786
WTNT45 KNHC 202050
TCDAT5
HURRICANE TAMMY DISCUSSION NUMBER 10
NWS NATIONAL HURRICANE CENTER MIAMI FL AL202023
500 PM AST FRI OCT 20 2023
MODERATE SHEAR THAT IS LIKELY UNDERCUTTING THE HIGHER-LEVEL OUTFLOW
LAYER HAS DISRUPTED THE INNER-CORE DEVELOPMENT SEEN EARLIER TODAY.
THE EYE THAT BECAME APPARENT IN RADAR IMAGERY FROM BARBADOS THIS
MORNING HAS DEGRADED SINCE THAT TIME, BUT THERE IS STILL A LARGE
CURVED BAND THAT WRAPS AROUND THE EASTERN AND SOUTHERN PORTIONS OF
THE CIRCULATION. THE AIR FORCE AIRCRAFT THAT INVESTIGATED TAMMY
THROUGH EARLY AFTERNOON MEASURED SFMR WINDS OF AROUND 65 KT DURING
ITS FINAL PASS THROUGH THE CENTER, AND IT REPORTED THAT THE MINIMUM
PRESSURE HAD FALLEN TO AROUND 991 MB. ALTHOUGH THERE HAS BEEN THE
RECENT DEGRADATION OF THE INNER CORE, THE INITIAL INTENSITY REMAINS
65 KT, AND IS BASED ON THE EARLIER AIRCRAFT DATA AND MORE RECENT
SUBJECTIVE SATELLITE ESTIMATES FROM TAFB AND SAB. THE NEXT
RECONNAISSANCE MISSION INTO TAMMY IS SCHEDULED FOR THIS EVENING.
VERY RECENT RADAR IMAGERY FROM BARBADOS SUGGEST THAT TAMMY MAY
FINALLY BE MAKING ITS ANTICIPATED NORTHWESTWARD TURN, HOWEVER THE
LONG-TERM MOTION IS STILL WEST-NORTHWESTWARD ABOUT 6 KT. A MORE
PRONOUNCED NORTHWESTWARD MOTION SHOULD BEGIN VERY SOON AS A
TROUGH MOVING OFF THE EAST COAST OF THE UNITED STATES ERODES THE
WESTERN PORTION OF A SUBTROPICAL RIDGE OVER THE CENTRAL ATLANTIC.
THE EXPECTED NORTHWESTWARD MOTION SHOULD BRING THE CENTER OF TAMMY
NEAR OR OVER PORTIONS OF THE LEEWARD ISLANDS LATER TONIGHT AND
SATURDAY. WHILE THE NUMERICAL MODEL GUIDANCE IS IN RELATIVELY GOOD
AGREEMENT DURING THE FIRST 48-72 HOURS OF THE FORECAST PERIOD, THE
SPREAD HAS GREATLY INCREASED AFTER THAT TIME. THE GFS SHOWS A
STRONGER AND MORE VERTICALLY DEEP CYCLONE RECURVING OVER
WEST-CENTRAL ATLANTIC AFTER DAY 3, WHILE THE LATEST UKMET AND ECMWF
MODELS DEPICT A WEAKER CYCLONE THAT IS LEFT BEHIND BY AN
EASTWARD-MOVING TROUGH OVER THE WESTERN ATLANTIC. THE NHC FORECAST
CONTINUES TO PREDICT RECURVATURE, ALTHOUGH THE LATTER PORTION HAS
BEEN ADJUSTED SLOWER ONCE AGAIN.
THE ENVIRONMENT AHEAD OF TAMMY IS NOT PREDICTED TO CHANGE MUCH
DURING THE NEXT DAY OR SO. WARM WATERS AND MODERATE SHEAR ARE
EXPECTED TO ALLOW FOR SOME MODEST STRENGTHENING DURING THE NEXT
COUPLE OF DAYS, AND THE NHC INTENSITY FORECAST DURING THAT TIME IS
SIMILAR TO THE PREVIOUS ADVISORY. AFTER TAMMY MOVES NORTH OF THE
LEEWARD ISLANDS, INCREASING VERTICAL WIND SHEAR IS LIKELY TO
INITIATE WEAKENING AS TAMMY MOVES NORTHWARD. THE NHC INTENSITY
FORECAST IS CLOSE TO THE IVCN INTENSITY AID. DUE TO THE
DIFFERENCES IN HOW FAST TAMMY RECURVES, THERE IS MORE UNCERTAINTY
THAN NORMAL ON WHEN TAMMY WILL BEGIN EXTRATROPICAL TRANSITION.
THE CURRENT NHC FORECAST MAINTAINS TAMMY AS A HURRICANE THROUGH DAY
5, BUT IF IT TRENDS TOWARD THE FASTER SIDE OF THE GUIDANCE IT COULD
BE POST-TROPICAL BY THAT TIME.
KEY MESSAGES:
1. HURRICANE CONDITIONS ARE EXPECTED IN PORTIONS OF THE LEEWARD
ISLANDS BY LATE TONIGHT THROUGH SATURDAY. TROPICAL STORM
CONDITIONS ARE EXPECTED TO BEGIN IN PORTIONS OF THE LESSER ANTILLES
WITHIN THE WARNING AREA THIS EVENING OR OVERNIGHT.
2. HEAVY RAINS FROM TAMMY WILL BEGIN TO AFFECT THE LEEWARD AND
NORTHERN WINDWARD ISLANDS TONIGHT AND SATURDAY SPREADING INTO THE
BRITISH AND U.S. VIRGIN ISLANDS AND PUERTO RICO AS EARLY AS SUNDAY.
THIS RAINFALL MAY PRODUCE ISOLATED FLASH AND URBAN FLOODING, ALONG
WITH ISOLATED MUDSLIDES IN AREAS OF HIGHER TERRAIN.
3. A STORM SURGE COULD PRODUCE COASTAL FLOODING IN AREAS OF ONSHORE
WINDS AS THE CENTER OF TAMMY MOVES NEAR OR OVER THE LEEWARD ISLANDS.
NEAR THE COAST, THE SURGE WILL BE ACCOMPANIED BY LARGE AND DANGEROUS
WAVES.
FORECAST POSITIONS AND MAX WINDS
INIT 20/2100Z 14.3N 59.2W 65 KT 75 MPH
12H 21/0600Z 15.3N 60.1W 65 KT 75 MPH
24H 21/1800Z 16.7N 61.2W 70 KT 80 MPH
36H 22/0600Z 18.1N 62.1W 75 KT 85 MPH
48H 22/1800Z 19.5N 63.0W 80 KT 90 MPH
60H 23/0600Z 20.8N 63.4W 80 KT 90 MPH
72H 23/1800Z 22.0N 63.4W 75 KT 85 MPH
96H 24/1800Z 24.5N 61.7W 70 KT 80 MPH
120H 25/1800Z 28.4N 57.7W 65 KT 75 MPH
FORECASTER BROWN
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