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

21 April 2013

BOBGRAM issued 21 April 2013

WEATHERGRAM
YOTREPS
Issued 21 Apr 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 SOI (30 day running mean) is based on the difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number. SOI has been erratic this year and on 20 April it was plus 0.62.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. 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 Pacific region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds around the equator and consequent changes in the latitude zones of weather around the planet.
During March there was a warming trend in the SEEP (surface of the eastern equatorial pacific) but this has reversed during April. So there is now a neutral signal also coming from the Ocean.

Madden Julian Oscillation MJO
The MJO is a cycle of enhanced tropical convection that occasionally moves from Indian Ocean across Australia into the Coral Sea, and it can trigger the formation of Tropical cyclones, as happened in early March. There was a weak MJO event moving into the pacific region over the past few weeks, with little impact. The next MJO cycle is likely to occur around mid–May.

Panama to Galapagos: There could be some useful NW/NE winds for departure from Tue 23 April, but anticipate a lot of motoring in light winds. Some help may be gleaned from a surface current by going counter-clockwise around Isla Mapelo. The International Convergence Zone ITCZ is stronger than last week and mainly located between 8N and 2N and mainly to west of 80W.

Galapagos to Marquesas: There are signs of a convergence zone between 122 and 132W but this is expected to fade during the coming week. Some good trade winds over Galapagos for a change and these should last for a week of so making this a good time to go. To get the most from wind and surface current head off for 1 deg 50min S 100W and then 4 deg 30min S 108 W and then go direct to Marquesas.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ is draped from the east end of the Solomons across Fiji/Samoa to the Southern cooks. A tropical Low MAY form over the area west of Fiji around Sun 28 April and then deepen as it moves SE on Mon/Tue 29/30 April. Still too early to tell at this stage, so be watchful.

Sub-tropical Ridge STR
The Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, has been forced to the north for a while, allowing a westerly flow to reach northern NZ. This is just right for those wanting to sail east of NZ to Tahiti or Southern Cooks. The window is only expected to last until around Thursday 25 April and then a high cell is expected to dip south to 30/40S to east of NZ, killing the wind on this route. So be quick for this.

Roaring 40s and New Zealand
A complex family of low pressure centres area in the Tasman Sea and NZ area at present. This system has been able to direct some moist air from the tropics onto northern NZ recently. This moisture has fed some thunderstorms resulting in surface flooding in Tauranga, landslides in Waihi (west Bay of Plenty) and a tornado + wind damage in Raglan (Waikato west coast). The system seems to have run out of tropical air now and by Wednesday its main low (L1 in above map) should be east of the South Island. The last of the family L2 above is expected to turn into a trough and cross the North Island on Thursday 25 April (ANZAC Day, a public holiday in Australia and NZ).

See my yotpak at http://lnk.ie/EVKD/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/EVKE/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only http://lnk.ie/EVKF/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/EVKG/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

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