New Caledonia on Tues 20 Dec, deepen and be slow-moving for the Christmas weekend then move SE across /near New Caledonia and weaken away by 29 Dec. Worth avoiding.

A sub-tropical low is expected to form between Minerva and Niue on Fri 23 Dec, move SE deepen and peak near 35S 160E on Sun 25 Dec and then move further SE and weaken, leaving behind a tropical trough/convergence zone worth avoiding.

Large High at 45S is slow-moving over NZ/180 area this week — there is a squash zone of enhanced easterly winds between this and the SPCZ, mainly near 20 to 30S this week. Next week the main part of this high is likely to get east of NZ, and the squash zone may turn into a strong NE flow onto northern NZ during the spring tides following the New moon. Be aware of this strengthening NE with the approach of New Year.

NE tail winds for start of Sydney-Hobart race on 26th, then, in typical fashion, a deep tropically enforced LOW is expected to cross Tasmania around Thu 29 Dec, followed by a intense cold high--- strong pressure gradient between high and low makes for a southerly buster around Friday 30 Dec … maybe too late to bother the race this year, maybe not - Depends on the timing.

Next Sunday is Christmas Day. I'll take time-out to feel the spirit. So, next edition of weathergram will be Monday 26 Dec.

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