Weathergram

WEATHERGRAM

YOTREPS

Issued 28 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 standardized 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 27 April it was plus 0.44.

image

SOI since 2010 as seen at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi

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 these in turn tweak 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 revered during April. So there is 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 has been a weak MJO event moving into the pacific region over the past few weeks, and in the past week there has been signs of increased activity in the South Pacific Convergence Zone.

CRUISING ROUTES

Panama to Galapagos: There could be some useful NW/NE winds for departure on Monday 29 April, then local winds should be light or southerly until around 5 May. The International Convergence Zone ITCZ is weaker 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 120 and 130W but this is getting weaker. Some good trade winds over Galapagos 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 6S110W and follow current to 7S124W and then go direct to Marquesas.

NZ to the Tropics.

The nominal start of the cruising season is 1 May, and that’s an OK day to go but with rather light winds from a passing ridge. Those departing then will encounter a passing trough on Friday/Saturday, and it should be an OK adventure. A good time to depart is with fading SW winds as on Monday/Tuesday 29/30 April or on Sun 5 May.

WEATHER ZONES

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Weather Zones (see text) as at Sunday28 April

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Fiji MetService analysis  Sun 28 Apr 0300UTC

see http://www.met.gov.fj/aifs_prods/0991.jpg

South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ is very active and broad from Solomons to between Vanuatu and Fiji. Fiji Met Service have found a tropical depression under those clouds SW of Solomons and upper air conditions are good for this feature to go west. The Fiji Met Service map also illustrates a good example of a squash zone of enhanced trade winds between the SPCZ and the STR. Avoid.

Another feature is expected to deepen into a Low over Fiji/Tonga area over Monday Tuesday and then gather the energy of SPCZ and take it SE. If you are cruising between NZ and Tahiti then it will be best to go clockwise around this low.

Sub-tropical Ridge STR

The Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is well north of its normal position over Australia/NZ, but not so to east of NZ. Those who are cruising between NZ and Tahiti may still be waiting for STR to come north.

High that, on Sunday, was over eastern Australia is expected to cross Tasman Sea on Tuesday and NZ on Wednesday and then go east of NZ along the STR, making a squash zone of easterlies between it and the Low from Fiji. Avoid.

Roaring 40s and New Zealand

An active trough is expected to cross the Tasman Sea on Wednesday move onto South island on Thursday and be followed by a cold SW flow over NZ on Saturday. Avoid.

After that another High should cross Tasman Sea/NZ on Sun 5 May and Monday 6 May .

FOR MORE

See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is https://metbob.wordpress.com

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