814
WTNT44 KNHC 291459
TCDAT4
TROPICAL STORM IMELDA DISCUSSION NUMBER 12
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092025
1100 AM EDT MON SEP 29 2025
IMELDA IS STRENGTHENING THIS MORNING. WE HAVE HAD PLENTY OF DATA
FROM BOTH THE NOAA AND AIR FORCE RESERVE HURRICANE HUNTERS, AND BOTH
OF THEIR OBSERVATIONS SHOW THAT THE TROPICAL STORM IS BECOMING
BETTER ORGANIZED. THIS MATCHES THE IMPROVING STRUCTURE ON SATELLITE
WHICH SHOWS A BURSTING CENTRAL DENSE OVERCAST. HOWEVER, TAIL DOPPLER
RADAR (TDR) ANALYSIS STILL SHOWS SOME MODEST TILT NORTH WITH HEIGHT
BETWEEN THE 1 KM AND 5 KM CENTERS. THE PEAK 750 MB FLIGHT LEVEL
WINDS FROM THE NOAA-P3 WAS UP TO 59 KT TO THE NORTHEAST OF THE
CENTER, AND THE MOST RECENT DROPSONDE FROM THE C-130 CENTER FIX WAS
990 MB WITH 20 KT OF WIND. THUS, THE INITIAL INTENSITY FOR IMELDA
HAS BEEN RAISED TO 50 KT THIS ADVISORY, WITH A MINIMUM PRESSURE OF
988 MB. THE NORTHWEST BAHAMAS CONTINUE TO GET TROPICAL STORM
CONDITIONS, AND THERE WAS A RECENT WIND GUST TO 72 KT AT TREASURE
CAY.
THE TROPICAL STORM CONTINUES TO MOVE NORTHWARD AT ABOUT 7 KT. THIS
MOTION IS FORECAST TO CONTINUE TODAY DUE TO STEERING ALONG THE
WESTERN SIDE OF THE SUBTROPICAL RIDGE AND EASTERN SIDE OF A MID TO
UPPER-LEVEL TROUGH LOCATED OVER THE SOUTHEASTERN U.S. THE STEERING
FROM THE SUBTROPICAL RIDGE IS ABOUT TO BECOME INTERRUPTED BY
HURRICANE HUMBERTO'S LARGE CIRCULATION AS THE UPSTREAM TROUGH STARTS
TO DIG IN BEHIND IMELDA. THE COMBINATION OF THESE CHANGES WILL CAUSE
THE TROPICAL CYCLONE'S TRACK TO SHIFT ABRUPTLY EAST-NORTHEASTWARD AS
IT ACCELERATES. THE LATEST TRACK GUIDANCE IS FAIRLY SIMILAR TO THE
PREVIOUS CYCLE, THOUGH THERE REMAINS A SUBSTANTIAL AMOUNT OF
ALONG-TRACK SPREAD OVER THE 36-60 H PERIOD, WHERE THE ECMWF IS ON
THE SLOW END OF THE GUIDANCE ENVELOPE, AND THE GFS AND GOOGLE
DEEPMIND ENSEMBLE MEAN (GDMI) ARE ON THE FASTER END. THE LATEST NHC
TRACK FORECAST IS ALONG THE PRIOR FORECAST TRACK, BUT A LITTLE
FASTER AFTER 36 H, BLENDING THE HCCA AND GDMI TRACK GUIDANCE. ON THE
CURRENT FORECAST TRACK, THE CENTER OF IMELDA IS FORECAST TO PASS
NEAR BERMUDA IN ABOUT 60-H, THOUGH THE EXACT TIMING OF THIS REMAINS
LOW CONFIDENCE.
EVIDENCE FROM BOTH THE AIRCRAFT AND SATELLITE OBSERVATIONS SHOW THAT
IMELDA IS BEGINNING TO DEVELOP AN INNER CORE, AND FURTHER
INTENSIFICATION IS EXPECTED IN A LOW TO MODERATE VERTICAL WIND SHEAR
ENVIRONMENT ALONG WITH SUFFICIENT MID-LEVEL MOISTURE AND PLENTY WARM
SEA-SURFACE TEMPERATURES. IN 48-60 H, THE SHEAR BEGINS TO INCREASE
DRAMATICALLY AND THE STORM COULD PASS NEAR THE COLD WAKE OF
HUMBERTO. HOWEVER, IMELDA IS ALSO LIKELY TO GET A INTENSITY BOOST
FROM A FAVORABLE TROUGH INTERACTION, WHICH COULD ENHANCE THE WINDS
ALONG THE CYCLONE'S NORTHWESTERN FLANK, AND A HIGHER 85 KT PEAK IS
NOW SHOWN DURING THIS TIME. THIS ENHANCEMENT COULD EVOLVE INTO A
"STING JET" LIKE FEATURE AS THE SYSTEM BECOMES EXTRATROPICAL. THE
TIMING OF THIS TRANSITION REMAINS DIFFICULT TO DETERMINE, SINCE THE
ECMWF FORECAST IS MUCH SLOWER AND FURTHER SOUTH, KEEPING THE CYCLONE
OVER WARMER SSTS. MY BEST GUESS IS THAT THIS PROCESS WILL BE
COMPLETE BETWEEN THE 72 TO 96 H FORECAST POINTS. THE OFFICIAL
FORECAST REMAINS AT THE HIGHER END OF THE MODEL GUIDANCE, BUT IS
PRETTY CLOSE TO THE HCCA CONSENSUS AID.
KEY MESSAGES:
1. IMELDA IS EXPECTED TO CONTINUE TO BRING TROPICAL STORM CONDITIONS
TO PORTIONS OF THE NORTHWESTERN BAHAMAS TODAY.
2. RAINFALL ASSOCIATED WITH TROPICAL STORM IMELDA WILL CONTINUE TO
IMPACT THE BAHAMAS THROUGH TUESDAY, WHICH WILL LIKELY PRODUCE FLASH
AND URBAN FLOODING. HEAVY RAINFALL ACROSS THE COASTAL CAROLINAS
COULD CAUSE ISOLATED FLASH AND URBAN FLOODING THROUGH TUESDAY.
3. INTERESTS IN BERMUDA SHOULD MONITOR THE PROGRESS OF IMELDA AS A
HURRICANE WATCH COULD BE REQUIRED THIS AFTERNOON.
4. SWELLS AND HIGH SURF FROM BOTH HUMBERTO AND IMELDA ARE EXPECTED
TO PRODUCE DANGEROUS MARINE CONDITIONS AND RIP CURRENTS ALONG MUCH
OF THE EAST COAST OF THE UNITED STATES DURING THE NEXT SEVERAL DAYS.
FORECAST POSITIONS AND MAX WINDS
INIT 29/1500Z 26.9N 77.1W 50 KT 60 MPH
12H 30/0000Z 28.0N 77.0W 60 KT 70 MPH
24H 30/1200Z 29.0N 75.9W 70 KT 80 MPH
36H 01/0000Z 29.7N 73.6W 75 KT 85 MPH
48H 01/1200Z 30.8N 70.2W 80 KT 90 MPH
60H 02/0000Z 32.3N 65.4W 85 KT 100 MPH
72H 02/1200Z 33.7N 60.0W 80 KT 90 MPH
96H 03/1200Z 38.0N 53.0W 60 KT 70 MPH...POST-TROP/EXTRATROP
120H 04/1200Z 43.0N 49.0W 55 KT 65 MPH...POST-TROP/EXTRATROP
FORECASTER PAPIN
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