347  
WTNT42 KNHC 140858  
TCDAT2  
 
HURRICANE OPHELIA DISCUSSION NUMBER 21  
NWS NATIONAL HURRICANE CENTER MIAMI FL AL172017  
500 AM AST SAT OCT 14 2017  
 
OPHELIA'S 20-NMI-DIAMETER EYE HAS CONTINUED TO BECOME MORE DISTINCT  
AND CLOUD-FREE, WITH THE EYE TEMPERATURE NOW REACHING 15 DEG C.  
SATELLITE INTENSITY ESTIMATES RANGE FROM T4.5/77 KT FROM TAFB TO  
T5.0/90 KT FROM SAB AND T5.5/102 KT FROM UW-CIMSS ADT. FOR NOW, THE  
INITIAL INTENSITY WILL REMAIN AT 85 KT, WHICH IS AN AVERAGE OF THE  
AVAILABLE INTENSITY ESTIMATES.  
 
THE INITIAL MOTION ESTIMATE IS 060/21 KT. OPHELIA IS EMBEDDED WITHIN  
DEEP-LAYER SOUTHWESTERLY FLOW ON THE EAST SIDE OF A BROAD  
MID-LATITUDE TROUGH. THE GLOBAL AND REGIONAL MODELS REMAIN IN  
EXCELLENT AGREEMENT ON THE TROUGH AMPLIFYING OVER THE NEXT 3-4 DAYS,  
WHICH WILL CAUSE THE HURRICANE TO ACCELERATE TOWARD THE NORTHEAST AT  
FORWARD SPEEDS NEAR 30 KT BY 48 HOURS. THE TIGHT CLUSTERING OF THE  
NHC MODEL GUIDANCE, WHICH SHOWS VERY LITTLE CROSS-TRACK OR  
ALONG-TRACK SPREAD, INCREASES THE CONFIDENCE IN THE OFFICIAL TRACK.  
AS A RESULT, NO SIGNIFICANT CHANGES WERE MADE TO THE PREVIOUS  
ADVISORY, AND THE NEW FORECAST TRACK REMAINS NEAR THE MIDDLE OF THE  
GUIDANCE ENVELOPE, CLOSE TO THE HCCA AND TVCX CONSENSUS MODELS.  
 
OPHELIA IS EXPECTED TO REMAIN IN RELATIVELY LOW VERTICAL WIND SHEAR  
ENVIRONMENT FOR THE NEXT 12 HOURS OR SO, WHICH SHOULD HELP THE  
HURRICANE RETAIN MUCH OF ITS CURRENT INTENSITY DURING THAT TIME,  
EVEN THOUGH SSTS ARE ONLY GOING TO BE 24-25C. HOWEVER, UPPER-LEVEL  
TEMPERATURES THAT ARE STILL ABOUT 2 DEG C COOLER THAN NORMAL, WHICH  
WILL HELP TO CREATE SUFFICIENT INSTABILITY TO CONTINUE TO DRIVE THE  
DEVELOPMENT OF INNER-CORE CONVECTION. BY 36 HOURS OR SO, THE SHEAR  
IS FORECAST TO INCREASE TO 30-40 KT AND THE TROPOSPHERE IS EXPECTED  
TO BECOME STABLE AS SEA-SURFACE TEMPERATURES DECREASE TO LESS THAN  
20 DEG C. HOWEVER, EVEN THOSE SST VALUES ARE ABOUT 2 DEG C WARMER  
THAN NORMAL FOR THIS TIME OF THE YEAR. THOSE ABOVE-AVERAGE OCEAN  
TEMPERATURES ARE FORECAST TO COMBINE WITH STRONG BAROCLINIC ENERGY  
ASSOCIATED WITH A POTENT, NEGATIVELY TILTED, UPPER-LEVEL TROUGH,  
CAUSING OPHELIA TO TRANSITION INTO A POWERFUL EXTRATROPICAL LOW  
PRESSURE SYSTEM. BY 48 HOURS, THE POST-TROPICAL CYCLONE IS FORECAST  
TO MAINTAIN SUSTAINED HURRICANE-FORCE WINDS AS IT APPROACHES  
IRELAND, WITH STRONGER WINDS EXPECTED OVER HIGHER TERRAIN.  
 
GIVEN THAT OPHELIA IS FORECAST TO BECOME EXTRATROPICAL, THE WIND  
FIELD SHOULD EXPAND, RESULTING IN IMPACTS OVER PORTIONS OF THE  
BRITISH ISLES REGARDLESS OF ITS EXACT LOCATION OR STRENGTH. BY 96  
HOURS, OPHELIA SHOULD HAVE WEAKENED DUE TO THE INTERACTION WITH  
LAND, CAUSING THE SURFACE CIRCULATION TO BECOME ILL-DEFINED, AND  
DISSIPATION IS EXPECTED SHORTLY THEREAFTER.  
 
ALTHOUGH THE CENTER OF OPHELIA IS NOT FORECAST TO REACH IRELAND OR  
THE UK FOR ANOTHER 48-60 HOURS, WIND AND RAIN EFFECTS WILL ARRIVE  
WELL IN ADVANCE OF THE CYCLONE CENTER. INDIVIDUALS IN THOSE  
LOCATIONS SHOULD CONSULT PRODUCTS FROM THEIR LOCAL METEOROLOGICAL  
SERVICE FOR MORE INFORMATION ON LOCAL IMPACTS.  
 
TROPICAL-STORM-FORCE WINDS ARE POSSIBLE THROUGHOUT THE AZORES  
BEGINNING TONIGHT, PRIMARILY DUE TO AN APPROACHING COLD FRONT.  
HOWEVER, ANY TRACK DEVIATION TO THE WEST COULD BRING STRONGER WINDS  
ASSOCIATED WITH OPHELIA'S CIRCULATION TO THOSE ISLANDS. INTERESTS  
IN THE AZORES SHOULD REFER TO PRODUCTS ISSUED BY THE AZORES WEATHER  
FORECAST AND WATCH CENTER.  
 
KEY MESSAGES:  
 
1. OPHELIA IS EXPECTED TO BECOME A HURRICANE-FORCE POST-TROPICAL  
CYCLONE BY MONDAY BEFORE IT MOVES NEAR IRELAND AND THE UNITED  
KINGDOM. DIRECT IMPACTS FROM WIND AND HEAVY RAIN IN PORTIONS OF  
THESE AREAS ARE LIKELY, ALONG WITH DANGEROUS MARINE CONDITIONS. FOR  
MORE DETAILS ON THE MAGNITUDE, TIMING, AND LOCATION OF IMPACTS FROM  
POST-TROPICAL OPHELIA, RESIDENTS IN IRELAND SHOULD REFER TO PRODUCTS  
ISSUED BY MET EIREANN, AND RESIDENTS IN THE UNITED KINGDOM SHOULD  
REFER TO PRODUCTS ISSUED BY THE MET OFFICE.  
 
FORECAST POSITIONS AND MAX WINDS  
 
INIT 14/0900Z 33.9N 28.6W 85 KT 100 MPH  
12H 14/1800Z 35.3N 25.4W 80 KT 90 MPH  
24H 15/0600Z 38.3N 20.4W 80 KT 90 MPH  
36H 15/1800Z 43.0N 16.0W 75 KT 85 MPH...POST-TROP/EXTRATROP  
48H 16/0600Z 48.4N 12.5W 75 KT 85 MPH...POST-TROP/EXTRATROP  
72H 17/0600Z 57.4N 6.3W 50 KT 60 MPH...POST-TROP/EXTRATROP  
96H 18/0600Z 63.1N .5W 40 KT 45 MPH...POST-TROP/EXTRATROP  
120H 19/0600Z...DISSIPATED  
 

 
FORECASTER STEWART  
 
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