ZCZC MIATCDAT4 ALL TTAA00 KNHC DDHHMM TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042006 1100 AM EDT TUE AUG 22 2006 DVORAK INTENSITY ESTIMATES AT 12Z WERE 30 KT FROM TAFB AND AFWA... AND 35 KT FROM SAB. SINCE THEN CONVECTION HAS DECAYED SOMEWHAT AND SO THE ADVISORY INTENSITY WILL REMAIN 30 KT. HIGHER VALUES IN THE 8Z QUIKSCAT PASS ARE BELIEVED TO BE RAIN CONTAMINATED. THERE IS CLEAR ROTATION IN THE MID-LEVEL CLOUDS THAT COULD BE REPRESENTATIVE OF THE SURFACE CENTER...BUT THE LOW CLOUD LINES SUGGEST THE CENTER COULD BE DISPLACED A BIT TO THE SOUTHEAST. THE QUIKSCAT AMBIGUITIES SLIGHTLY FAVOR A DISPLACED CENTER. SO WITH SOME UNCERTAINTY...THE INITIAL MOTION IS ESTIMATED TO BE 295/14. GLOBAL MODEL GUIDANCE IS UNANIMOUS IN SHIFTING THE SUBTROPICAL RIDGE EASTWARD WITH INCREASED MID- TO UPPER-LEVEL TROUGHING IN THE CENTRAL ATLANTIC OVER THE NEXT SEVERAL DAYS. THIS SHOULD ALLOW THE DEPRESSION TO MAINTAIN ITS PRESENT WEST-NORTHWESTWARD HEADING FOR AT LEAST THE NEXT THREE DAYS. AFTER THAT...THERE SEEM TO BE TWO PRIMARY OPTIONS. THERE IS GOOD AGREEMENT IN THE GLOBAL MODELS THAT THE CYCLONE WILL ENCOUNTER INCREASED SOUTHERLY SHEAR. THIS COULD TURN THE CYCLONE NORTHWARD...IF THE SYSTEM CAN HANG TOGETHER. ALTERNATIVELY...THE SYSTEM COULD SHEAR OFF AND THE WEAKENED REMNANTS TURN WESTWARD. THE FORMER SCENARIO IS FAVORED BY THE GFS AND GFDL...WHILE THE LATTER SCENARIO IS THE CHOICE OF THE UKMET AND ECMWF MODELS. FOR NOW...I'VE MADE NO CHANGES TO THE PREVIOUS ADVISORY TRACK...WHICH SPLITS THE DIFFERENCE. THERE IS A LOT OF STABLE AIR IN THE ENVIRONMENT AHEAD OF THE DEPRESSION. NEVERTHELESS...BOTH THE SHIPS AND GFDL GUIDANCE INDICATE STRENGTHENING TO AT LEAST A STRONG TROPICAL STORM. GIVEN THAT THE DEPRESSION HAS NOT STRENGTHENED YET OVER THE WARMEST WATERS IT WILL SEE FOR THE NEXT FOUR DAYS...I WONDER IF THE GUIDANCE IS A LITTLE TOO AGGRESSIVE. FORECAST POSITIONS AND MAX WINDS INITIAL 22/1500Z 13.6N 26.1W 30 KT 12HR VT 23/0000Z 14.6N 28.1W 35 KT 24HR VT 23/1200Z 16.1N 30.7W 40 KT 36HR VT 24/0000Z 17.6N 33.4W 45 KT 48HR VT 24/1200Z 19.2N 36.3W 50 KT 72HR VT 25/1200Z 22.0N 42.5W 60 KT 96HR VT 26/1200Z 25.0N 48.0W 65 KT 120HR VT 27/1200Z 28.0N 53.0W 65 KT $$ FORECASTER FRANKLIN NNNN
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Page last modified: Monday, 11-Sep-2006 11:28:35 UTC