[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 251454
TCDAT1

TROPICAL STORM OPHELIA DISCUSSION NUMBER  19
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL162011
1100 AM AST SUN SEP 25 2011

OPHELIA IS EXPERIENCING SOUTHWESTERLY VERTICAL SHEAR OF 25-30 KT...
WHICH HAS DISPLACED MOST OF THE DEEP CONVECTION WELL TO THE
NORTHEAST AND EAST OF THE ALLEGED LOW-LEVEL CENTER. HOWEVER...SOME
CONVECTION HAS RECENTLY BEGUN TO DEVELOP CLOSER TO THE CENTER OF
THE CYCLONE. THE INITIAL INTENSITY IS BEING MAINTAINED AT 35 KT
BASED ON AN EARLIER ASCAT PASS AT 0410Z THAT INDICATED A LARGE AREA
OF 35-40 KT WINDS WINDS BETWEEN 150-225 NMI EAST OF THE CENTER IN A
BAND OF DEEP CONVECTION. GIVEN THAT THAT BAND OF CONVECTION HAS
PERSISTED SINCE THE ASCAT PASS...IT IS ASSUMED THAT AT LEAST 35-KT
WINDS STILL EXIST IN THAT RAINBAND.

THE INITIAL MOTION ESTIMATE IS 280/08 KT OVER THE PAST 12 HOURS.
OPHELIA HAS ACTUALLY MADE A SHARP JOG TO THE WEST OR EVEN SOUTH OF
DUE WEST OVER THE PAST 6 HOURS. THIS IS DUE TO THE VERTICAL
CIRCULATION BECOMING MORE SHALLOW AS A RESULT OF THE LOSS OF INNER
CORE CONVECTION. HOWEVER...THE SHIPS MODEL IS FORECASTING THE
CURRENT STRONG SOUTHWESTERLY SHEAR TO ABATE TO AROUND 15-20 KT
DURING THE NEXT 6-48 HOURS...WHICH IS EXPECTED TO ALLOW MORE
CONVECTION TO REDEVELOP NEAR THE CENTER AND CREATE A MORE
VERTICALLY DEEP CIRCULATION.  AS A RESULT...OPHELIA SHOULD
GRADUALLY TURN TOWARD THE NORTHWEST OVER THE NEXT COUPLE OF DAYS AS
AN EASTWARD-MOVING UPPER-LEVEL TROUGH ERODES THE RIDGE TO THE NORTH
OF THE CYCLONE. ALTHOUGH THE ECMWF DISSIPATES THE CURRENT LOW-LEVEL
CIRCULATION AND SPINS UP ANOTHER CENTER TO THE EAST OF OPHELIA...
THERE IS MUCH LESS SPREAD IN THE MODEL GUIDANCE COMPARED TO 24
HOURS AGO. THE ECMWF SOLUTION IS BEING DISCOUNTED DUE TO VERY HIGH
SURFACE PRESSURES IN THE NORTHEAST QUADRANT OF THE OPHELIA
CIRCULATION...AND THE OFFICIAL FORECAST TRACK LIES A LITTLE WEST OF
THE PREVIOUS TRACK AND IS NEAR THE MIDDLE OF THE GUIDANCE ENVELOPE.

AS MENTIONED PREVIOUSLY...THE VERTICAL SHEAR IS FORECAST TO DECREASE
TO NEARLY HALF ITS CURRENT VALUE DURING THE NEXT 48 HOURS OR SO...
WHICH SHOULD ALLOW FOR SOME DEEP CONVECTION TO REDEVELOP NEAR THE
CENTER. THE RESULT SHOULD BE AT LEAST A MAINTENANCE OF THE CURRENT
INTENSITY. ALTHOUGH THE VERTICAL SHEAR IS EXPECTED TO INCREASE
AGAIN BY 72 HOURS AND BEYOND...SOME FAVORABLE UPPER-LEVEL DYNAMICS
MAY ALSO COME INTO PLAY...WHICH COULD INDUCE SOME MODEST
STRENGTHENING AS INDICATED BY ALL OF THE GLOBAL MODELS. THE NHC
INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND REMAINS
ABOVE THE SHIPS/LGEM MODELS AND THE CONSENSUS MODEL...ICON.
HOWEVER...IF DEEP CONVECTION DOES NOT REDEVELOP SOON NEAR THE
ALLEGED CENTER...THE SYSTEM COULD DEGENERATE INTO JUST AN OPEN WAVE
BY THIS AFTERNOON.

FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 18.3N  60.4W   35 KT  40 MPH
 12H  26/0000Z 18.8N  61.5W   35 KT  40 MPH
 24H  26/1200Z 19.6N  62.8W   35 KT  40 MPH
 36H  27/0000Z 20.6N  64.1W   35 KT  40 MPH
 48H  27/1200Z 21.4N  65.0W   35 KT  40 MPH
 72H  28/1200Z 22.9N  66.0W   40 KT  45 MPH
 96H  29/1200Z 25.0N  67.0W   45 KT  50 MPH
120H  30/1200Z 27.5N  67.5W   45 KT  50 MPH

$$
FORECASTER STEWART



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