898
WTNT44 KNHC 061447
TCDAT4
TROPICAL STORM DEXTER DISCUSSION NUMBER 11
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042025
1100 AM AST WED AUG 06 2025
THIS MORNING, DEXTER'S STRUCTURE CONTINUES TO BE QUITE RESILIENT
AGAINST AN ESTIMATED 35-40 KT OF WESTERLY VERTICAL WIND SHEAR OVER
THE STORM. DEEP CONVECTION CONTINUES TO BURST, STILL PRIMARILY
DOWN-SHEAR, BUT AN EARLIER AMSR2 MICROWAVE PASS SHOWED A BIT MORE
BANDING-TYPE STRUCTURE ON THE 37 GHZ CHANNEL. SUBJECTIVE DVORAK
ESTIMATES FROM TAFB AND SAB ARE NOW BOTH T3.0/45 KT, AND THERE WERE
A FEW GOES-19 DERIVED MOTION WIND VECTORS UP TO 52 KT NEAR 850 MB ON
THE WEST SIDE OF DEXTER'S LOW-LEVEL CIRCULATION. THESE DATA SUPPORT
RAISING THE INITIAL INTENSITY TO 45 KT FOR THIS ADVISORY.
DEXTER REMAINS ON AN EAST-NORTHEASTWARD HEADING, A BIT FASTER THAN
EARLIER AT 060/13 KT. THE TROPICAL STORM IS BEING STEERED BY THE
MID-LATITUDE WESTERLY FLOW ON THE DOWNSTREAM SIDE OF A SOUTHWARD
DIGGING SHORTWAVE TROUGH MOVING OUT OF ATLANTIC CANADA. THE GUIDANCE
HAS FINALLY COME INTO GOOD AGREEMENT ON THIS TROUGH CAPTURING DEXTER
IN 36 H, LEADING TO FURTHER ACCELERATION EAST-NORTHEASTWARD UNTIL
THE TROUGH AND DEXTER FULLY PHASE. THE TRACK GUIDANCE THIS CYCLE IS
A TOUCH FASTER THAN EARLIER, AND THE LATEST NHC TRACK FORECAST WAS
NUDGED A LITTLE FASTER COMPARED TO THE PRIOR FORECAST CYCLE.
DEXTER'S RECENT INTENSIFICATION IS LIKELY NOT FULLY TROPICAL IN
NATURE. IN FACT, DEXTER IS CURRENTLY POSITIONED IN THE RIGHT
ENTRANCE REGION OF A 120 KT UPPER-LEVEL JET STREAK, WHICH IS LIKELY
PROVIDING SOME ENHANCED UPPER-LEVEL DIVERGENCE AND AIDING THE DEEP
CONVECTION OVER THE CYCLONE. THE DIGGING SHORTWAVE TROUGH WILL ALSO
START IMPINGING ON THE CYCLONE, INITIATING EXTRATROPICAL TRANSITION.
THIS PROCESS SHOULD BE COMPLETE IN ABOUT 36 H, WHEN BOTH THE GFS AND
ECMWF MODELS SHOW DEEP CONVECTION SEPARATING FROM DEXTER'S CORE.
HOWEVER, THIS WILL BE A DYNAMIC EXTRATROPICAL TRANSITION, WITH ALL
THE VARIOUS HURRICANE-REGIONAL MODELS SUGGESTING THE DEVELOPMENT OF
A STING JET, OR BRANCH OF RAPIDLY DESCENDING DRY AIR, CURVING
CYCLONICALLY AROUND THE CYCLONE'S WESTERN FLANK. THE NHC INTENSITY
FORECAST SHOWS DEXTER PEAKING AS A 60-KT EXTRATROPICAL CYCLONE IN
ABOUT 36 H IN RESPONSE TO THIS FEATURE, AND BOTH HAFS-A/B SUGGEST
THIS INTENSITY COULD BE CONSERVATIVE. AFTER THE EXTRATROPICAL
CYCLONE OCCLUDES, THE WINDS SHOULD DECREASE AGAIN AS IT LOSES ITS
BAROCLINIC FORCING, WITH THE LOW OPENING UP TO A TROUGH BY THE END
OF THE FORECAST PERIOD OVER THE FAR NORTHEASTERN ATLANTIC.
FORECAST POSITIONS AND MAX WINDS
INIT 06/1500Z 39.9N 58.2W 45 KT 50 MPH
12H 07/0000Z 40.4N 55.4W 50 KT 60 MPH
24H 07/1200Z 41.5N 51.4W 55 KT 65 MPH
36H 08/0000Z 42.9N 47.3W 60 KT 70 MPH...POST-TROP/EXTRATROP
48H 08/1200Z 44.4N 43.6W 55 KT 65 MPH...POST-TROP/EXTRATROP
60H 09/0000Z 45.2N 39.5W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 09/1200Z 45.6N 34.8W 40 KT 45 MPH...POST-TROP/EXTRATROP
96H 10/1200Z 47.0N 26.2W 35 KT 40 MPH...POST-TROP/EXTRATROP
120H 11/1200Z...DISSIPATED
FORECASTER PAPIN
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