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

23 November 2014

Bob Blog

WEATHERGRAM
YOTREPS
Issued 23 November 2014
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.

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, slightly relaxed for a while in October, and dived below
-10 early in November, but is now relaxing a little.

TROPICAL TOPICS
Tropical cyclone activity has now reduced to one possible area in the Indian
Ocean, and an area of interest to the NW of Samoa.

The latest tropical disturbance summary at www.usno.navy.mil/NOOC/nmfc-
ph/RSS/jtwc/ab/abpwweb.txt
has this to say about this system:
ABPW10 PGTW 230600
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/SIGNIFICANT
TROPICAL WEATHER ADVISORY FOR THE WESTERN AND /SOUTH PACIFIC
OCEANS/230600Z-240600ZNOV2014// RMKS/ 1. WESTERN NORTH PACIFIC AREA (180 TO
MALAY PENINSULA):
A. TROPICAL CYCLONE SUMMARY: NONE.
B. TROPICAL DISTURBANCE SUMMARY: NONE.
2. SOUTH PACIFIC AREA (WEST COAST OF SOUTH AMERICA TO 135 EAST):
A. TROPICAL CYCLONE SUMMARY: NONE.
B. TROPICAL DISTURBANCE SUMMARY:
(1) THE AREA OF CONVECTION PREVIOUSLY LOCATED NEAR 8.2S 177.1W, IS NOW
LOCATED NEAR 11.5S 174.4W, APPROXIMATELY 204 NM NORTH OF PAGO PAGO, AMERICAN
SAMOA. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS UNORGANIZED FLARING
CONVECTION ASSOCIATED WITH AN ELOGATED LOW-LEVEL CIRCULATION CENTER. A
230354Z NOAA-19 MICROWAVE IMAGE REVEALS VERY FRAGMENTED CONVECTIVE BANDING
ON THE EASTERN SIDE BROADLY WRAPPING INTO THE CENTER. ALSO, A CIMMS 230300Z
850MB RELATIVE VORTICITY PRODUCT SHOWS AN ELONGATED VORTICITY SIGNATURE.
UPPER-LEVEL ANALYSIS INDICATES THE DISTURBANCE IS LOCATED NORTH OF THE RIDGE
AXIS IN A FAVORABLE ENVIRONMENT WITH LOW TO MODERATE (05 TO 15 KNOT)
VERTICAL WIND SHEAR AND EXCELLENT POLEWARD OUTFLOW.
ADDITIONALLY, SEA SURFACE TEMPERATURES WITH A RANGE OF 26 TO 28 DEGRESS
CELCIUS IN THE AREA ARE FAVORABLE FOR DEVELOPMENT. MAXIMUM SUSTAINED SURFACE
WINDS ARE ESTIMATED AT 20 TO 25 KNOTS. MINIMUM SEA LEVEL PRESSURE IS
ESTIMATED TO BE NEAR 1004 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A
SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS REMAINS LOW.
(2) NO OTHER SUSPECT AREAS.//

It is judged as having a low chance of any further deepening. The GFS
models and their GRIB files are picking that this system may deepen into a
depression on Mon/Tuesday and bring clockwise gale winds between the Niuas
and Niue, then Southern Cooks on Wednesday/Thursday- but the more reliable
ECMWF model suggests this isn't likely.

Weekly rain maps over the past fortnight show an increase in convection over
the past week in the Indian Ocean. This is possibly part of a MJO cycle
that will gradually make its way into the Pacific Ocean over the next few
weeks. For the next week or so, it increases the risk of a tropical cyclone
formation in the area to NW of Aussie. For the following weeks this
increased risk spreads east. This is shown at
www.meteo.nc/cyclone/coin-des-experts

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ has strengthened in the past week and is expected to drift south onto
northern Tonga and SE onto NIUE, then later onto southern cooks.
The GFS model is picking that a depression may form on the SPCZ and move
along it to the SE, but other models only have a weak feature. The GFS
model also has a tropical low forming SE of French Polynesia. These may not
actually happen but if you are sailing in these areas this week then brace
for strong winds anyway.

STR= Sub-tropical Ridge
The STR is strong and well defined and mostly at its normal latitude for the
time of year, but north of normal in the Tasman Sea/ NZ area.
HIGH at 30S between New Caledonia and New Zealand is expected to stay put
until Wednesday and then slowly fade away as it travels east from Thursday
to Saturday.
NEXT High cell is expected to travel east along 45S from Saturday 29 Nov ,
reaching North island around Wed 3 December - followed by a northerly flow
-that is GOOD for arriving in NZ  late next week (Thursday to Saturday 4-6
Dec).

Between the tropics and NZ
At North Minerva:
Trade winds until Thursday, light and variable on Fri 28 to Mon 1 Dec, then
more trade winds. Good idea to depart around Thursday so as to catch a
northerly flow when approaching NZ  voyage will encounter a period of
southerly winds around Sun 30 Nov/Mon 1 Dec and requires waypoints.

Over northern NZ:
Westerly flow on Monday/Tuesday, then an active trough on Wednesday followed
by a SW/S flow on Thursday 27 to Tues 2 Dec. NOT the best week (after
Tuesday) for arriving in NZ.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
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