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

28 October 2012

BOBGRAM 28 Oct 2012

Issued 28 Oct 2012
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 Ocean: Equatorial sea surface temperatures of our widest ocean act something like a thermostat for our weather engine—when they are sufficiently warmer than normal it is called an El Nino episode. Well they have been above average across much of the tropical Pacific over the last few months but are now relaxing.

Both NZ's NIWA and Australia's Bureau of Meteorology agree that the indicators are now pointing to less chance of an El Nino this summer than there has been recently. There is more chance that the coming cyclone season is likely to be in neutral territory, neither La Nina nor El Nino. This means that the weather patterns for the coming summer should be way-different from what was delivered by the La Nina we had last summer.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is relaxing after a bounce back from its low of -1.01 back on 25 Aug. It has been hovering near zero for the past few weeks at around plus 0.2 to 0.3 during October. It's now in neutral mode.

Tropical cyclones have had a busy time in the Northern hemisphere in the past week. TC SON-TINH caused landslides claiming the lives of 25 in the Philippines. 15,000 people have been evacuated in the Philippines and 80,000 in south China. There is also a cyclone off the east coast of USA called SANDY, with a death toll of 58 when it was in the Caribbean.

As for our coming cyclone season. Here's a comment from NIWA: as given by Newstalk ZB:
Forecasting centres predict the number of tropical cyclones in the 2012 to 2013 season will be near or just above the average of 10.
NIWA says at least one category three or higher cyclone could occur anywhere across the Southwest Pacific during the season and all communities should remain vigilant.
New Zealand can expect at least one ex-tropical cyclone to pass within 550km of the country.
South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ has remained in scattered clumps in the past week and is mainly stretching from Solomons to east of northern Vanuatu, with another clump along 10S between Northern Cooks and Marquesas.
During the coming week the SPCZ should remain slow-moving, but its western branch may shift south and visit Fiji on Fri/Sat/Sun 2/3/4 Nov. Uncertain at this stage.

One High is expected to move northeast across the Tasman Sea from Monday, fading away north of NZ on Thursday 1 November, with its remains following the normal STR path along 30S from Thursday to Sunday 4 Nov.
Next High is expected to take a more southern path, one typical of summer from Tasmania on Fri 2 Nov towards North Island by Sun 4 Nov--- but it may get blocked in the Tasman Sea. We shall see.

NZ/Tasman Sea
One Low is moving northeast-wards onto Northern NZ and should continue NE and then off to the east of the date line by Wed 31 Oct. There is a southerly to SW flow on its western side that may get as far as 20S between NZ and Fiji by Tuesday, with 3 metre swells as far as 25S. Not a major disruption but a cause of uncomfortable sailing.

The next trough should cross NZ on Thursday and Friday as a cold front followed by a SW flow.
After that there are two scenarios: the GFS is showing the High following this front to stretch along 40S to east of NZ , so that the SW flow gets converted into a SE flow stretching from NZ to Fiji/Tonga during next weekend of 3 and 4 Nov and early next week. ECMWF model, usually the more reliable, has the front stalling over northern NZ and forming a Low, and the High blocking in the Tasman Sea, so that the SE flow between these systems sits over NZ. SO if you are sailing this week, you'll need updates,

There is a scarcity of wind for sailing from the Tropics to NZ this week, so it'll be slow. A sailing breeze is expected to return to the Niuas in Tonga by Thursday and to Tongatapu by around Fri/sat and to Minerva by Sat 3 November. If you want, you can use this time to motor south and get ready for breeze.

Rather than sailing into the SW/S winds on the back side of the low departing from NZ early this week (blue arrow in illustrated edition above), it may be better to try and capture the N/NW flow ahead of the next trough (jade arrow in illustrated edition above showing Wed position). The flow isn't expected to last long but it is the best on offer for sailing this week, and should be between New Caledonia and NZ on Thu 1 Nov, Fiji and NZ on Fri 2 Nov and Tonga and NZ on Fri/Sat 2/3 Nov.

The front that follows this N/NW flow may either stall near NZ until mid-next week (as seen in latest ECMWF model at, or be followed by SE then E winds (as seen in the GFS model that most GRIB viewers use). These are both OK patterns for sailing to NZ, with a few waypoints to avoid headwinds.

If you sail via the Kermadecs there may still be some pumice rafts around.

The submarine volcano that produced these last July has recently been studied by NIWA.
Havre Volcano , NW of Curtis Island in the Kermadecs, had a massive eruption affecting the seafloor.
Illustrated edition shows NIWA's multibeam echo sounder image of Havre after its eruption in July. Crater is 5 kilometres wide and 800metres deep,

The pumice is no longer covering the entire sea, but rafts of it may still be encountered in quantities sufficient to block water filters. Some boulders are up to beach ball size. Illustrated edition some photos taken from Yacht Melina 4-12 Oct, within 500 nautical miles NW of Kermadecs (photos supplied with permission):

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