BOBGRAM issued 22 Dec

WEATHERGRAM
YOTREPS
Issued 22 December 2013
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 (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.
SOI is currently steady after a recent rise. Its 30-day running mean was
0.66 on 22 Dec.

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 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.

Sea surface temperatures are now warmer than normal at Galapagos. But NOT
along the Peru Coast. This Peru Coast deserves special scrutiny during
December =96 because it is the arrival of warmer-than-normal water every
so often there in December that triggers the onset of an El Nino episode-
El Nino gets its name for the El Nino di Navidad (Christmas) festival in
Peru. This warm-water-arrival is NOT happening there this December.

TROPICAL TOPICS
Tropical cyclones AMARA and BRUCE formed in the India Ocean in the past
week as predicted in Weathergram issued last Sunday. They are moving SW
and expected to recurve to the SE at 20s. They have been formed from an
area on enhanced convection that is moving slowly east from the NW Indian
Ocean onto the Timor Sea during the next week or so. This pulse of extra
convection may continue eastwards and reach the northern Coral Sea area
early in the new year, increasing the chance of development there of
tropical depressions

WEATHER ZONES
As a Christmas present to any budding meteorologists on my list , I'd like
to share with you a cool Website for viewing the wind flow of the GFS
model at various levels on a weird collection of map projections go to
earth.nullschool.net, click on the word earth and then play with the
variations. Have a happy meteorological Christmas!

South Pacific Convergence Zone SPCZ
The SPCZ is in its normal position roughly between Solomons and
Fiji/Tonga.
A subtropical low is expected to form on the SPCZ over or around Fiji by
Friday 27 Dec and move across Tonga on Sat/Sun /Mon/Tuesday. Avoid

Sub-tropical Ridge STR
The STR is near its normal solstice position over the Australian Bight but
is currently weak and more fluid over the Tasman Sea/NZ area. On Boxing
Day and the following days a cold dry HIGH is expected to move NE out of
the Southern Ocean (south of NZ) and to the east of NZ and then east along
45S next week.

Tasman Sea /New Zealand
The Low that is over Tasmanian tonight is expected to deepen as it crosses
the Tasman Sea on Monday and then onto southern NZ on Tuesday. This
should trigger a new low to form off the Canterbury coast on Christmas Eve
and then travel NE to north of Chatham Islands on Christmas day. This
scenario brings a transition of wind flow over NZ from NW on Tuesday to W
then SW then S then SE for Boxing Day and then E for Friday. At some
stage in this transition there may be an OK period for having that
Christmas BBQ, but it will come and go, and some fine-tuning may be
needed to get the timing right.
See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at
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