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

29 April 2012

BOBGRAM issued 29 April

Issued 29 Apr 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.

Production a little later than normal this evening because I was driving from Wellington to Auckland this afternoon. All OK here.

The Southern Oscillation Index or SOI is now holding at around -0.6… That was its 30day running mean value on 8 April and 22 to 29 April – there was a slight dip to -0.8 around 15 April. Since it is holding at -0.6 we are on the cusp of describing it as a weak El Nino or still in neutral territory. Either way, this means that the tropics are having only a minor impact on South Pacific weather patterns at present—so OK you can use those Routing text books which pick early May as the end of the cyclone season as far as Yacht Insurance is concerned and so an OK time for departing NZ and sailing to the Islands.

South Pacific Convergence Zone SPCZ has been weakening during the past week and now there is only scattered convection across the Coral Sea.
There has been a line of convection between northern Vanuatu and Fiji, but this has mostly weakened away. There is a steady line of convection from Tuvalu to Samoa to Southern Cooks and over French Polynesia. A small tropical Low is likely to form on the SPCZ over Southern Cooks around 5 May and this should go SE to southern parts of French Polynesia on Sun 6 May.

Galapagos to Marquesas: Much the same as last week. There are squally patches between 115 and 118W otherwise OK. Depart quickly SSW from Galapagos until 3S then WSW to 5S then west to 125W then direct.

The weak ridge to NE of NZ today is expected to go east and fade away on Monday. Next ridge is likely to be held in place in Tasman on Monday and Tuesday to allow the passage of the northern semicircle of a huge Southern ocean Low. This STR Hugh should cross NZ from Wed to Friday 2 to 4 May and leave a ridge over South Island on Sat/Sun 5/6 May.
There will be an enhancement in the trade winds on the north side of this High near 23S on Friday but it isn't expected to be much of a squash zone.

NZ /Tasman Sea troughs
There is a Low between Brisbane and Lord Howe tonight and its clouds do look menacing on sat pictures… but models are picking all this to die 'in situ' during Monday as the next High sets up in south Tasman Sea. Basically it i8is a good looking seed of a trough, but lacks a good jet stream and so hasn't as much upper support as does the South Tasman High.

That Southern ocean LOW that has been moving along 60S from 150E to 180 over the weekend has a northern semicircle of wind and swell that already extends as far north as 40S = southern North Island. As the Low makes its way further east on Monday is may throw northwards a secondary low as far as 45S 170W by wed 2 May—Along with SW swell reaching 9 occasionally 13 metres east of NZ . Avoid.

Next Tasman low is likely to deepen off Sydney on Friday 4 May, as the High over NZ moves off. This Low should deepen and stay in the Tasman on weekend of 5/6 May, allowing a strong N to NE flow to form from New Caledonia to western NZ. Avoid.

NZ to Tropics: There is a nice window-of-departure for a good voyage based on the incoming High between 1 and 3 May. It's a balance between the squash zone on north side of this high (which peaks at 25S on Thursday/Friday 3/4 May) and encountering the NE winds of the following trough (which makes departures from Thurs 3 May onwards no good)… so a lot depends on vessels individual circumstances of speed and destination.

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