431 WTNT43 KNHC 270249 TCDAT3 Hurricane Sam Discussion Number 18 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Sun Sep 26 2021 A NOAA Hurricane Hunter aircraft investigating Sam this afternoon and evening found that the major hurricane likely peaked in intensity at around 135 kt with a central pressure of about 929 mb between 1900-2200 UTC when the eye contracted down to about 7 nmi in diameter. Since that time, however, wind and radar data from the aircraft reveal that Sam has been undergoing an eyewall replacement cycle (ERC), including significant erosion of the eyewall convection in the eastern semicircle, which has resulted in the central pressure increasing by at least 14 mb in only a few hours. The eye diameter has also doubled in size now. The ERC is also confirmed by the ragged inner-core convection and cloud-filled eye that have developed in satellite imagery. The advisory intensity of 125 kt is based on a blend of the available reliable SFMR surface wind data and flight-level wind data, and also using the pressure-wind relationship for a 943-mb central pressure. The initial motion estimate is now 315/06 kt. A strong subtropical ridge to the north and northeast of Sam is forecast by the global and regional models to remain entrenched across the central and eastern Atlantic for the next several days, resulting in the hurricane moving northwestward for the 72 hours. Thereafter, a mid- to upper-level trough is forecast to dig southward just off the U.S. east coast, creating a deep southerly flow regime that will act to accelerate Sam northward on days 4 and 5. The new NHC track forecast is similar the previous advisory, and lies close to the tightly packed consensus models through 72 hours. In the 96-120 hour period, the models diverge significantly on when and where Sam will begin moving out toward the north, and the NHC track forecast favors the more eastward GFS solution since that model has been performing quite well with Sam thus far. Now that Sam is undergoing an ERC, fluctuations in intensity could occur for the next day or two while the vertical wind shear remains low at least than 10 kt. Thereafter, a slow weakening trend is expected due to a modest increase in the wind shear from the southwest and occasional intrusions of drier mid-level air. The NHC intensity forecast is a little lower than the previous advisory, mainly due to the lower initial intensity, and closely follows the HCCA and IVCN consensus model intensity forecasts. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 14.7N 50.8W 125 KT 145 MPH 12H 27/1200Z 15.3N 51.6W 125 KT 145 MPH 24H 28/0000Z 16.2N 52.8W 120 KT 140 MPH 36H 28/1200Z 17.0N 53.9W 120 KT 140 MPH 48H 29/0000Z 17.8N 55.0W 120 KT 140 MPH 60H 29/1200Z 18.7N 56.3W 115 KT 130 MPH 72H 30/0000Z 20.0N 58.0W 115 KT 130 MPH 96H 01/0000Z 23.4N 61.0W 110 KT 125 MPH 120H 02/0000Z 28.4N 62.6W 105 KT 120 MPH $$ Forecaster Stewart