Subtropical Depression SEVENTEEN

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SUBTROPICAL DEPRESSION SEVENTEEN DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL172010
500 AM AST WED OCT 06 2010
 
SATELLITE IMAGES INDICATE THAT THE AREA OF LOW PRESSURE NORTH OF
PUERTO RICO HAS DEVELOPED BANDS OF DEEP AND CURVED CONVECTION IN
THE NORTHERN AND EASTERN QUADRANTS.  IN ADDITION...NIGHT VISIBLE
IMAGES SUGGEST THAT THE CENTER HAS BECOME BETTER-DEFINED SINCE
YESTERDAY.  SATELLITE CLASSIFICATIONS ARE A TROPICAL 2.0 FROM
TAFB...AND A SUBTROPICAL 1.5 FROM TAFB.  WHILE THE LOW IS NOT FAR
FROM BEING MORE TROPICAL THAN SUBTROPICAL...ITS CURRENT LARGE
RADIUS OF MAXIMUM WINDS...AT LEAST 120 N MI...AND ITS ENTANGLEMENT
WITH AN UPPER LOW LEAD ME TO START THE SYSTEM AS A SUBTROPICAL
DEPRESSION.  THE INITIAL WINDS ARE SET TO 30 KT...IN AGREEMENT WITH
A FEW SHIP AND BUOY OBSERVATIONS DURING THE PAST SEVERAL HOURS.

MOST OF THE MODEL GUIDANCE SHOW THE UPPER-LEVEL LOW TO THE
WEST-SOUTHWEST OF THE DEPRESSION MOVING SLOWLY SOUTHEASTWARD AND
WEAKENING OVER THE NEXT DAY OR SO.  THIS SHOULD ALLOW THE
DEPRESSION TO MOVE ON THE NORTHERN SIDE OF THAT LOW...WHICH WOULD
PLACE IT INTO A MORE FAVORABLE AREA FOR STRENGTHENING WITH LIGHTER
WIND SHEAR.  SINCE THE SYSTEM IS OVER WARM WATERS AND THE MODELS
WEAKEN THE UPPER-LEVEL LOW...TRANSITION TO A TROPICAL CYCLONE SEEMS
LIKELY IN A DAY OR SO.  BECAUSE OF THE CURRENT STRUCTURE OF THE
CYCLONE...ONLY A SLOW INTENSIFICATION IS EXPECTED BECAUSE IT COULD
TAKE SOME TIME TO SHED ALL OF ITS SUBTROPICAL CHARACTERISTICS. 
VERTICAL WIND SHEAR IS LIKELY TO INCREASE IN A COUPLE OF DAYS...
WHICH WILL PROBABLY HALT ANY STRENGTHENING AFTER THAT TIME.  THE
NHC INTENSITY FORECAST IS ON THE LOW SIDE OF THE GUIDANCE GIVEN THE
INITIAL SUBTROPICAL STRUCTURE...AND IS GENERALLY BETWEEN THE MODEL
CONSENSUS AND THE LGEM.  

BEST ESTIMATE OF INITIAL MOTION IS ABOUT 325/7.  THE DEPRESSION IS
MOVING NORTHWESTWARD BETWEEN THE UPPER-LEVEL LOW AND A LARGE RIDGE
OVER THE CENTRAL ATLANTIC OCEAN.  THE RIDGE SHOULD BE ERODED AWAY
IN A DAY OR TWO BY A SERIES OF MID-LATITUDE TROUGHS MOVING ACROSS
THE WESTERN ATLANTIC OCEAN.  THIS PATTERN SHOULD CAUSE THE SYSTEM
TO SLOW DOWN BY LATE TONIGHT...THEN TURN TOWARD THE NORTHEAST AND
ACCELERATE BY EARLY FRIDAY DUE TO THE INCREASING MID-LATITUDE FLOW. 
OVERALL THE MODEL AGREEMENT IS PRETTY GOOD FOR A WEAK SYSTEM AND
THE NHC FORECAST IS NEAR OR SLIGHTLY EAST OF THE DYNAMICAL MODEL
CONSENSUS.  EXTRATROPICAL TRANSITION SEEMS LIKELY BY 96 HR...IF NOT
SOONER...AS DEPICTED BY MOST OF THE GLOBAL MODELS.    
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      06/0900Z 22.2N  67.0W    30 KT
 12HR VT     06/1800Z 23.0N  67.7W    35 KT
 24HR VT     07/0600Z 23.6N  68.0W    40 KT
 36HR VT     07/1800Z 24.2N  67.5W    45 KT
 48HR VT     08/0600Z 25.0N  66.1W    45 KT
 72HR VT     09/0600Z 28.0N  60.5W    45 KT
 96HR VT     10/0600Z 32.0N  50.0W    40 KT...POST-TROP/EXTRATROP
120HR VT     11/0600Z 36.5N  37.5W    35 KT...POST-TROP/EXTRATROP
 
$$
FORECASTER BLAKE
 
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