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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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