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

30 June 2013

BOBGRAM issued 30 June 2013

Issued 30 June 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.

Background influences

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin. It rose to over plus 1 briefly in April, dropped to minus 0.39 in early May, and has since then bounced back positive, maintaining a value over reaching 1 for most of June and at 1.06 on 30 June. So the signal coming from the Atmosphere at present is hinting at a La Nina.

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus the Sea surface temperatures SST across the equatorial Pacific may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet. So far this year NINO3.4 has been on the cool side, but not strongly so, and continues in this trend. Certainly not as cool as in the big recent La Nina events in 2010/11 and 2011/12.

Tropical cyclones and Monsoon
Two tropical cyclones around at present. DALILA going northwest off the west coast of Central America, and RUMBIA which has just crossed the Philippines and is heading for China making landfall about 126 n miles west of Hong Kong. The Monsoon rain has still been heavy in north central India and over Bay of Bengal, but hasn't yet reached the bush fires of Indonesia.

A look at the air pressure anomaly map for mid-May to mid-June reveals lower than normal pressures over Europe, Eastern Russia, and the Tasman Sea. It also shows that the STR in the southern hemisphere is shifted a little to the south and stronger than normal (except across the Tasman Sea).

South Pacific Convergence Zone SPCZ
The SPCZ is extending eastwards along 5-10S, at the northern end of its range. By Wednesday the forecast is that there may be two smaller convergence zone extensions CZ, one from eastern Coral Sea to New Caledonia and t'other over French Polynesia. These CZ branches are expected to be liked with Lows at 35S and should be avoided. The one over New Caledonia is expected to weaken and move onto Fiji on Friday or Saturday.

Sub-tropical Ridge STR
The STR has recovered from last week and is reasonably strong along 30 to 40S, but remains weak in the Tasman Sea. So the large High that is over Eastern Australia by Wednesday is expected to weaken as it crosses the Tasman Sea on Thursday and just bring one day of light winds to northern NZ on Friday.

Roaring 40s and New Zealand
In NZ this week the action mainly comes from a low that is expected to form to north of Lord Howe on Monday and then cross northern NZ on Wednesday. This is likely to be followed by a broad trough bring a NW flow to NZ on Saturday, a SW flow on Sunday and then a possible weather pattern for good departures from NZ to the tropics on Monday 8 July.

Tahiti to Tonga:
Between Monday and Wednesday UTC there is a CZ affecting Tahiti. On Thursday to Saturday there are expected to be enhance trade winds on the north side of a large high moving east along 30S. Better to wait for Sunday 7 July.

Sad about SV NINA
A crew member called me via sat phone from SV NINA during afternoon on Monday 3 June, when they started to encounter a NW gale and wanted to know where best to go. I got them to call me back in 30mins, downloaded some weather data and, when they called back, passed on a forecast, then sent it by txt and email to their sat phone.
They were caught is a rapidly deepening Low. It was too late to run for cover- that would just prolong the incoming storm , so I suggested they go south a bit then hove to and brace for a period of storm force southerly winds followed by a period of swells around 7 or 8 significant metres.
On Tues 4 June they sent me one more txt position via sat phone but since then there has been no coms. Discussing our concern with friends and family and the fact that the emergency beacon on NINA has not yet been deployed,--- so radio searches were done until their last likely ETA in Australia arrived last Sunday. Then, with concern exceeding expectations, Rescue Coordination Centre has done a full plane search and coastal search. So far this has been negative. We still have our fingers crossed for them.

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