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

20 November 2016

Bob Blog 20 Nov

WEATHERGRAM
YOTREPS
Compiled 20 November 2016

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.

Thanks to Kevin Fawcett (of Charlotte, New Carolina) for informing me that the
Niue Yacht club website at www.NYC.NU has a webcam looking out over Alofi Wharf,
with a library of 30 minute images. If there is weather data from the airport
that is added to the image.

Our week of perigean tides from that super moon is now over. I hope you all took
adequate precautions.

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) slipped into La Nina territory for a few weeks in
September and this triggered a few meteorological web sites to say "We are now
having a La Nina". Well, I'll let you judge the veracity of that: during October
it relaxed and in the past few weeks the SOI has been negative.
SOI trend (x10) since 2013 showing us in neutral territory may be seen at
www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour
into the air, where is rises and cools into cloud. The equatorial Pacific region
hosts the widest and warmest sea on the planet. Thus its sea surface
temperatures SST may be thought of as a factor in the running of planetary
weather engine. When SST in the target zone (equatorial Pacific between dateline
and Galapagos, called NINO3.4) are notably cooler than normal, this is called a
La Nina episode.
Sea surface temperature anomaly are cooler than normal, but not yet in La Nina
territory, and may be seen at
www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly
Compare the recent El Nino (warm seas) with its extreme predecessor in 1998.

The Tropics
The tropics are still taking a rest.
Rain accumulation maps show an increase in activity in the Coral Sea, and around
Panama, and a decrease around Fiji/Samoa. Rain maps are at
trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
The MJO has move out of the Pacific and is over the Americas are heading for the
Indian Ocean, so we should expect a few quiet weeks in the South Pacific
now..and activity to peak late in December.
A tropical low [1004] has formed today in Coral sea to NW of New Caledonia and
this is expected to travel south into Tasman Sea where it is expected to merge
or get captured by a deepening trough/low forming east of New South Wales on
Thursday and to travel mainly over southern NZ on Friday.
Tropical accumulated rainfall for next week may be seen at windyty.com

Subtropical ridge (STR)
The subtropical ridge is becoming stronger and wider this week.
The HIGH which is travelling east along 35S tonight past NZ and further east on
Mon/Tue/wed may briefly have a small squash zone of strong SE winds on its
northern side on Monday near 20S.
Next High in Tasman Sea is expected to form on Saturday and then travel slowly
along 30S and 35S during next week, enjoy.
These subtropical ridges offer reasonable weather for any yachts still seeking
to get from tropics to NZ/Australia, however there are light winds in the centre
of the ridge, and there are troughs in-between the ridges, so travelling in the
winds around the back side of a passing ridge may require sailing thru a
trough-that's OK if you do it at a good latitude mid trip, but may be
disconcerting if both you and the trough arrive at the destination the same day.

Tasman Sea/NZ Area troughs.
The tropical low [1004] NW of New Caledonia tonight is expected to travel south
into Tasman Sea on Monday and fade on Tuesday and merge or get captured by a
deepening trough/low forming east of New South Wales on Thursday and all this
should travel mainly over southern NZ on Friday. Associated trough is expected
to reach Opua around Friday afternoon- about the same time that I am scheduled
to give a talk about weather to the ICA seminar at Opua . hopefully it won't be
too much of a distraction, and will be over by BBQ time.

As of sailing Noumea to Australia: try and avoid that low which is expected to
form off New South Wales on Thursday morning. It looks like it may bring a
southerly buster.

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