780  
WTNT43 KNHC 050838  
TCDAT3  
 
POST-TROPICAL CYCLONE SAM DISCUSSION NUMBER 51  
NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021  
900 AM GMT TUE OCT 05 2021  
 
SAM'S LOW-LEVEL CIRCULATION CENTER HAS BECOME EXPOSED ON THE SOUTH  
SIDE OF AN EAST-TO-WEST ORIENTED CLOUD BAND CONTAINING CLOUD TOPS  
THAT HAVE WARMED TO AROUND -50 DEG C. THE CYCLONE'S OVERALL CLOUD  
PATTERN IN SATELLITE IMAGERY RESEMBLES THAT OF AN OCCLUDED  
EXTRATROPICAL LOW PRESSURE SYSTEM, AND SURFACE ANALYSES FROM THE  
NOAA OCEAN PREDICTION CENTER INDICATE THAT SAM'S INNER-CORE IS NOW  
INTERACTING WITH A FRONTAL SYSTEM. FURTHERMORE, THE SYSTEM IS  
LOCATED OVER SEA-SURFACE TEMPERATURES OF AROUND 14 DEG C. ALTHOUGH  
THE SYSTEM LIKELY STILL HAS A WARM-CORE THERMAL STRUCTURE IN THE  
MID- AND UPPER-LEVELS OF THE TROPOSPHERE BASED ON EARLIER 04/2321Z  
AMSU-A/-B DATA, THE LOW-LEVEL CIRCULATION NOW IS COMPRISED OF AN  
EXTENSIVE FIELD OF COLD-AIR STRATOCUMULUS CLOUDS. BASED ON THESE  
DATA, SAM HAS MADE THE TRANSITION TO AN EXTRATROPICAL CYCLONE OVER  
THE FAR NORTH ATLANTIC. THE INITIAL WIND SPEED HAS BEEN LOWERED TO  
70 KT BASED ON A STANDARD DECAY RATE.  
 
THE INITIAL MOTION ESTIMATE IS NORTHWARD OR 010/20 KT.  
EXTRATROPICAL-SAM HAS BEEN MOVING NORTH-NORTHEASTWARD AT 35 KT  
SINCE THE PREVIOUS ADVISORY, BUT THE LATEST GLOBAL AND REGIONAL  
MODELS ARE ALL IN VERY GOOD AGREEMENT THAT THE SYSTEM WILL SLOW  
DOWN SIGNIFICANTLY DURING THE NEXT 6-12 HOURS AS IT INTERACTS WITH  
AN APPROACHING BAROCLINIC MID- TO UPPER-LEVEL TROUGH/LOW. BY LATE  
TUESDAY, THE POST-TROPICAL CYCLONE SHOULD TURN EAST-NORTHEASTWARD,  
AND THEN MAKE A COUNTER-CLOCKWISE LOOP SOUTHWEST OF ICELAND ON  
WEDNESDAY. BY THURSDAY, THE LARGE CYCLONE IS EXPECTED TO MOVE  
EAST-NORTHEASTWARD OR EASTWARD AND PASS VERY NEAR THE SOUTHERN COAST  
OF ICELAND ON DAYS 4 AND 5 AS A WEAKENING EXTRATROPICAL LOW PRESSURE  
SYSTEM. THE NEW NHC TRACK FORECAST IS VERY SIMILAR TO THE PREVIOUS  
ADVISORY TRACK, AND LIES CLOSE TO THE TIGHTLY PACKED SIMPLE- AND  
CORRECTED-CONSENSUS TRACK MODELS.  
 
EXTRATROPICAL-SAM IS EXPECTED TO MAINTAIN HURRICANE-FORCE WINDS FOR  
THE NEXT 12-18 HOURS, FOLLOWED BY A SLOW WEAKENING TREND AS THE  
CYCLONE LOSES ITS BAROCLINIC FORCING. HOWEVER, POST-TROPICAL-SAM  
IS STILL EXPECTED TO PASS CLOSE TO ICELAND IN THE 96-120-H PERIOD AS  
AN EXPANSIVE LOW PRESSURE SYSTEM PRODUCING GALE-FORCE WINDS OVER A  
VERY LARGE AREA.  
 
THIS IS THE LAST ADVISORY ISSUED BY THE NATIONAL HURRICANE CENTER ON  
SAM. ADDITIONAL INFORMATION ON THIS SYSTEM CAN BE FOUND IN HIGH  
SEAS FORECASTS ISSUED BY THE NATIONAL WEATHER SERVICE, UNDER AWIPS  
HEADER NFDHSFAT1, WMO HEADER FZNT01 KWBC, AND ONLINE AT  
OCEAN.WEATHER.GOV/SHTML/NFDHSFAT1.PHP.  
 
KEY MESSAGES:  
 
1. SWELLS GENERATED BY SAM WILL IMPACT THE NORTHERN LEEWARD  
ISLANDS, THE GREATER ANTILLES, PORTIONS OF THE BAHAMAS, AND  
SOUTHEASTERN NEWFOUNDLAND THROUGH MIDWEEK. THESE SWELLS  
COULD CAUSE LIFE-THREATENING SURF AND RIP CURRENT CONDITIONS.  
PLEASE CONSULT PRODUCTS FROM YOUR LOCAL WEATHER OFFICE.  
 
 
FORECAST POSITIONS AND MAX WINDS  
 
INIT 05/0900Z 51.0N 39.4W 70 KT 80 MPH...POST-TROP/EXTRATROP  
12H 05/1800Z 52.1N 38.9W 65 KT 75 MPH...POST-TROP/EXTRATROP  
24H 06/0600Z 52.8N 35.1W 60 KT 70 MPH...POST-TROP/EXTRATROP  
36H 06/1800Z 54.3N 30.0W 55 KT 65 MPH...POST-TROP/EXTRATROP  
48H 07/0600Z 57.9N 24.6W 50 KT 60 MPH...POST-TROP/EXTRATROP  
60H 07/1800Z 61.8N 23.9W 45 KT 50 MPH...POST-TROP/EXTRATROP  
72H 08/0600Z 62.7N 27.4W 45 KT 50 MPH...POST-TROP/EXTRATROP  
96H 09/0600Z 61.7N 26.2W 40 KT 45 MPH...POST-TROP/EXTRATROP  
120H 10/0600Z 63.6N 15.0W 40 KT 45 MPH...POST-TROP/EXTRATROP  
 

 
FORECASTER STEWART  
 
 
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