Tropical Storm ALVIN

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TROPICAL STORM ALVIN DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP012013
800 AM PDT THU MAY 16 2013
 
ALVIN APPEARS TO HAVE DEGRADED IN STRUCTURE OVERNIGHT.  INFRARED
SATELLITE IMAGERY AND SCATTEROMETER DATA INDICATE THAT THE
CIRCULATION HAS HAD DIFFICULTY SEPARATING FROM THE ITCZ...AND THE
LOW-LEVEL CENTER IS LIKELY LOCATED ON THE WESTERN EDGE OF THE DEEP
CONVECTION.  THE INTENSITY IS BEING HELD AT 45 KT BASED ON THE
EARLIER ASCAT DATA...BUT SATELLITE ESTIMATES SUGGEST THAT ALVIN
COULD BE A BIT WEAKER.

ALTHOUGH THE HISTORICALLY TWO BEST GLOBAL MODELS...THE GFS AND
ECMWF...HAVE HAD SOME DISAGREEMENT ON ALVIN...EACH APPEARS TO HAVE
BEEN RIGHT TO SOME DEGREE.  THE GFS HAD A BETTER HANDLE ON
GENESIS...BUT THE ECMWF MAY HAVE BEEN RIGHT THAT ALVIN WOULD NOT
STRENGTHEN SIGNIFICANTLY.  AS OPPOSED TO YESTERDAY...THE SHIPS
GUIDANCE NOW INDICATES THAT 15 TO 20 KT OF VERTICAL SHEAR WILL
AFFECT ALVIN DURING THE NEXT FIVE DAYS...AND NEITHER OF THE
STATISTICAL MODELS NOW SHOWS THE CYCLONE REACHING HURRICANE
STRENGTH.  THE PROSPECTS FOR SIGNIFICANT INTENSIFICATION APPEAR TO
BE DWINDLING...AND THE OFFICIAL FORECAST ESSENTIALLY HOLDS THE
INTENSITY STEADY FOR THE NEXT 48 HOURS.  A MORE PRONOUNCED INCREASE
IN WESTERLY SHEAR SHOULD BEGIN A DEFINITE WEAKENING TREND BY DAY
3...AND ALVIN COULD DEGENERATE TO A REMNANT LOW NEAR THE END OF THE
FORECAST PERIOD.  NEEDLESS TO SAY...THERE IS STILL LOW CONFIDENCE IN
THE INTENSITY FORECAST.

SINCE THE OFFICIAL INTENSITY FORECAST HAS BEEN LOWERED...THE NHC
TRACK FORECAST HAS BEEN SHIFTED A BIT FARTHER SOUTH AND WEST WITH
THE ASSUMPTION THAT LOWER-LEVEL FLOW WILL HAVE A GREATER INFLUENCE
ON STEERING.  THE TRACK MODELS ARE IN BETTER AGREEMENT ON THE
GENERAL DIRECTION TODAY ALTHOUGH THERE IS STILL A SPEED
DISCREPANCY...ESPECIALLY BETWEEN THE GFS AND ECMWF.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  16/1500Z  9.4N 107.7W   45 KT  50 MPH
 12H  17/0000Z  9.9N 108.8W   50 KT  60 MPH
 24H  17/1200Z 10.7N 110.5W   50 KT  60 MPH
 36H  18/0000Z 11.2N 112.3W   50 KT  60 MPH
 48H  18/1200Z 11.6N 113.6W   50 KT  60 MPH
 72H  19/1200Z 13.0N 116.0W   45 KT  50 MPH
 96H  20/1200Z 13.5N 118.5W   30 KT  35 MPH
120H  21/1200Z 13.5N 120.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER BERG
 
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