Bob blog 18 Sec 2016

WEATHERGRAM
YOTREPS
Compiled 18 December 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.

There is a burst of near equatorial west to NW winds stretching from Indian
Ocean to the Western South Pacific. The associated monsoonal trough is expected
to drift close to North Australia and may deepen and contribute to tropical low
development.
The MJO index is currently weak and not really contributing much to this, but
when we get near equatorial west to NW winds in mid-December, something usually
develops within a week or so.

The Tropics
TC VARDAH was nearing its peak this time last week as it approached Chennai.
During the past week it went west completely across India and the Arabian Sea
and is now approaching the Somalia area as a depression.

TD04F/99P formed last week to NW of Fiji and has been a slow-moving tropical
depression, dropping its heavy rains over the Fiji area all week and, so far,
not quite making the required wind flow to be a Category 1 tropical cyclone.

This is a good time to show the various web sites that provide information about
the thinking of the monitoring meteorologists as you look at the satellite
imagery:

Fiji Met Service have the primary role for South Pacific cyclones east of 160
East and they issue daily a 3-day outlook at www.met.gov.fj/tc_outlook.pdf

Their latest discussion is at www.met.gov.fj/aifs_prods/20036.txt

Tonight this says:
Tropical Disturbance Summary For area Equator to 25S, 160E to 120WISSUED FROM
RSMC NADI Dec 172316 UTC.TROPICAL DEPRESSION 04F CENTRE [1000HPA] WAS ANALYSED
NEAR 16.5S174.9E AT 172200UTC. POSITION GOOD BASED ON HIMAWARI-8 VIS
IMAGERY.TD04F SLOW MOVING. CONVECTION DISPLACED TO SOUTHEAST OF LLCC. SYSTEM
LIES IN AN UPPERDIVERGENCE REGION AND IN A MODERATE TO HIGH SHEARED ENVIRONMENT.
GLOBAL MODELS MOVE TD04F INTIALLY TOWARDS THE WEST THEN SOUTHWARDSWITH LITTLE
INTENSIFICATION. POTENTIAL FOR THIS SYSTEM TO DEVELOP INTO A TROPICAL CYCLONE
IN THENEXT 24 TO 48 HOURS IS LOW.
********************************************************************
NO OTHER SIGNIFICANT TROPICAL DISTURBANCE ANALYSED OR FORECAST IN THEAREA.

If a feature is worthy enough, an advisory is issued at
www.met.gov.fj/aifs_prods/20032.txt
TROPICAL DISTURBANCE ADVISORY NUMBER A10 ISSUED FROM RSMC NADIDec 162013 UTC.
TROPICAL DEPRESSION TD04F CENTRE 1000HPA WAS ANALYSED NEAR 15.9S174.5E AT
161800UTC. POSITION GOOD BASED ON HIMAWARI-8 IR IMAGERY.DEPRESSION SLOW MOVING.
MAXIMUM 10-MINUTE AVERAGE WINDS NEAR THECENTRE ESTIMATED AT ABOUT 25 KNOTS.
LLCC EXPOSED. CONVECTION DISPLACED TO SOUTHEAST OF LLCC. OUTFLOW GOODTO THE
SOUTH AND EAST BUT RESTRICTED ELSEWHERE. SST AROUND 29 DEGREE.SYSTEM LIES IN A
LOW UPPER DIVERGENCE REGION AND IN A MODERATESHEARED ENVIRONMENT. DVORAK
ANALYSIS BASED ON SHEAR PATTERN WITH LLCCABOUT LESS THAN 3/4 FROM DEEP
CONVECTION YIELDS DT=2.5, MET=2.0 ANDPT=2.0. FT BASED ON PT. THUS YIELDING,
T2.0/2.0/S0.0/24HRS. GLOBAL MODELS HAVE PICKED UP THE SYSTEM AND KEEP IT SLOW
MOVING WITHSLIGHT INTENSIFICATION. POTENTIAL FOR THIS SYSTEM TO DEVELOP INTO A
TROPICAL CYCLONE IN THENEXT 24 TO 48 HOURS IS LOW. FORECASTS :AT 12 HRS VALID
AT 170600 UTC 15.7S 174.5E SLOW MOVING WITH 25KTCLOSE TO CENTREAT 24 HRS VALID
AT 171800 UTC 15.7S 174.4E SLOW MOVING WITH 30KTCLOSE TO CENTRE. OUTLOOK: AT 36
HRS VALID AT 180600 UTC 16.1S 174.4E SLOW MOVING WITH 30KTCLOSE TO CENTREAT 48
HRS VALID AT 181800 UTC 16.9S 174.2E SLOW MOVING WITH 30KTCLOSE TO CENTRE. THIS
WILL BE THE LAST TROPICAL DISTURBANCE ADVISORY ON TD04F. TD04FWILL CONTIUNE TO
BE MONITORED AND ADVISORY WILL BE REISSUED IF CHANCES INCREASES FOR IT TO
DEVELOP INTO A TROPICAL CYCLONE.
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And a TRACK and THREAT map are produced when a cyclone is being tracked.
The track map is at www.met.gov.fj/aifs_prods/65661.html

MetService in NZ have a cyclone monitoring website at
www.metservice.com/warnings/tropical-cyclone-activity
tonight it says:
Tropical Cyclone Potential Bulletin
TROPICAL CYCLONE POTENTIAL BULLETIN ISSUED BY METEOROLOGICAL SERVICE OF NEW
ZEALAND
AT 0236 UTC 18-Dec-2016
CURRENT STATUS OF CYCLONE ACTIVITY
There are presently no tropical cyclones in the Coral Sea or South
Pacific areas.
FORECAST TO 1200 UTC 19-Dec-2016
Tropical depression TD04F, currently near 16.5S 174.9E, or 160nm
northwest of Viti Levu, is in a moderate shear envoronment,and is
expected to remain stationary or slowly move south-southeast. An
approaching shortwave trough is expected to increase the vertical
wind shear of the system, and the potential for TD04F to strengthen
into a tropical cyclone remains LOW.
A weak low northwest of Rarotonga is not expected to strengthen into
a tropical cyclone. A weak low may develop over the Solomon Sea
during the next 48 hours, but is also not expected to develop into a
TC.
OUTLOOK TO 1200 UTC 22-Dec-2016
TD04F is expected to slowly move south tracking close to Fiji while
gradually easing. Tropical cyclone development is NOT expected
elsewhere in the forecast area during the forecast period.
The next bulletin will be issued by 0500 UTC Mon 19-Dec-2016
(C) Copyright Meteorological Service of New Zealand Ltd 2016

Streamline Analysis
This streamline analysis map indicates wind flow near the earth's surface. The
areas where the arrowed streamlines coloured in teal move together (convergence)
and move away from each other (divergence) help determine the location of
features within the wind pattern, such as lows or lines of significant weather.
The map also shows surface pressure coloured in black, for areas south of the
Tropics.

This page also has an animated and colour coded IR satellite imagery to show the
convective clouds.
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Other sites watching tropical cyclones are
www.tropicaltidbits.com/storminfo/
and ruc.noaa.gov/tracks/

Rain accumulation weekly maps from
trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif shows the intense
prolonged rainfall from the depression near Fiji, and, to a lesser extent, the
rack of TC VARDAH. The Intertropical convergence zone seems to show decreasing
activity.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
TD04F/99P is expected to drift slowly south to 20S by Thursday and then move off
to the SE.
There is also likely to be a tropical low travelling SW along the coast from
Darwin towards Broome (NW Australia) from Tuesday to Saturday and this may then
go south and inland over the weekend.

Subtropical ridge (STR)
The subtropical high in west Tasman sea tonight is expected to travel northeast
and cross to north of NZ by Wednesday and then to east of NZ. This High is
blocking the travels of TD04F. Its seems that TD04F now has to wait and t and
finally go south after this ridge has gone east.
Just like last week, a new HIGH is expected to squirt from Australian Bight into
South Tasman sea on Thursday and then NE across central Tasman sea and North
Island on early next week. This High should be in central Tasman Sea on
Christmas day, so that the most likely option for NZ Christmas/Boxing day
weather is a SW flow, and an inland trough= some inland afternoon showers + for
the Auckland area, in a word, EASING. (but the SW winds may pick up again on the
27th in the next passing front).

Tasman Sea/NZ Area troughs.
A Front and SW flow is moving off the North Island on Monday.
Another front is expected on Thursday and Friday, followed by up to strong SW
winds on Saturday.
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