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Subtropical Storm MELISSA Forecast Discussion Number 6
2013-11-19 21:46:43| National Hurricane Center (Atlantic)
Issued at 500 PM AST TUE NOV 19 2013 000 WTNT44 KNHC 192046 TCDAT4 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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