Subtropical Storm MELISSA

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SUBTROPICAL STORM MELISSA DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL142013
1100 PM AST MON NOV 18 2013
 
DEEP CONVECTION IN MELISSA REMAINS QUITE ASYMMETRIC WITH A NARROW
BAND WRAPPING AROUND THE CENTER AT A RADIUS OF ABOUT 60-90 NM ALONG
WITH A LARGER BAND IN THE EASTERN SEMI-CIRCLE A FEW HUNDRED MILES
FROM THE CENTER. WATER VAPOR IMAGERY INDICATES THAT MELISSA REMAINS
INTERTWINED WITH AN UPPER-LEVEL LOW...THUS A SUBTROPICAL STORM
STATUS STILL APPEARS WARRANTED AT THIS TIME.  A TIMELY 0030 UTC
PARTIAL ASCAT-A PASS PROVIDED PEAK WINDS OF ABOUT 45 KT.  THIS
ALONG WITH THE TAFB HEBERT-POTEAT SUBTROPICAL CLASSIFICATION OF
3.0...45-50 KT...HELPED TO DETERMINE THE INITIAL INTENSITY OF 50
KT.

MELISSA IS MOVING AT 325 DEGREES AT 7 KT.  THE CYCLONE SHOULD VEER
TOWARD THE NORTHEAST AND ACCELERATE DURING THE NEXT TWO DAYS AS A
SHORTWAVE TROUGH OVER EASTERN CANADA CURRENT HELPS KICK OUT MELISSA
INTO THE MID-LATITUDE WESTERLIES.  THE TRACK GUIDANCE IS TIGHTLY
CLUSTERED THROUGH 72 HR...BUT THEN DIVERGES DEPENDING IN PART ON
HOW MUCH EXTRATROPICAL TRANSITION OCCURS.  THE GFS AND DEPENDENT
MESOSCALE HURRICANE MODELS INDICATE AN EARLIER TRANSITION...WHICH
THEN WRAPS MELISSA NORTHWARD QUICKER.  THE ECMWF AND NAVGEM KEEP
MELISSA FARTHER EAST IN THE WARM SECTOR OF THE APPROACHING
EXTRATROPICAL LOW AND DISSIPATE IT BEFORE EXTRATROPICAL TRANSITION. 
THE OFFICIAL FORECAST IS A BLEND OF THESE SOLUTIONS AND IS CLOSE TO
THE PREVIOUS ADVISORY THROUGH THREE DAYS...BUT THEN FASTER AND
FARTHER EAST THEREAFTER.  THE EXTRATROPICAL TRACK IS BASED IN LARGE
PART UPON INPUT FROM THE OCEAN PREDICTION CENTER.

MELISSA HAS A SHORT WINDOW OF OPPORTUNITY FOR FURTHER
INTENSIFICATION.  THE SHEAR IS PROJECTED TO LESSEN FOR ABOUT A DAY
WHILE THE CYCLONE REMAINS OVER LUKEWARM WATERS.  THIS SHOULD ALLOW
ENOUGH CONVECTION TO FORM OVER THE CENTER AND A DEEP WARM CORE TO
DEVELOP FOR TROPICAL TRANSITION TO OCCUR...AS ALSO INDICATED BY THE
FSU CYCLONE PHASE SPACE DIAGRAMS.  IN ABOUT 36 TO 48 HOURS...THE
RAPIDLY DROPPING SSTS THAT MELISSA WILL ENCOUNTER COUPLED WITH MUCH
STRONGER VERTICAL SHEAR SHOULD CAUSE THE DEEP CONVECTION TO CEASE. 
THUS A TRANSITION TO A POST-TROPICAL LOW IS INDICATED AT 48
HOURS...EVEN THOUGH A FULL EXTRATROPICAL STAGE IS PREDICTED TO
OCCUR AROUND THREE DAYS.  BY DAY FIVE...MELISSA IS ANTICIPATED TO
BECOME ABSORBED BY THE LARGER EXTRATROPICAL LOW.
 
THE AFOREMENTIONED ASCAT PASS ALSO ALLOWED FOR A MORE CONFIDENT
ASSESSMENT OF THE 34 AND 50 KT WIND RADII.
  
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0300Z 30.3N  54.7W   50 KT  60 MPH
 12H  19/1200Z 31.3N  54.5W   50 KT  60 MPH
 24H  20/0000Z 33.6N  52.4W   55 KT  65 MPH
 36H  20/1200Z 36.4N  48.5W   60 KT  70 MPH
 48H  21/0000Z 39.8N  43.4W   55 KT  65 MPH...POST-TROPICAL
 72H  22/0000Z 46.0N  33.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  23/0000Z 53.5N  33.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  24/0000Z...DISSIPATED
 
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FORECASTER LANDSEA
 
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