ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM 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 NNNN
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Page last modified: Wednesday, 13-Sep-2006 08:40:02 UTC