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Post-Tropical Cyclone Sam Forecast Discussion Number 51
2021-10-05 10:38:47| National Hurricane Center (Atlantic)
Issued at 900 AM GMT Tue Oct 05 2021 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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Post-Tropical Cyclone Sam Graphics
2021-10-05 10:37:10| National Hurricane Center (Atlantic)
5-Day Uncertainty Track last updated Tue, 05 Oct 2021 08:37:10 GMT Wind Speed Probabilities last updated Tue, 05 Oct 2021 08:37:10 GMT
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Summary for Post-Tropical Cyclone Sam (AT3/AL182021)
2021-10-05 10:34:19| National Hurricane Center (Atlantic)
...SAM TRANSITIONS TO A POWERFUL POST-TROPICAL CYCLONE OVER THE FAR NORTH ATLANTIC BETWEEN NEWFOUNDLAND AND ICELAND... ...THIS IS THE LAST ADVISORY... As of 9:00 AM GMT Tue Oct 5 the center of Sam was located near 51.0, -39.4 with movement N at 23 mph. The minimum central pressure was 970 mb with maximum sustained winds of about 80 mph.
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Post-Tropical Cyclone Sam Public Advisory Number 51
2021-10-05 10:34:19| National Hurricane Center (Atlantic)
Issued at 900 AM GMT Tue Oct 05 2021 000 WTNT33 KNHC 050834 TCPAT3 BULLETIN Post-Tropical Cyclone Sam Advisory Number 51 NWS National Hurricane Center Miami FL AL182021 900 AM GMT Tue Oct 05 2021 ...SAM TRANSITIONS TO A POWERFUL POST-TROPICAL CYCLONE OVER THE FAR NORTH ATLANTIC BETWEEN NEWFOUNDLAND AND ICELAND... ...THIS IS THE LAST ADVISORY... SUMMARY OF 900 AM GMT...0900 UTC...INFORMATION ---------------------------------------------- LOCATION...51.0N 39.4W ABOUT 690 MI...1110 KM ENE OF CAPE RACE NEWFOUNDLAND ABOUT 1110 MI...1785 KM SW OF REYKJAVIK ICELAND MAXIMUM SUSTAINED WINDS...80 MPH...130 KM/H PRESENT MOVEMENT...N OR 10 DEGREES AT 23 MPH...37 KM/H MINIMUM CENTRAL PRESSURE...970 MB...28.64 INCHES WATCHES AND WARNINGS -------------------- There are no coastal watches or warnings in effect. DISCUSSION AND OUTLOOK ---------------------- At 900 AM GMT (0900 UTC), the center of Post-Tropical Cyclone Sam was located near latitude 51.0 North, longitude 39.4 West. The post-tropical cyclone is moving toward the north near 23 mph (37 km/h), and this general motion should continue today with a further decrease in forward speed. A slow east-northeastward motion is forecast to begin by late today, and a northeastward motion is expected Wednesday and Wednesday night. Maximum sustained winds have decreased to near 80 mph (130 km/h) with higher gusts. Although gradual weakening is forecast during the next few days, Sam is expected to remain a powerful post-tropical cyclone over the north Atlantic today. Hurricane-force winds extend outward up to 80 miles (130 km) from the center and gale-force winds extend outward up to 460 miles (740 km). The estimated minimum central pressure is 970 mb (28.64 inches). HAZARDS AFFECTING LAND ---------------------- Key messages for Sam can be found in the Tropical Cyclone Discussion under AWIPS header MIATCDAT3 and WMO header WTNT43 KNHC, and on the web at hurricanes.gov/graphics_at3.shtml?key_messages. SURF: 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. NEXT ADVISORY ------------- This is the last public 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. $$ Forecaster Stewart
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Post-Tropical Cyclone Sam Wind Speed Probabilities Number 51
2021-10-05 10:34:16| National Hurricane Center (Atlantic)
Issued at 0900 UTC TUE OCT 05 2021 000 FONT13 KNHC 050834 PWSAT3 POST-TROPICAL CYCLONE SAM WIND SPEED PROBABILITIES NUMBER 51 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 0900 UTC TUE OCT 05 2021 AT 0900Z THE CENTER OF POST-TROPICAL CYCLONE SAM WAS LOCATED NEAR LATITUDE 51.0 NORTH...LONGITUDE 39.4 WEST WITH MAXIMUM SUSTAINED WINDS NEAR 70 KTS...80 MPH...130 KM/H. Z INDICATES COORDINATED UNIVERSAL TIME (GREENWICH) ATLANTIC STANDARD TIME (AST)...SUBTRACT 4 HOURS FROM Z TIME EASTERN DAYLIGHT TIME (EDT)...SUBTRACT 4 HOURS FROM Z TIME CENTRAL DAYLIGHT TIME (CDT)...SUBTRACT 5 HOURS FROM Z TIME WIND SPEED PROBABILITY TABLE FOR SPECIFIC LOCATIONS CHANCES OF SUSTAINED (1-MINUTE AVERAGE) WIND SPEEDS OF AT LEAST ...34 KT (39 MPH... 63 KM/H)... ...50 KT (58 MPH... 93 KM/H)... ...64 KT (74 MPH...119 KM/H)... FOR LOCATIONS AND TIME PERIODS DURING THE NEXT 5 DAYS PROBABILITIES FOR LOCATIONS ARE GIVEN AS OP(CP) WHERE OP IS THE PROBABILITY OF THE EVENT BEGINNING DURING AN INDIVIDUAL TIME PERIOD (ONSET PROBABILITY) (CP) IS THE PROBABILITY OF THE EVENT OCCURRING BETWEEN 06Z TUE AND THE FORECAST HOUR (CUMULATIVE PROBABILITY) PROBABILITIES ARE GIVEN IN PERCENT X INDICATES PROBABILITIES LESS THAN 1 PERCENT PROBABILITIES FOR 34 KT AND 50 KT ARE SHOWN AT A GIVEN LOCATION WHEN THE 5-DAY CUMULATIVE PROBABILITY IS AT LEAST 3 PERCENT. PROBABILITIES FOR 34...50...64 KT SHOWN WHEN THE 5-DAY 64-KT CUMULATIVE PROBABILITY IS AT LEAST 1 PERCENT. - - - - WIND SPEED PROBABILITIES FOR SELECTED LOCATIONS - - - - FROM FROM FROM FROM FROM FROM FROM TIME 06Z TUE 18Z TUE 06Z WED 18Z WED 06Z THU 06Z FRI 06Z SAT PERIODS TO TO TO TO TO TO TO 18Z TUE 06Z WED 18Z WED 06Z THU 06Z FRI 06Z SAT 06Z SUN FORECAST HOUR (12) (24) (36) (48) (72) (96) (120) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - LOCATION KT $$ FORECASTER STEWART