Subtropical Storm MELISSA

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SUBTROPICAL STORM MELISSA DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL142013
500 PM AST TUE NOV 19 2013
 
MELISSA APPEARS TO BE ON THE CUSP OF MAKING THE TRANSITION FROM A
SUBTROPICAL TO A TROPICAL CYCLONE. TWO EARLIER ASCAT OVERPASSES
CAPTURED THE WIND FIELD IN THE EASTERN SEMICIRCLE...WHICH INDICATED
THAT THE OUTER WIND FIELD HAD WEAKENED AND THAT THE INNER-CORE WIND
FIELD HAD CONTRACTED INWARD WITH THE RADIUS OF MAXIMUM WINDS NOW AT
ABOUT 40-50 NMI. IN ADDITION...AN 1119 UTC AMSU OVERPASS SHOWED
THAT A PRONOUNCED UPPER-LEVEL WARM CORE HAD DEVELOPED OVER THE
LOW-LEVEL CENTER. THESE SATELLITE INDICATORS WOULD SUGGEST THAT
MELISSA WAS NOW A TROPICAL CYCLONE. HOWEVER...THE AMSU DATA ALSO
REVEALED THAT THERE WAS STILL A STRATOSPHERIC CONNECTION TO THE
WARM CORE. THIS...PLUS THE LACK OF ANY DEEP CONVECTION NEAR THE
WELL-DEFINED LOW-LEVEL CENTER...ARGUES FOR KEEPING MELISSA A
SUBTROPICAL STORM. SINCE THE ASCAT PASSES MISSED THE WESTERN
SEMICIRCLE WHERE THE STRONGEST WINDS WERE LIKELY OCCURRING IN SOME
VIGOROUS CONVECTION...THE INTENSITY IS BEING MAINTAINED AT 55 KT
BASED ON A SUBTROPICAL SATELLITE CLASSIFICATION FROM TAFB...AND
EARLIER 53-56 KT AMSU INTENSITY ESTIMATES FROM CIRA AND CIMSS.
 
THE INITIAL MOTION ESTIMATE IS NOW 040/14 KT. MELISSA HAS FINALLY
MADE THE MUCH ANTICIPATED TURN TO THE NORTHEAST AHEAD OF A STRONG
COLD FRONT AND ASSOCIATED DEEP-LAYER TROUGH. NHC MODEL GUIDANCE IS
IN EXCELLENT AGREEMENT ON THE CYCLONE ACCELERATING NORTHEASTWARD
OVER THE NEXT 2-3 DAYS BEFORE TURNING NORTHWARD AND MERGING WITH A
FRONTAL SYSTEM IN 4-5 DAYS OVER THE FAR NORTH ATLANTIC. THE
OFFICIAL FORECAST TRACK WAS SHIFTED TO THE RIGHT OF THE PREVIOUS
ADVISORY TRACK AFTER 36 HOURS...AND LIES CLOSE TO THE MULTI-MODEL
CONSENSUS TVCA AND THE FLORIDA STATE SUPERENSEMBLE MODEL.
 
THE VERTICAL SHEAR HAS DROPPED SHARPLY TO ABOUT 7 KT...WHICH HAS
ALLOWED THE LOW- AND MID-/UPPER-LEVEL CIRCULATION CENTERS TO
FINALLY BECOME VERTICALLY ALIGNED. THE ONLY THING PREVENTING
MELISSA FROM MAKING THE TRANSITION TO A TROPICAL CYCLONE IS THE
LACK OF SIGNIFICANT DEEP CONVECTION ORGANIZING AROUND THE LOW-LEVEL
CENTER. HOWEVER...SHALLOW TO MODERATE CONVECTION HAS BEEN 
DEVELOPING NEAR THE CENTER AND IN THE DRY SLOT IN THE EASTERN
SEMICIRCLE DURING THE PAST COUPLE OF HOURS...AND ONLY A SLIGHT
INCREASE IN THE DEPTH AND COVERAGE OF THE CONVECTION WOULD ALLOW
MELISSA TO STRENGTHEN SLIGHTLY AND ALSO QUICKLY TRANSITION TO A
TROPICAL STORM. THE CYCLONE IS EXPECTED TO MOVE OVER COOLER WATER
AND INTO A DRIER AND MORE STABLE AIRMASS IN 24-36 HOURS...WHICH
SHOULD INDUCE STEADY WEAKENING. BY 48 HOURS...INCREASING VERTICAL
WIND SHEAR SHOULD ENHANCE THE WEAKENING PROCESS AND ALSO CAUSE
MELISSA TO TRANSITION TO A POST-TROPICAL EXTRATROPICAL CYCLONE.
THE FORECAST CLOSELY FOLLOWS THE INTENSITY CONSENSUS MODEL ICON.
 
TRACK...INTENSITY... AND WIND RADII FORECASTS DURING THE
EXTRATROPICAL STAGE WERE COORDINATED WITH THE NOAA OCEAN PREDICTION
CENTER.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/2100Z 33.0N  53.7W   55 KT  65 MPH
 12H  20/0600Z 34.9N  51.4W   60 KT  70 MPH
 24H  20/1800Z 37.4N  46.7W   55 KT  65 MPH
 36H  21/0600Z 40.1N  41.1W   50 KT  60 MPH
 48H  21/1800Z 42.4N  35.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  22/1800Z 47.0N  26.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  23/1800Z 53.5N  24.5W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  24/1800Z...DISSIPATED
 
$$
FORECASTER STEWART
 
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