659
WTPZ44 KNHC 241433
TCDEP4
TROPICAL STORM ISELLE DISCUSSION NUMBER 5
NWS NATIONAL HURRICANE CENTER MIAMI FL EP092026
800 AM PDT MON AUG 24 2026
DESPITE BEING AFFECTED BY MODERATE WEST-NORTHWESTERLY SHEAR,
ISELLE'S CLOUD PATTERN HAS CONTINUED TO IMPROVE, WITH A CORE OF VERY
DEEP CONVECTION THAT HAS NOW ROTATED OVER AND NORTH OF THE CENTER,
AND A DEEP CONVECTIVE BAND EXTENDING SOUTHWARD ON THE EAST SIDE OF
THE CYCLONE. A 24/0903 UTC AMSR-2 PASS ALSO SHOWS THE IMPROVING
STRUCTURE OF THE INNER CORE. THE LATEST SUBJECTIVE DVORAK
CLASSIFICATION FROM TAFB IS T3.5/55 KT, AND OBJECTIVE SATELLITE
INTENSITY ESTIMATES RANGE FROM 50 TO 59 KT. THE INITIAL INTENSITY
HAS BEEN SET AT 55 KT FOR THIS ADVISORY, IN AGREEMENT WITH THE
ESTIMATES.
THE INITIAL MOTION ESTIMATE IS NORTHWESTWARD, OR 310/11 KT. A
MID-LEVEL RIDGE NORTH OF ISELLE IS FORECAST TO STRENGTHEN, CAUSING
ISELLE TO TURN WEST-NORTHWESTWARD ON TUESDAY, AND THAT MOTION SHOULD
THEN CONTINUE THROUGH THE END OF THE WEEK. THE LATEST NHC TRACK
FORECAST IS A LITTLE FASTER AND TO THE LEFT OF THE PREVIOUS
FORECAST, AND LIES VERY CLOSE TO THE HFIP CORRECTED CONSENSUS (HCCA)
AND THE GOOGLE DEEPMIND ENSEMBLE MEAN (GDMI).
ISELLE REMAINS OVER WARM WATERS AND WITHIN A MOIST ENVIRONMENT.
HOWEVER, SHIPS AND UW-CIMSS BOTH CURRENTLY ANALYZE 17 KT OF VERTICAL
WIND SHEAR. WATER TEMPERATURES AND MOISTURE WILL REMAIN QUITE
FAVORABLE FOR ANOTHER 24 H, BUT THE SHEAR IS FORECAST TO INCREASE TO
AT LEAST 20 KT BETWEEN THE 12 TO 24 H PERIOD. GIVEN THE RECENTLY
IMPROVING STRUCTURE, THE NHC FORECAST CONTINUES TO CALL FOR
STRENGTHENING AND SYSTEM TO BECOME A HURRICANE IN 12 TO 24 H, WHICH
IS AT THE HIGH END OF THE INTENSITY MODEL GUIDANCE SUITE. HOWEVER,
GIVEN THE INCREASING SHEAR, THERE IS SOME UNCERTAINTY AS TO WHETHER
ISELLE WILL BECOME A HURRICANE. BEYOND 24 H, GRADUALLY DECREASING
OCEAN TEMPERATURES AND MOISTURE WILL LEAD TO STEADY WEAKENING, AND
THE NHC INTENSITY FORECAST LIES IN THE MIDDLE OF THE GUIDANCE
ENVELOPE FROM HOUR 48 ONWARD. THE LATEST ECMWF AND GFS SIMULATED
SATELLITE IMAGERY SUGGEST THAT ISELLE COULD BECOME A REMNANT LOW AS
EARLY AS THURSDAY. I HAVE MOVED FORWARD THE FORECAST TIME FOR
ISELLE BECOMING POST-TROPICAL TO HOUR 96, BUT IT'S POSSIBLE THE
CYCLONE COULD LOSE ITS CONVECTION SOONER THAN FORECAST. AFTER THAT,
THE MODELS INDICATE THE REMNANT LOW OPENING UP INTO A TROUGH BY DAY
5.
FORECAST POSITIONS AND MAX WINDS
INIT 24/1500Z 18.5N 117.1W 55 KT 65 MPH
12H 25/0000Z 19.4N 118.3W 65 KT 75 MPH
24H 25/1200Z 20.5N 120.2W 65 KT 75 MPH
36H 26/0000Z 21.1N 122.1W 60 KT 70 MPH
48H 26/1200Z 21.5N 123.8W 50 KT 60 MPH
60H 27/0000Z 22.0N 125.6W 40 KT 45 MPH
72H 27/1200Z 22.5N 127.6W 35 KT 40 MPH
96H 28/1200Z 23.1N 131.9W 30 KT 35 MPH...POST-TROP/REMNT LOW
120H 29/1200Z...DISSIPATED
FORECASTER HAGEN
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