Tropical Depression EIGHTEEN

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TROPICAL DEPRESSION EIGHTEEN DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 PM EDT SAT SEP 17 2005
 
SATELLITE IMAGERY AND NEARBY BUOY AND SHIP OBSERVATIONS INDICATE THE
TROPICAL DISTURBANCE LOCATED EAST OF THE TURKS AND CAICOS HAS
ACQUIRED ENOUGH CONVECTIVE ORGANIZATION TO BE CLASSIFIED AS A
TROPICAL DEPRESSION. THE INITIAL INTENSITY OF 25 KT IS BASED ON A
SATELLITE INTENSITY ESTIMATE OF 25 KT FROM TAFB AND EARLIER SHIP
REPORTS. BANDING HAS INCREASED IN THE EASTERN SEMICIRCLE AND
SEVERAL MODERATE CONVECTIVE CELLS HAVE RECENTLY DEVELOPED AROUND
THE SURFACE CENTER.

THE INITIAL MOTION ESTIMATE IS 290/09. TD-18 IS EXPECTED TO MOVE
WEST-NORTHWESTWARD FOR THE NEXT 48 HOURS TOWARD A WEAKNESS IN THE
SUBTROPICAL RIDGE THAT LIES JUST EAST OF FLORIDA. AFTER THAT...
MOST OF THE GLOBAL MODELS AGREE THAT THE WEAKNESS WILL GRADUALLY
FILL AS THE RIDGE OVER TEXAS AND THE GULF COAST BUILDS EASTWARD.
HOWEVER...THE EXACT TIMING OF THE FILLING RDIGE WEAKNESS IS
UNCERTAIN AND THAT WILL DETERMINE HOW FAR NORTH THE CYCLONE MOVES
BEFORE IT TURNS WESTWARD. THE GLOBAL MODELS AND THE GFDL MODEL KEEP
THE SYSTEM OVER OR SOUTH OF THE FLORIDA KEYS. ONLY THE CANADIAN
MODEL AND THE TIGHTLY PACKED BAM MODELS BRING THE STORM ACROSS THE
SOUTHERN PORTION OF THE FLORIDA PENINSULA IN 60-72 HOURS. SINCE THE
DEPRESSION IS STILL IN THE FORMATIVE STAGE...THE CIRCULATION CENTER
COULD EASILY DEVELOP ANY WHERE WITHIN THE CONVECTIVE CLOUD MASS...
WHICH COULD RESULT IN NORTH-SOUTH SHIFTS IN THE FORECAST TRACK
DURING THE NEXT DAY OR SO. ALL INTERESTS IN THE BAHAMAS...SOUTH
FLORIDA...THE FLORIDA KEYS...AND CENTRAL AND WESTERN CUBA SHOULD
PAY CLOSE ATTENTION TO THIS SYSTEM OVER THE NEXT FEW DAYS.

THE INTENSITY FORECAST IS MUCH MORE OF A CHALLENGE THAN THE TRACK
FORECAST IS. THE VERTICAL SHEAR IS FORECAST TO DECREASE TO LESS
THAN 10 KT IN THE 36-60 HOUR TIME FRAME...WHILE THE CYCLONE IS OVER
30C AND WARMER SSTS. THIS WOULD FAVOR RAPID INTENSIFICATION IF DRY
MID-LEVEL AIR DOES NOT GET ENTRAINED INTO THE CENTER. AFTERWARDS...
THE GFS-BASED SHIPS INTENSITY MODEL GRADUALLY INCREASES THE
WESTERLY SHEAR TO MORE THAN 20 KT AND LEVELS OFF THE INTENSITY AT
73 KT. HOWEVER...THE NOGAPS...UKMET...AND CANADIAN MODELS SHOW A
200 MB ANTICYCLONE WITH AN EAST-WEST ORIENTED RIDGE AXIS EXTENDING
ACROSS THE FLORIDA KEYS BY 48 HOURS. EVEN THE GFS DEVELOPS A
SIMILAR UPPER-LEVEL PATTERN...ONLY ACROSS THE SOUTHERN FLORIDA
PENINSULA. THE CLOSE PROXIMITY TO SUCH A FAVORABLE UPPER-LEVEL
OUTFLOW PATTERN SUGGESTS THAT THIS TROPICAL CYCLONE COULD BE MUCH
STRONGER BY 72 HOURS THAN WHAT IS CURRENTLY FORECAST BY THE SHIPS
MODEL. THE 18Z GFDL MODEL RAPIDLY INTENSIFIES THE SYSTEM INTO A
120-KT HURRICANE IN 72 HOURS. WHILE THAT RATE OF INTENSIFICATION
MAY BE A LITTLE EXCESSIVE...THIS SYSTEM REACHING STRONG CATEGORY 2
STATUS WITHIN THE NEXT 72 HOURS IS CERTAINLY A VIABLE SCENARIO.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0300Z 22.0N  69.7W    25 KT
 12HR VT     18/1200Z 22.5N  71.5W    35 KT
 24HR VT     19/0000Z 23.1N  73.6W    40 KT
 36HR VT     19/1200Z 23.6N  75.5W    50 KT
 48HR VT     20/0000Z 24.0N  77.7W    60 KT
 72HR VT     21/0000Z 24.5N  82.0W    70 KT
 96HR VT     22/0000Z 25.0N  86.0W    75 KT
120HR VT     23/0000Z 25.0N  90.5W    80 KT

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