000
WTNT41 KNHC 300844
TCDAT1

HURRICANE OPHELIA DISCUSSION NUMBER  31
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL162011
500 AM AST FRI SEP 30 2011

OPHELIA HAS BEEN RAPIDLY STRENGTHENING DURING THE PAST 12 HOURS
BASED ON AN EYE FEATURE OCCASIONALLY APPARENT ON BOTH INFRARED AND
NIGHT-VISIBLE SATELLITE IMAGES. MICROWAVE IMAGERY INDICATES A
WELL-DEVELOPED AND CLOSED DEEP-LAYER EYE...ALBEIT TILTED SLIGHTLY
TO THE NORTHEAST. A BLEND OF SATELLITE INTENSITY ESTIMATES OF
T5.0/90 KT FROM SAB...T4.5/77 KT FROM TAFB...AND UW-CIMSS AVERAGE
ADT OF T4.7/82 KT IS THE BASIS FOR THE 85-KT ADVISORY INTENSITY.

THE INITIAL MOTION ESTIMATE IS 345/08 KT. THERE IS NO SIGNIFICANT
CHANGE TO THE PREVIOUS FORECAST TRACK REASONING. OPHELIA IS
CURRENTLY ROUNDING THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE SITUATED ALONG 32N LATITUDE. AN UNSEASONABLY
STRONG MID-TROPOSPHERIC TROUGH IS FORECAST TO DEEPEN AND MOVE
EASTWARD TO THE U.S. EAST COAST BY 48 HOURS...AND WILL ACT TO
ACCELERATE AND LIFT OPHELIA NORTHWARD OVER THE NEXT 72 HOURS. BY
DAYS 3-4...OPHELIA IS FORECAST TO MERGE WITH A FRONTAL SYSTEM AND
BE OVER MUCH COLDER WATER...WHICH WILL RESULT IN THE CYCLONE
BECOMING A STRONG POST-TROPICAL WINTER-TYPE LOW PRESSURE SYSTEM
THAT WILL TURN EASTWARD NEAR THE CANADIAN MARITIMES. THE ONLY
NOTICEABLE CHANGE IN THE MODEL GUIDANCE IS A SOUTHWARD SHIFT IN THE
GUIDANCE TRACKS AFTER 72 HOURS. IN FACT...THE NEW ECMWF RUN BARELY
TAKES OPHELIA AS FAR NORTH AS 45N LATITUDE. THE OFFICIAL FORECAST
TRACK WAS NUDGED SLIGHTLY TO THE LEFT THROUGH 72 HOURS...AND THEN
BROUGHT FARTHER SOUTH IN LINE WITH THE CONSENSUS MODEL...TVCN...BUT
NOT AS FAR SOUTH AS THE ECMWF MODEL TRACK.

THE OUTFLOW OF OPHELIA REMAINS QUITE IMPRESSIVE...EXCEPT THAT THE
WESTERN EDGE OF THE CIRRUS OUTFLOW HAS STARTED TO FLATTEN AND
DISSIPATE AS WESTERLY UPPER-LEVEL WINDS ENCROACH ON THE CYCLONE.
HOWEVER...THE LIGHT TO MODERATE WESTERLY VERTICAL WIND SHEAR THAT
IS CURRENTLY AFFECTING THE CYCLONE IS FORECAST TO TURN MORE FROM
SOUTH-SOUTHWEST BY 36-48 HOURS...WHICH SHOULD ALLOW FOR SOME
ADDITIONAL STRENGTHENING TO OCCUR. ALTHOUGH FURTHER RAPID
STRENGTHENING IS A POSSIBILITY...THE VERTICAL SHEAR IS FORECAST BY
THE SHIPS MODEL TO FLUCTUATE BETWEEN 5 AND 15 KT DURING THE NEXT 48
HOURS...WHICH SHOULD ACT TO KEEP THE EYE TILTED AT TIMES AND LIKELY
DISRUPT THE RAPID INTENSIFICATION PROCESS. AS A RESULT...ONLY
MODEST STRENGTHENING IS FORECAST UNTIL OPHELIA REACHES 22C AND
COLDER SSTS SHORTLY AFTER THE 48-HOUR TIME PERIOD...WHICH IS ALSO
COINCIDENT WITH AN EXPECTED SHARP INCREASE IN THE VERTICAL SHEAR.
THE INTENSITY FORECAST IS SIMILAR TO THE INTENSITY CONSENSUS
MODEL...ICON...THROUGH 72 HOURS...AND THEN LEANS CLOSE TO THE LGEM
MODEL AFTER THAT. IT IS WORTH NOTING THAT SOME OF THE GLOBAL MODELS
ARE NOW HOLDING ON TO OPHELIA AS A ROBUST EXTRATROPICAL LOW
PRESSURE SYSTEM BEYOND 96 HOURS.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0900Z 22.4N  62.6W   85 KT 100 MPH
 12H  30/1800Z 24.2N  63.1W   90 KT 105 MPH
 24H  01/0600Z 27.0N  63.4W   90 KT 105 MPH
 36H  01/1800Z 30.1N  62.9W   95 KT 110 MPH
 48H  02/0600Z 34.6N  61.7W   95 KT 110 MPH
 72H  03/0600Z 44.3N  57.0W   65 KT  75 MPH...POST-TROP/EXTRATROP
 96H  04/0600Z 51.5N  38.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  05/0600Z...DISSIPATED

$$
FORECASTER STEWART