Tropical Depression EIGHT

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TROPICAL DEPRESSION EIGHT DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 AM EDT WED SEP 13 2006
 
SATELLITE IMAGERY SHOWS THAT THE DEPRESSION IS POORLY
ORGANIZED...WITH DEEP CONVECTION REMAINING DISPLACED WEST OF THE
CENTER.  GIVEN THE ABSENCE OF CONVECTION NEAR THE CIRCULATION
CENTER...THE INITIAL INTENSITY IS HELD AT 30 KT IN ACCORDANCE WITH
LATEST DVORAK ESTIMATES.

THE CONVECTIVE PATTERN APPEARS TO BE DUE TO A STRONG LOWER TO
MID-LEVEL JET IMPINGING UPON THE DEPRESSION. UPPER-AIR SOUNDINGS
DURING THE PAST 24 HOURS FROM DAKAR AND SAL CAPE VERDE...ALONG WITH
SPECIAL SOUNDINGS FROM THE AMMA PROJECT...SHOW THAT A STRONG 700 MB
JET ACCOMPANIED THE SYSTEM AFTER IT EMERGED FROM AFRICA.
ADDITIONALLY...THE METEOSAT-8 SPLIT WINDOW CHANNEL DIFFERENCING
PRODUCT PRODUCED BY UW-CIMSS SHOWS A LARGE SAHARAN AIR LAYER NORTH
OF THE SYSTEM. THERMAL WIND BALANCE WOULD ARGUE FOR A MID-LEVEL JET
ALONG THE SOUTHERN EDGE OF THAT VERY WARM AND STABLE AIRMASS. UNTIL
THE DEPRESSION CAN ESCAPE THIS SEEMINGLY UNFAVORABLE ENVIRONMENT...
SIGNIFICANT STRENGTHENING APPEARS LESS LIKELY. ADDITIONALLY...SUCH
A BROAD CIRCULATION PRECLUDES SIGNIFICANT STRENGTHENING IN THE
SHORT-TERM. THE 850-200 MB WIND SHEAR CALCULATION USED IN THE SHIPS
MODEL CANNOT RESOLVE THE APPARENT MID-LEVEL SHEAR IN THIS CASE.
ACCORDINGLY...THE INTENSITY FORECAST IS BELOW THE SHIPS
GUIDANCE...WHICH FORECASTS A HURRICANE WITHIN 48 HOURS...AND IS
CLOSER TO THE MORE CONSERVATIVE GFDL. HOWEVER...TOWARD THE END OF
THE FORECAST...THE GLOBAL MODELS INDICATE AN IMPROVED ENVIRONMENT
FOR STRENGTHENING. THE NEW FORECAST IS A LITTLE LOWER THAN THE
PREVIOUS ADVISORY...BUT STILL ANTICIPATES A HURRICANE LATE IN THE
PERIOD. 
 
THE BROAD CIRCULATION OF THE SYSTEM MAKES IT DIFFICULT TO PRECISELY
IDENTIFY A CENTER. TRACKING THE MEAN CENTER OF THE LARGE GYRE YIELDS
AN INITIAL MOTION ESTIMATE OF 275/14.  GLOBAL MODELS ARE IN GOOD
AGREEMENT THAT THE CYCLONE WILL BE STEERED AROUND THE SOUTHERN
PERIPHERY OF THE SUBTROPICAL RIDGE DURING THE NEXT 2-3 DAYS. 
THEREAFTER...THE CYCLONE WILL ENCOUNTER A BREAK IN THE RIDGE
RESULTING IN A GRADUAL NORTHWARD TURN AND DECELERATION.  THE
OFFICIAL FORECAST IS SHIFTED NORTH OF THE PREVIOUS ADVISORY AND
LIES JUST SOUTH OF THE DYNAMICAL MODEL CONSENSUS. 

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/0900Z 11.9N  26.7W    30 KT
 12HR VT     13/1800Z 12.2N  29.0W    30 KT
 24HR VT     14/0600Z 12.8N  32.3W    35 KT
 36HR VT     14/1800Z 13.7N  35.2W    40 KT
 48HR VT     15/0600Z 14.7N  37.9W    45 KT
 72HR VT     16/0600Z 17.5N  42.5W    55 KT
 96HR VT     17/0600Z 19.5N  45.0W    65 KT
120HR VT     18/0600Z 21.5N  47.5W    80 KT
 
$$
FORECASTER RHOME/KNABB
 
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