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

14 December 2014

Bob Blog

WEATHERGRAM
YOTREPS
Issued 14 December 2014
Bob McDavitts ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the
patterned world.

The Ocean: extra has been stored in the Pacific Ocean for a while now and
all the Oceanic indices have crossed the threshold into El Nino territory.

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
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.
It has been negative since July and dived below -10 (Australian units) for
much of September, and again for a week in November, but has since been
relaxing again the atmospheric El Nino is stuttering In an El Nino event
the latitudes of the normal weather zones are drawn closer to the equator,
causing the South Pacific Convergence zone to shift further north and east
of its normal position.

TROPICAL TOPICS
A MJO (Madden Julian Oscillation of enhanced convection) is making its way
now eastwards across the South Pacific Ocean this week, and this has formed
near-equatorial west/NW winds over northern Australia and in the northern
Coral Sea. These may reach 180 in the next week or so, making for
challenging conditions for fishing and diving in Tuvalu and Kiribati.
MJO cycles are usually paced about 6 weeks apart and are known to increase
the risk of tropical cyclone formation as they pass. This MJO has triggered
a small Tropical Cyclone over the open sea to SW of Indonesia called BAKUNG
near 9S 87E. However this MJO is early in the season and is NOT expected to
form a TC in the Pacific.

Weekly rain maps over the past fortnight show an increase in convection over
the past week spreading eastwards across Northern Australia. Also the rain
that was over Philippines with HAGIPUT about a week ago has gone now.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ is now split into several distinct convergence zones, and this makes
the next week more volatile than normal. The MJO does this, but I think this
MJO might not trigger a TC in our region. That just raises the ante for the
next MJO, which is likely in January. I suppose thats good news for
Christmas /New Year travellers.
The MJO has triggered a series of sub-tropical troughs in the northern
Tasman SeaOne is passing by northern NZ tonight and gave wind and rain here
for past 24 hours, upsetting our Christmas in the Park in Auckland last
night and cancelling at least one local youth group picnic today.
Yuck. But it is a sign of summer, I suppose.
After this week there should be two Convergence Zones: One visiting New
Caledonia and Fiji for Christmas and another, likely to be intense, between
Samoa and Southern Cooks/ French Polynesia. Disturbing Christmas weather
here.

STR= Sub-tropical Ridge
The troughs forming in the northern Tasman Sea have dislodged the STR to
southern NZ (first time in weeks, a nice change for them). One High is
expected to move off to NE of NZ over next few days as another Low crosses
the Tasman Sea from Thursday to Sunday 21st (over South Island).
This should set things up so that next HIGH crosses central Tasman Sea 40S
from Sunday to Wednesday (fading over North Island) and the following HIGH
travels along 45S from Wed crossing central NZ on Friday/Saturday.
Happy settled weather for Christmas to NZ.

Over northern NZ
Strong East/NE/NW winds on Monday/Tuesday/Wednesday from passing trough,
then quieter northerly winds until a weak trough is expected on Sun/Mon
21/22 Dec, and then SE winds.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
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