Bob Blog 25 Sep 2016
Posted on September 26, 2016
WEATHERGRAM
YOTREPS
Compiled 25 September 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.
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) has slipped into La Nina territory during past two
weeks. It was above 10 units on this Australian scale. If it can hold this high
for a month in a row the we can say we are having a La Nina.
SOI trend (x10) since 2013 showing us in La Nina 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) are notably cooler than normal, this is called a La Nina episode.
Sea surface temperature anomaly may be seen at
http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthl
y
We now have cooling seas in the target area, and there MAY be a weak La Nina in
the next few months
The Tropics
TC MEGA is in the NW Pacific, there are two tropical systems between Hawaii and
Mexico, and TC KARL and LISA are in the North Atlantic.
Cyclone tracks seen on ruc.noaa.gov/tracks/
Rain for the past fortnight may be seen at
trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif
and these rain accumulation maps show an increase in convection over Indonesia
and Philippines from the west, and an easing in convection over the South
Pacific.
WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ this week is expected to stay quiet in the Solomon islands to Tokelau area
No squash zones. A Passing trough may affect from Tonga to Niue late in the
week.
Subtropical ridge (STR)
This is expected to remain weak and along about 20 to 30S.It is NOT often this
far north and is weakening all the trade winds, offering rare opportunities for
getting east in an Island hopping fashion if you like.
Rain accumulation may be seen at windyty.com
Next High is expected to spread to east of NZ from the SOUTH after Wednesday,
shovelling polar chilled southerly winds northwards so that they deepen a LOW
near 35S 165W late in the week. Avoid travelling direct between NZ and Tahiti.
Travel from Tahiti to Tonga:
The weak winds that have been affecting Tahiti recently should be over from
tomorrow. Go for it. No squash zones, and no strong convergence zones, but
voyages are likely to encounter a passing trough with maybe some showers and
varying winds for a day or two. It is looking OK to venture via Suwarrow this
week.
Travel between the Tropics and NZ:
A series of LOWS continue to travel across Tasman Sea/Northern NZ area over next
10 days, with a brief break for a passing ridge on Thu/Fri. These lows are not
really making gales, but the fronts do have strong winds. Avoid
arrival/departure on Sat 1 Oct or Thu 6 Oct.
Travel between New Caledonia and Australia:
Intense TROUGH crossing north Tasman sea on Mon 26 Sep, then OK weather for
Australia to New Caledonia. Another intense trough in N Tasman sea on Thu/Fri
29/30 Sep to avoid.
As for getting form New Caledonia to Australia: not this week, too much
headwind, and probably not next week. Maybe OK from around 8-10 Oct.
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See my yotpak at boatbooks.co.nz/weather.html for terms used.
See my website http://www.metbob.com for information on tailor-made voyage
forecasts- Feedback to bob@metbob.com.
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