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Hurricane Sam Forecast Discussion Number 35
2021-10-01 10:46:48| National Hurricane Center (Atlantic)
Issued at 500 AM AST Fri Oct 01 2021 000 WTNT43 KNHC 010846 TCDAT3 Hurricane Sam Discussion Number 35 NWS National Hurricane Center Miami FL AL182021 500 AM AST Fri Oct 01 2021 An Air Force Reserve Hurricane Hunter aircraft investigated Sam a few hours ago and found that the already-intense hurricane had strengthened a bit since yesterday afternoon's reconnaissance flight. The plane measured a peak 700-mb flight-level wind of 147 kt in the northeastern eyewall, and using a typical 90-percent reduction from that altitude, the intensity is estimated to be 130 kt. Dropsondes released in the eye also indicated that the central pressure had fallen to 934 mb. This drop in pressure is likely due to a combination of the strengthening and an expansion of the tropical-storm- and hurricane-force wind fields, which was confirmed by the aircraft and scatterometer data from last evening. Sam is moving a little faster toward the north-northwest (345/15 kt) as it curves around the western periphery of a mid-tropospheric ridge located over the eastern and central subtropical Atlantic. The hurricane is expected to turn northward later today and then northeastward by Saturday night--passing well to the east of Bermuda--when it begins to move between the ridge and a mid- to upper-level low located over the northeastern United States and Atlantic Canada. Interaction and a possible merger with the aforementioned low during extratropical transition is likely to cause a sudden north-northeastward acceleration to the southeast of Newfoundland on day 4, followed by a slower motion toward the east-northeast on day 5. The track models are very tightly clustered for much of the 5-day forecast period, yielding high confidence in the NHC track forecast. The new NHC forecast is relatively unchanged from the previous forecast, and closely follows the GFEX and TVCA consensus aids. Although fluctuations in intensity will still be possible, a general weakening trend is anticipated in the coming days as Sam moves over a gradually cooler ocean, reaching sub-26C waters by day 3. However, the weakening is not expected to be rapid due to continued upper-level divergence and only light-to-moderate deep-layer shear, and Sam could remain a major hurricane at least through Saturday night. Global models now indicate that Sam should complete extratropical transition by day 4, and baroclinic forcing is likely to keep the system as a potent hurricane-force post-tropical cyclone through the end of the forecast period. The NHC intensity forecast is essentially an update of the previous forecast, lying close to the HCCA and IVCN consensus aids. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next couple of days. Swells are expected to reach Bermuda and the Bahamas today, and then spread to the United States east coast and Atlantic Canada 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 tonight or early Saturday, and a Tropical Storm Warning is in effect for the island. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 26.6N 61.7W 130 KT 150 MPH 12H 01/1800Z 28.8N 62.0W 125 KT 145 MPH 24H 02/0600Z 31.7N 61.2W 115 KT 130 MPH 36H 02/1800Z 34.2N 59.7W 105 KT 120 MPH 48H 03/0600Z 36.0N 57.8W 95 KT 110 MPH 60H 03/1800Z 37.7N 55.0W 90 KT 105 MPH 72H 04/0600Z 40.1N 51.4W 85 KT 100 MPH 96H 05/0600Z 47.7N 44.8W 75 KT 85 MPH...POST-TROP/EXTRATROP 120H 06/0600Z 50.1N 38.1W 65 KT 75 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg
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