491
WTNT45 KNHC 200856
TCDAT5
TROPICAL STORM TAMMY DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL202023
500 AM AST FRI OCT 20 2023
TAMMY IS STILL AN ASYMMETRIC TROPICAL STORM, ALTHOUGH THE SURFACE
CENTER IS EMBEDDED BENEATH A PERSISTENT AREA OF DEEP CONVECTION.
RADAR DATA FROM BARBADOS ALSO SUGGEST THAT THE CIRCULATION REMAINS
TILTED FROM WEST TO EAST WITH HEIGHT. AN AIR FORCE RESERVE
HURRICANE HUNTER AIRCRAFT INVESTIGATED TAMMY A FEW HOURS AGO AND
MEASURED MAXIMUM 850-MB FLIGHT-LEVEL WINDS OF 61 KT AND SFMR
SURFACE WINDS AROUND 50 KT, INDICATING THAT THE STORM'S INTENSITY
REMAINS 50 KT. THE CENTRAL PRESSURE HAS FALLEN SLIGHTLY TO 1000 MB.
THE AIRCRAFT CENTER FIXES INDICATE THAT TAMMY HAS CONTINUED TO SLOW
DOWN AND IS MOVING TOWARD THE WEST-NORTHWEST (290 DEGREES) AT 7 KT.
AN AMPLIFIED DEEP-LAYER TROUGH MOVING ACROSS THE EASTERN UNITED
STATES IS BEGINNING TO PUSH THE ATLANTIC SUBTROPICAL RIDGE
EASTWARD, AND THIS PATTERN EVOLUTION SHOULD ALLOW TAMMY TO TURN
NORTHWESTWARD BY THIS EVENING, WITH ITS CENTER PASSING VERY NEAR OR
OVER THE LEEWARD ISLANDS TONIGHT AND ON SATURDAY. THE NEW NHC
FORECAST IS VERY CLOSE TO THE PREVIOUS TRACK PREDICTION DURING THE
FIRST 36 HOURS AND LIES NEAR THE HCCA AND TVCA CONSENSUS AIDS.
AFTER 36 HOURS, THE TRACK GUIDANCE HAS SHIFTED A BIT WEST AND
SLOWED DOWN, APPARENTLY DUE TO THE AFOREMENTIONED TROUGH LIFTING TO
THE NORTHEAST AND LEAVING TAMMY BEHIND. AS A RESULT, THE OFFICIAL
FORECAST CONTINUES TO SHOW RECURVATURE OVER THE CENTRAL ATLANTIC BY
DAY 5, BUT AT A RELATIVELY SLOW FORWARD SPEED.
SHIPS GUIDANCE SUGGESTS THAT DEEP-LAYER SHEAR OVER TAMMY SHOULD
DECREASE A BIT OVER THE NEXT 24 HOURS, BUT MODEL FIELDS INDICATE
THERE COULD BE SOME MID-LEVEL SHEAR BELOW THE OUTFLOW LEVEL.
STILL, ENVIRONMENTAL CONDITIONS APPEAR CONDUCIVE TO SUPPORT GRADUAL
STRENGTHENING, AND THE NHC INTENSITY FORECAST BRINGS TAMMY TO
HURRICANE INTENSITY BY 36 HOURS AS THE CENTER PASSES NEAR OR OVER
THE LEEWARD ISLANDS. IT IS POSSIBLE THAT TAMMY COULD BECOME A
HURRICANE BEFORE THAT TIME, AS SUGGESTED BY SEVERAL MODELS
INCLUDING SHIPS, HAFS-B, HWRF, AND THE HCCA CORRECTED CONSENSUS. A
PEAK IN INTENSITY IS FORECAST IN 3-4 DAYS, BEFORE SOUTHWESTERLY
VERTICAL SHEAR INCREASES IN EARNEST AHEAD OF THE TROUGH. MODEL
FIELDS SUGGEST THAT EXTRATROPICAL TRANSITION COULD BEGIN TOWARD THE
END OF THE FORECAST PERIOD, BUT FOR NOW TAMMY IS STILL SHOWN AS A
HURRICANE ON DAY 5.
KEY MESSAGES:
1. TAMMY IS EXPECTED TO STRENGTHEN TO A HURRICANE BY SATURDAY
WHILE IT MOVES NEAR OR OVER PORTIONS OF THE LEEWARD ISLANDS.
TROPICAL STORM CONDITIONS ARE EXPECTED TO BEGIN IN PORTIONS OF THE
LESSER ANTILLES WITHIN THE TROPICAL STORM WARNING AREA THIS
AFTERNOON OR EVENING, WITH HURRICANE CONDITIONS POSSIBLE WITHIN THE
HURRICANE WATCH AREA ON SATURDAY. ADDITIONAL WATCHES AND WARNINGS
COULD BE REQUIRED LATER TODAY.
2. HEAVY RAINS FROM TAMMY WILL BEGIN TO AFFECT THE LEEWARD AND
NORTHERN WINDWARD ISLANDS TODAY, SPREADING INTO THE BRITISH AND
U.S. VIRGIN ISLANDS AND PUERTO RICO OVER THE WEEKEND. THIS RAINFALL
MAY PRODUCE ISOLATED FLASH AND URBAN FLOODING, ALONG WITH ISOLATED
MUDSLIDES IN AREAS OF HIGHER TERRAIN.
FORECAST POSITIONS AND MAX WINDS
INIT 20/0900Z 14.0N 58.3W 50 KT 60 MPH
12H 20/1800Z 14.7N 59.4W 55 KT 65 MPH
24H 21/0600Z 15.9N 60.7W 60 KT 70 MPH
36H 21/1800Z 17.3N 61.8W 65 KT 75 MPH
48H 22/0600Z 18.8N 62.6W 70 KT 80 MPH
60H 22/1800Z 20.3N 63.2W 75 KT 85 MPH
72H 23/0600Z 21.7N 63.4W 80 KT 90 MPH
96H 24/0600Z 24.3N 61.9W 80 KT 90 MPH
120H 25/0600Z 27.8N 58.8W 75 KT 85 MPH
FORECASTER BERG
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page