Issued 27 July 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 trend (x10) since 2011 showing the recent negative dive as at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi
The SOI has dived negatively in the past month but needs to remain in the pink area of the graph (shown in illustrated version of this blog) for more than a month for this to be called an El Nino episode. Even though we are not there yet, there have been some signs of El Nino –like weather in the South pacific recently: the westerly winds of the roaring 40s have extended further north than normal over the New Caledonia area.
Looking at the 30 day averaged pressure anomaly map shows this clearly, and also shows near neutral conditions around French Polynesia. For an El Nino to kick in, Darwin needs to have a sustained above normal barometer, and this is happening—Also Tahiti needs to have a sustained below normal barometer—and that is not happening yet.
Barometer anomaly map for past 30 days as seen at http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html
Action this week seems to be shifting to the NE Pacific
GENEVIVE approaching the south side of Hawaii HI
And HERNAN is lurking off Mexico
These features are expected to fade over the next few days. There are also a few features that may develop into cyclone sin the NW Pacific.
From http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/wp1014.gif Refer to website to get the latest
The weekly rain maps for the past fortnight show a burst of tropical activity spreading eastward along the ITCZ International Tropical Convergence Zone. The South Pacific Zone SPCZ has also shown signs of redeveloping after a few quiet weeks.
Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif
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, light brown arrows are wave directions. .
SPCZ= South Pacific Convergence Zone
The SPCZ is intensifying and is expected to drift south this week so that it visits Samoa on Tuesday UTC and the Southern Cooks on Wed/Thu UTC.
STR= Sub-tropical Ridge
The High moving east off NZ on Monday is expected to travel along 30 to 40 S. There should be a zone of enhanced easterly winds along 15 to 20S on its north side from Wed to Sunday.
Departing westwards from Tahiti:
OK to depart before Tuesday local but any later in the week and there are likely to be some strong SE winds encountered on the voyage, except maybe for a trip to Suwarrow.
Departing from NZ to the tropics
A broad trough is likely to take all week to cross the Tasman Sea and NZ. Its first feature should be preceded by northerly winds and followed by NW winds and cross the South Island on Monday and the North Island on Tuesday/ Tuesday night. The second feature is likely to be more intense, preceded by NW winds on Thursday and Friday, accompanied by a front on Saturday and followed by SW winds that may be strong on Sunday 3 August. SO it is not a good week to depart from NZ.
See my yotpak at boatbooks.co.nz/weather.html for terms used.
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