[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 280836
TCDAT3

TROPICAL STORM RINA DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL182011
400 AM CDT FRI OCT 28 2011

AFTER AN EARLIER BURST OF VERY COLD-TOPPED CONVECTION OVER THE
LOW-LEVEL CENTER...WATER VAPOR IMAGERY INDICATES A SURGE OF 40-KT
SOUTHERLY UPPER-LEVEL WINDS HAS STARTED TO SHEAR AWAY THE
CONVECTION. AS A RESULT OF THE SHARP DEGRADATION IN THE CONVECTIVE
STRUCTURE...THE INITIAL INTENSITY HAS BEEN DECREASED TO 45 KT...
WHICH IS CONSISTENT WITH THE DOWNWARD TREND IN WIND SPEEDS NOTED IN
NEARBY SURFACE OBSERVATION SITES.

THE INITIAL POSITION IS BASED ON A COUPLE OF SURFACE OBS AND POOR
RADAR SIGNATURES FROM BOTH CUBAN AND MEXICAN RADARS...WHICH MAKES
THE INITIAL MOTION ESTIMATE AN UNCERTAIN 015/04 KT. THE RAGGED
APPEARANCE IN RADAR IMAGERY SUGGESTS THAT THE SMALL CIRCULATION OF
RINA MAY ALREADY BE SUCCUMBING TO THE INCREASING SOUTHERLY VERTICAL
WIND SHEAR. SOME MID-LEVEL ROTATION HAS BECOME APPARENT JUST
NORTHEAST OF THE TIP OF YUCATAN...WHEREAS SURFACE OBS INDICATE THE
SURFACE CENTER IS STILL OVER LAND. WITH THE VERTICAL CIRCULATIONS
FORECAST TO COMPLETELY DECOUPLE DURING THE NEXT 12 HOURS OR SO...
THE LOW-LEVEL CENTER IS EXPECTED TO GRADUALLY TURN SLOWLY EASTWARD
AND THEN TURN SOUTHWARD BACK OVER THE NORTHWESTERN CARIBBEAN SEA BY
24 HOURS. THE OFFICIAL FORECAST TRACK IS SIMILAR TO THE PREVIOUS
ADVISORY AND IS WELL TO THE RIGHT OF THE CONSENSUS MODELS...TVCN
AND TVCA.

SINCE THE VERTICAL SHEAR IS FORECAST TO GRADUALLY INCREASE...THE
MID-/UPPER-LEVEL CIRCULATION IS EXPECTED TO DECOUPLE FROM THE
LOW-LEVEL CIRCULATION AND LIFT OUT TO THE NORTHEAST WHILE THE
LOW-LEVEL CIRCULATION REMAINS BEHIND OVER THE NORTHWESTERN
CARIBBEAN SEA. THIS STRONG SHEAR PATTERN SHOULD CAUSE RINA TO
STEADILY WEAKEN THROUGHOUT THE FORECAST PERIOD...AND THE CYCLONE
COULD BECOME A TROPICAL DEPRESSION BY LATER TODAY OR EARLY
SATURDAY. RINA IS EXPECTED TO BECOME A REMNANT LOW OVER THE
NORTHWESTERN CARIBBEAN BY 48 HOURS...BUT WITH A SLIGHT CHANCE OF
REGENERATING INTO A TROPICAL CYCLONE IF IT DOES NOT GET ABSORBED
INTO THE BROAD LOW PRESSURE SYSTEM LOCATED EAST OF NICARAGUA.

FORECAST POSITIONS AND MAX WINDS

INIT  28/0900Z 21.1N  87.0W   45 KT  50 MPH
 12H  28/1800Z 21.5N  86.8W   40 KT  45 MPH
 24H  29/0600Z 21.3N  86.4W   35 KT  40 MPH
 36H  29/1800Z 20.6N  86.1W   30 KT  35 MPH
 48H  30/0600Z 20.0N  86.1W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  31/0600Z 19.2N  86.3W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  01/0600Z 17.9N  86.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  02/0600Z 17.0N  87.1W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
FORECASTER STEWART


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