771  
WTNT42 KNHC 092037  
TCDAT2  
 
TROPICAL STORM OPHELIA DISCUSSION NUMBER 3  
NWS NATIONAL HURRICANE CENTER MIAMI FL AL172017  
500 PM AST MON OCT 09 2017  
 
OPHELIA'S CONVECTIVE PATTERN HAS CONTINUED TO STEADILY IMPROVE WITH  
THE FORMATION OF STRONG THUNDERSTORMS WITH CLOUD TOPS NEAR -80C NEAR  
THE CENTER, ALONG WITH THE DEVELOPMENT OF TIGHTER CURVED BANDING  
FEATURES IN THE SOUTHEASTERN SEMICIRCLE. THERE HAS ALSO BEEN A  
NOTICEABLE INCREASE IN LIGHTNING ACTIVITY WITHIN 30 NMI OF THE  
CENTER. THE TAFB DVORAK INTENSITY ESTIMATE AT 1800Z WAS T2.5/35 KT.  
HOWEVER, NHC OBJECTIVE DVORAK ESTIMATES ARE RANGING FROM T3.2/49 KT  
USING A CURVED BAND PATTERN TO T3.5/55 KT FOR A SHEAR PATTERN. BASED  
ON THE AFOREMENTIONED ESTIMATES AND THE CONTINUED IMPROVEMENT IN  
OPHELIA'S CONVECTIVE ORGANIZATION SINCE THE 1800Z TAFB ESTIMATE, THE  
INITIAL INTENSITY IS INCREASED TO 40 KT.  
 
THE INITIAL MOTION ESTIMATE IS 055/03 KT. THERE IS NO SIGNIFICANT  
CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. OPHELIA IS  
EXPECTED TO REMAIN EMBEDDED WITHIN A WEAK FLOW REGIME FOR THE NEXT  
72 HOURS DUE TO THE CYCLONE BEING CUT OFF FROM THE MID-LATITUDE  
WESTERLIES, RESULTING IN A SLOW AND GRADUAL TURN TOWARD THE EAST AND  
SOUTHEAST DURING THAT TIME FRAME. THEREAFTER, A BROAD MID- TO  
UPPER-LEVEL TROUGH TO THE NORTHWEST, IN CONJUNCTION WITH A BUILDING  
SUBTROPICAL RIDGE TO THE SOUTH, IS FORECAST TO GRADUALLY ACCELERATE  
OPHELIA TOWARD THE NORTHEAST AT A SLIGHTLY FASTER FORWARD SPEED OF  
NEAR 10 KT. THE NEW NHC FORECAST TRACK IS CLOSE TO THE NORTHERN EDGE  
OF THE GUIDANCE ENVELOPE OUT OF RESPECT FOR THE ECMWF MODEL, BUT THE  
FORWARD SPEED IS SLOWER THAN THE ECMWF AND SIMILAR TO A BLEND OF THE  
HCCA AND TVCX CONSENSUS MODELS.  
 
OPHELIA IS EXPECTED TO REMAIN OVER MARGINALLY WARM SSTS OF 26.5-27C,  
BUT TEMPERATURES ALOFT THAT ARE 2-3 DEG C COLDER THAN NORMAL SHOULD  
CONTINUE TO PRODUCE STEEP LAPSE RATES AND STRONG INSTABILITY, WHICH  
WILL ALLOW FOR DEEP CONVECTION TO DEVELOP FOR THE NEXT 96 H OR SO.  
BOTH THE GFS AND ECMWF MODELS CONTINUE TO FORECAST THE VERTICAL WIND  
SHEAR TO DECREASE SHARPLY AFTER 24 H, WITH THE ECMWF FORECASTING  
SHEAR VALUES TO DECREASE TO LESS THAN 10 KT FROM 36-96 HOURS. BASED  
ON THE AVAILABILITY OF STRONG INSTABILITY AND LOWER SHEAR, SLOW BUT  
STEADY STRENGTHENING IS EXPECTED FOR THE NEXT 3-4 DAYS, WITH ONLY  
THE OCCASIONAL ENTRAINMENT OF VERY DRY MID-LEVEL AIR PREVENTING  
RAPID INTENSIFICATION FROM OCCURRING, AT LEAST IN THE SHORT TERM.  
AFTER 96 H, OPHELIA WILL BE ENCOUNTERING INCREASING SOUTHWESTERLY  
WIND SHEAR OF 15-20 KT AND ALSO BE MOVING OVER 25C SEA-SURFACE  
TEMPERATURES, A COMBINATION THAT SHOULD CAP THE INTENSIFICATION  
PROCESS AND POSSIBLY EVEN INDUCE WEAKENING. THE SHIPS AND LGEM  
STATISTICAL MODELS, ALONG WITH THE HCCA AND IVCN CONSENSUS MODELS,  
HAVE COME INTO MUCH BETTER AGREEMENT ON THEIR INTENSITY FORECASTS.  
THEREFORE, THERE IS A HIGHER DEGREE OF CONFIDENCE IN THE OFFICIAL  
INTENSITY FORECAST, WHICH BASICALLY REMAINS UNCHANGED FROM THE  
PREVIOUS ADVISORY. NOTE, IT IS POSSIBLE THAT OPHELIA COULD REACH A  
HIGHER PEAK INTENSITY THAN INDICATED BETWEEN 72-96 HOURS WHEN THE  
WIND SHEAR WILL BE AT ITS LOWEST VALUE.  
 
FORECAST POSITIONS AND MAX WINDS  
 
INIT 09/2100Z 31.5N 39.6W 40 KT 45 MPH  
12H 10/0600Z 31.6N 39.1W 45 KT 50 MPH  
24H 10/1800Z 31.4N 38.5W 50 KT 60 MPH  
36H 11/0600Z 30.8N 37.9W 55 KT 65 MPH  
48H 11/1800Z 30.1N 37.6W 60 KT 70 MPH  
72H 12/1800Z 29.7N 36.1W 65 KT 75 MPH  
96H 13/1800Z 31.0N 33.7W 65 KT 75 MPH  
120H 14/1800Z 32.4N 30.0W 65 KT 75 MPH  
 

 
FORECASTER STEWART  
 
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