955
WTNT45 KNHC 231446
TCDAT5
HURRICANE TAMMY DISCUSSION NUMBER 21
NWS NATIONAL HURRICANE CENTER MIAMI FL AL202023
1100 AM AST MON OCT 23 2023
TAMMY APPEARS TO BE HOLDING STEADY IN SPITE OF THE MODERATE
SOUTHWESTERLY VERTICAL WIND SHEAR. THERE HAVE BEEN REGULAR BURSTS
OF DEEP CONVECTION OVER THE CENTER AND A FRAGMENTED BAND AROUND THE
EASTERN PORTION OF THE CIRCULATION. THE LATEST INTENSITY IS HELD AT
70 KT TO REPRESENT A BLEND OF THE SUBJECTIVE SATELLITE INTENSITY
ESTIMATES FROM TAFB AND SAB, WHICH WERE 77 KT AND 65 KT,
RESPECTIVELY.
THE HURRICANE IS MOVING NORTHWARD AT 005/6 KT. TAMMY SHOULD
CONTINUE TO TURN NORTH-NORTHEASTWARD TO NORTHEASTWARD IN THE FLOW
BETWEEN A MID-LATITUDE TROUGH OVER THE WESTERN ATLANTIC AND A
MID-LEVEL RIDGE OVER THE CENTRAL SUBTROPICAL ATLANTIC. BEYOND 60 H,
THERE IS STILL A SIGNIFICANT SPREAD IN THE MODEL GUIDANCE. MOST OF
THE REGIONAL MODELS AND THE GFS MODEL FORECAST TAMMY TO CONTINUE
NORTHWARD AROUND THE WESTERN SIDE OF THE RIDGE. A DIFFERENT CLUSTER
OF MODEL SOLUTIONS, INCLUDING THE ECMWF, MERGE TAMMY WITH A CUTOFF
LOW THAT FORMS FROM THE TROUGH AND TURNS THE CYCLONE NORTHWESTWARD
TO WESTWARD. THE LATEST NHC TRACK FORECAST FAVORS THE LATTER
SCENARIO AND HAS BEEN NUDGED SLIGHTLY NORTH OF THE PREVIOUS
PREDICTION.
SOUTHWESTERLY SHEAR APPEARS TO BE INFLUENCING TAMMY'S STRUCTURE.
THE VERTICAL WIND SHEAR IS EXPECTED TO PERSIST AND LIKELY INCREASE
IN ABOUT 36 H, FORCING SOME DRY MID-LEVEL AIR INTO THE CYCLONE'S
CIRCULATION. THESE ENVIRONMENTAL CONDITIONS SHOULD LEAD TO GRADUAL
WEAKENING. MODEL GUIDANCE PREDICTS THAT TAMMY SHOULD UNDERGO
STRUCTURAL CHANGES OVER THE NEXT COUPLE OF DAYS AS IT INTERACTS WITH
THE AFOREMENTIONED CUTOFF LOW. THE LATEST OFFICIAL INTENSITY
FORECAST NOW SHOWS TAMMY BECOMING EXTRATROPICAL IN 72 HOURS.
KEY MESSAGES:
1. HEAVY RAIN FROM TAMMY WILL REMAIN POSSIBLE OVER THE BRITISH
VIRGIN ISLANDS AND LEEWARD ISLANDS THROUGH TODAY. THIS RAINFALL MAY
PRODUCE ISOLATED FLASH AND URBAN FLOODING, ALONG WITH ISOLATED
MUDSLIDES IN AREAS OF HIGHER TERRAIN.
FORECAST POSITIONS AND MAX WINDS
INIT 23/1500Z 21.9N 63.9W 70 KT 80 MPH
12H 24/0000Z 22.7N 63.4W 70 KT 80 MPH
24H 24/1200Z 23.6N 62.4W 75 KT 85 MPH
36H 25/0000Z 24.5N 61.2W 80 KT 90 MPH
48H 25/1200Z 25.8N 59.9W 75 KT 85 MPH
60H 26/0000Z 27.3N 58.9W 70 KT 80 MPH
72H 26/1200Z 29.1N 58.6W 60 KT 70 MPH...POST-TROP/EXTRATROP
96H 27/1200Z 31.2N 60.2W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 28/1200Z 32.4N 63.6W 45 KT 50 MPH...POST-TROP/EXTRATROP
FORECASTER BUCCI
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