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Hurricane Sam Forecast Discussion Number 34
2021-10-01 04:53:43| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Thu Sep 30 2021 000 WTNT43 KNHC 010253 TCDAT3 Hurricane Sam Discussion Number 34 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Thu Sep 30 2021 Sam's satellite structure on infrared remains quite impressive this evening, with a circular eye staying quite warm (+10-20C) in comparison to the ring of cold cloud tops associated with the eyewall (-60 to -70C). The most recent microwave imagery was a SSMIS pass at 1957 UTC, which indicated the eyewall remains quite intense, if just a little asymmetric with a stronger western semicircle. This eyewall asymmetry is consistent with a touch of light southerly shear affecting the cyclone as diagnosed by ECMWF-SHIPS. However, this shear does not appear to be having a significant impact on the hurricane's intensity. While there have not been any recent aircraft data to determine Sam's intensity since the last mission departed the hurricane at around 2000 UTC, the satellite intensity estimates have not changed much this evening. Thus, the initial intensity has been held at 125-kt this advisory. Another Air Force Reserve Reconnaissance Mission will be flying into Sam overnight to get a better sense of the current intensity. Following Sam's eye tonight, the hurricane is beginning to make a gradual turn northward at a faster motion, with the latest estimate now north-northwest at 345/14 kt. The track guidance in the short-term remains in good agreement that Sam should continue to turn to the north, north-northeast, and then northeastward as it is steered around the western edge of a large subtropical ridge over the central Atlantic. This track should take the hurricane east of Bermuda. While there is a large deep-layer trough located north of Sam, this feature will not initially capture the cyclone, as this trough first de-amplifies and merges with a larger deep-layer trough located over the far eastern Provinces of Canada. In fact, Sam's forward motion to the northeast in 48-72 hours may even slow a tad as it remains just south of the stronger upper-level westerlies. However, by 96-120 hours, the deep-layer low over Canada is expected to dig southeastward into North Atlantic, and will likely capture Sam as the hurricane rapidly accelerates ahead of this amplifying trough. It is in the 72-120 hour time-frame that a lot of the track guidance diverges, mostly related to the timing and degree of Sam's interaction of with this trough ejecting out of Canada. For now, the official NHC track forecast has elected to stay close to the track consensus TVCN, which has preformed well with this hurricane so far. This latest track is just a bit northeast and faster than the previous one after 72 hours. Sam still has another 24 hours under warm 28-29 C sea-surface temperatures while the southerly vertical wind shear is expected to remain light. Thus, the hurricane is expected to maintain Category 4 intensity over the next day or so, with short-term intensity changes likely to be controlled by inner core dynamics (such as eyewall replacement cycles). Thereafter, sea surface temperatures decrease markedly as southerly shear is also expected to gradually increase. This combination should lead to steady weakening. After 72 hours, the rate of weakening has actually been slowed a tad, since the ECMWF, GFS, and Canadian models all show Sam receiving significant baroclinic support as it undergoes a dynamic extratropical transition while the hurricane phases with the large baroclinic trough emerging off Canada. Simulated satellite imagery from these models suggest that Sam's transition to an extratropical cyclone should take place between 96-120 hours, with the resulting cyclone producing a large wind footprint across the North Atlantic basin. Key Messages: 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by tomorrow, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are expected on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Warning is in effect for the island. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 25.0N 61.2W 125 KT 145 MPH 12H 01/1200Z 27.2N 61.8W 125 KT 145 MPH 24H 02/0000Z 30.2N 61.6W 115 KT 130 MPH 36H 02/1200Z 32.9N 60.5W 105 KT 120 MPH 48H 03/0000Z 35.0N 58.9W 95 KT 110 MPH 60H 03/1200Z 36.6N 56.8W 85 KT 100 MPH 72H 04/0000Z 38.7N 53.8W 80 KT 90 MPH 96H 05/0000Z 44.6N 47.2W 75 KT 85 MPH 120H 06/0000Z 48.9N 42.5W 70 KT 80 MPH...POST-TROP/EXTRATROP $$ Forecaster Papin
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