Subtropical Storm Melissa - 4pm Tuesday Discussion

By: National Hurricane Center
By: National Hurricane Center

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.


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