Sam transitions to a powerful post-tropical cyclone over the far north Atlantic between newfoundland and iceland.
1. php.Key Messages1. 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.
There are no coastal watches or warnings in effect.
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.
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).
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.
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