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| #4078 (Received by flhurricane at: 5:38 PM 09.Aug.2004) TCDAT2 TROPICAL STORM BONNIE DISCUSSION NUMBER 7 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 PM EDT MON AUG 09 2004 AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT INVESTIGATING THE REMNANTS OF FORMER TROPICAL DEPRESSION TWO FOUND A SMALL BUT VERY TIGHT SURFACE CIRCULATION AND 56 KT 1500 FT FLIGHT-LEVEL WINDS. NORMALLY THIS WOULD EQUATE TO 48 SURFACE WINDS...BUT THE CONVECTION OVER THE CENTER IS SMALL AND NOT PARTICULARLY WELL ORGANIZED. THE RAPID SPIN UP IN THE LOW-LEVEL WIND FIELD COULD EASILY SPINDOWN IF THE INNER-CORE CONVECTION WEAKENS OR DISSIPATES. SO THE INITIAL INTENSITY IS A BLEND OF THE RECON WINDS AND A DVORAK SATELLITE INTENSITY ESTIMATE OF T2.5...OR 35 KT...FROM TAFB. THE INITIAL MOTION ESTIMATE IS 310/10. THERE IS LIMITED MODEL GUIDANCE AVAILABLE FOR THIS FORECAST PACKAGE SINCE NONE OF THE GLOBAL MODELS INITIALIZED THE BONNIE CIRCULATION VERY WELL AT 12Z. HOWEVER...ALL OF THE GLOBAL AND REGIONAL MODELS AGREE ON TAKING A WEAK MID-LEVEL TROUGH FROM OVER THE SOUTHERN PLAINS SOUTHEASTWARD TO THE NORTH-CENTRAL GULF OF MEXICO AND WEAKENING THE SUBTROPICAL RIDGE TO THE NORTH OF BONNIE. THE RESULT SHOULD BE A GRADUAL TURN TOWARD THE NORTH WITH A DECREASE IN FORWARD SPEED AS THE STEERING CURRENTS WEAKEN. BY 48 HOURS...BONNIE IS EXPECTED TO TURN NORTHEASTWARD AND GRADUALLY ACCELERATE AHEAD OF A SECOND AND STRONGER MID-LEVEL TROUGH THAT IS FORECAST TO PUSH SOUTHEASTWARD INTO THE WESTERN GULF OF MEXICO IN 48 TO 72 HOURS. THE OFFICIAL FORECAST TRACK IS A BLEND OF THE GFDL...DEEP AND MEDIUM BAM...AND THE GFS 10-MEMBER ENSEMBLE MODEL FORECASTS. THE INTENSITY FORECAST IS SOMEWHAT PROBLEMATIC. BONNIE HAS A SMALL AND TIGHT CIRCULATION...AND WILL BE MOVING INTO A LOWER VERTICAL SHEAR REGION OVER THE CENTRAL GULF OF MEXICO...WHICH WOULD NORMALLY FAVOR SIGNIFICANT INTENSIFICATION TO OCCUR. THE SHIPS MODEL BRINGS BONNIE TO 83 KT IN 72 HOURS...WHILE THE GFDL TAKES THE CYCLONE UP TO 74 KT AT THE SAME TIME PERIOD. THE ARGUMENT AGAINST FORECASTING ANY ROBUST INTENSIFICATION FROM OCCURRING...AT LEAST IN THE SHORT TERM...IS THE ABUNDANCE OF VERY DRY AIR SURROUNDING THE CYCLONE... AS SEEN IN WATER VAPOR IMAGERY...THAT BONNIE WILL HAVE TO NAVIGATE THROUGH FOR THE NEXT 24 TO 48 HOURS. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 09/2100Z 23.2N 88.7W 40 KT 12HR VT 10/0600Z 24.3N 90.0W 45 KT 24HR VT 10/1800Z 25.3N 90.3W 45 KT 36HR VT 11/0600Z 26.5N 90.2W 50 KT 48HR VT 11/1800Z 27.4N 89.5W 50 KT 72HR VT 12/1800Z 30.0N 86.5W 50 KT 96HR VT 13/1800Z 34.0N 80.5W 25 KT...INLAND 120HR VT 14/1800Z...DISSIPATED |
