Bob’s Blog

WEATHERGRAM

YOTREPS

Issued 27 April 2014

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.

SOI. The Southern Oscillation Index SOI sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin.

SOI (30 day running mean) was in the pink (below minus 10 in the farmonlineweather.com graph for much of April but is now blue again.

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SOI trend (x10) since 2011 showing the recent bounce as at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi

TROPICAL TOPICS—

No tropical cyclones around tonight, but the International Tropical Convergence zone has been active across the entire Pacific and over Central America. Also there was an almost-cyclone that brought a period of wind and rain to Gulf of Carpentaria and threatened the Timor Sea.

The recent deadly avalanches on Everest are noteworthy. The glacial melt lakes in that region have been growing since the 1980s. Most (but not all) glaciers in the region are shrinking because of a drop off in regional rains during the monsoon and winter months. Apa Sherpa used to have a farm until it was claimed by a glacial lake outburst flood in 1985 and then he became a climbing guide. According to Apa: “In 1989 when I climbed Everest there was a lot of snow and ice, but now most of it has just become bare rock.”

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Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

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Forecast rain for the coming week, as seen at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/strack_SH.shtml

WEATHER ZONES

 

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing isobars, Lows and Highs, wind, waves, rain, current, SPCZ and STR . Purple lines are upper flow.

Panama to Galapagos: The wind flow over Panama and Las Perlas is expected to be light and variable over the next few days. Also the wind flow for much of the distance between Panama and Galapagos is expected to be from the SW this week, as was the case last week, no good for this voyage. And so it may be worthwhile waiting another week (or more) for this trip, sad to say.

Galapagos to Marquesas

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This map shows winds that might be encountered on a voyage departing Isabella over next few days

If departing in the next few days with a vessel cruising at up to 6 knots then head off to south of the direct path at the start along 244M/248T to 2deg 30min S then WSW along 250M/262T for around 1888nm then go direct to Fatu Hiva.

SPCZ= South Pacific Convergence Zone

This stretches from Solomons to Fiji. There is also a convergence zone from Tonga/Samoa to French Polynesia, found on the north end of the squash zone of enhanced trade winds and NE winds that mark the north end of a large intense High with centre near 45S.

A tropical LOW may form south of Tonga by Wednesday 30 April and travel south along 170W.

STR= Sub-tropical Ridge

The STR remains in its normal winter position, stretching from the deserts of inland Australia across the North Tasman Sea and generally along 35 to 45S to the east of NZ.

New Zealand area

A front is on its way to cross Northland on Monday and then a squally trough is expected on Tuesday.

This is followed by a W/SW flow on Wednesday—the best weather pattern for departing to the north this week, and then a passing high with light winds on Thursday.

A large trough/low is expected to deepen rapidly off Tasmania on Saturday 3 May and then cross the Tasman Sea /NZ area early next week. The NW flow ahead of this system should start over Northland from Friday.

Departing from NZ to the tropics

30 April is the nominal end of the Southern Hemisphere Cyclone season so many yachts may want to depart Northland then or in early May. It seems to me that the best day to depart this week is with the W/SW winds on Wednesday.

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

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe. Weathergram text only (and translator) is at weathergram.blogspot.co.nz

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