WEATHERGRAM
YOTREPS
Issued 13 September 2015
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.
Where are we with El Nino?
SOI (the atmosphere)
The SOI (Southern Oscillation Index) measures the (normalized) air pressure difference between Tahiti and Darwin.
This parameter has been in El Nino territory since July. El Nino has recently been encouraging disturbed SW wind condition in the Tasman Sea/NZ area, but, as we shall see, the opposite is about to happen for a week or so, as some sort of atmospheric fling.
The SOI can be seen at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly
TROPICAL TOPICS
There are some tropical lows around as seen at http://tropic.ssec.wisc.edu/) but no tropical cyclones at this time:
WEATHER ZONES
SPCZ=South Pacific Convergence zone.
This extends from Solomons area to the Fiji/Samoa/Tonga area by mid-week.
STR= Sub-tropical Ridge
The HIGH that is in the Tasman Sea on Monday 14 UTC is expected to travel east along 30 to 35S, crossing northern NZ on Tuesday and then off to east of NZ for the remainder of the week.
It is an intense high and is expected to grow to over 1028hPa by Thursday. There is expected to be a squash zone of strong winds on it northern side over the Tonga area from late Wednesday and affecting the in the Niue/Southern Cooks areas on Thu/Fri UTC Avoid.
Next HIGH is expected to cross Tasmania on Friday and then southern NZ on Saturday and linger over Chatham Islands area for the next week, with a squash zone of strong E /NE winds over northern NZ. This is the opposite of the El Nino pattern so is just an atmospheric fling. It is a good pattern for sailing from New Caledonia/Vanuatu to Australia.
Travelling Tahiti to Tonga
The north end of a passing front is expected to bring a period of changing winds and maybe a few showers to Society Islands on local Sunday (Monday UTC) followed by good SE winds for a departure to the west on local Monday/Tuesday. However a departure on local Monday is likely to encounter period of strong SE winds on local wed/Thu. A departure on local Tuesday/Wednesday should only have a brief encounter with strong SE winds on Thursday. And a Thursday departure should miss the strong SE winds.
NZ
TROUGH moving across Tasmania on Monday UTC is expected to travel across NZ on Wednesday/Thursday/Friday. The north end of this trough is expected to bring a change to light southerlies and showery weather to New Caledonia/Vanuatu on Friday , and this is then expected to deepen into a LOW near 25S 175E by Saturday UTC, with strong to gale NE winds in a zone between NZ and Tonga. This Low is forecast to travel SE and bring some sort of easterly gale to northern NZ by Sunday UTC, possibility lingering for a few days.
Between Tonga and NZ:
Southern Tonga is expected to have strong ESE winds in a squash zone from Wednesday and between Tonga and NZ there is likely to be strong to gale NE winds from Friday to Sunday, with strong easterly winds persisting near Northland into next week. SO not a good week to sail from Tonga to NZ. If you are going the other way then go north to west of around 175E, so that you can go clockwise around the developing Low.
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See my yotpak at boatbooks.co.nz/weather.html for terms used.
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