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Hurricane Sam Forecast Discussion Number 19
2021-09-27 10:49:06| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Sep 27 2021 000 WTNT43 KNHC 270848 TCDAT3 Hurricane Sam Discussion Number 19 NWS National Hurricane Center Miami FL AL182021 500 AM AST Mon Sep 27 2021 The satellite presentation of Sam deteriorated overnight, as GOES-16 infrared imagery showed periodic disruptions to Sam's inner core convection. The eye has not been readily apparent in conventional satellite imagery for much of the night, although very recent imagery suggests an eye could be re-emerging. Data from the reconnaissance aircraft last night indicated that an eyewall replacement cycle (ERC) was underway, and some dry mid-level air impinging on the western side of the circulation could also be negatively impacting Sam. No recent high-resolution microwave data is available, but an earlier scatterometer pass revealed that the tropical-storm-force wind radii had expanded just a bit in the eastern semicircle of the hurricane. The initial intensity is lowered to 115 kt for this advisory based on a blend of the objective and subjective Dvorak current intensity estimates. A NOAA hurricane hunter aircraft is scheduled to investigate Sam later this morning, which will provide crucial data to assess changes in Sam's structure and intensity. The initial motion is estimated to be northwestward, or 315/7 kt. A subtropical ridge to the north and northeast of Sam will remain the primary steering mechanism over the next several days, and Sam is expected to maintain a northwestward heading through midweek. Then, a mid- to upper-level trough is forecast to dig southward over the western Atlantic late this week. The deep-layer southerly flow ahead of this feature should cause Sam to move faster toward the north-northwest by 96 h, followed by a northward acceleration thereafter. The NHC forecast track is shifted slightly to the right of the previous one through the first 48-60 h of the forecast period, which accounts for the more northwestward initial motion observed during the past 12 h. Otherwise, the NHC track is virtually unchanged as the guidance has remained fairly consistent. Sam's intensity has likely peaked now that an ERC has commenced, but environmental conditions should allow Sam to persist as a major hurricane for the next several days. Sea-surface temperatures are expected to be 28 deg C or greater along Sam's track through 120 h, and the vertical wind shear is forecast to remain weak (< 10 kt) through at least midweek. Thus, the official NHC intensity forecast only shows very gradual weakening during the next several days, generally following the HCCA and IVCN consensus aids. Given the lower initial intensity of Sam, the latest NHC forecast is once again lower than the previous one. By 120 h, southerly shear associated with the deep-layer trough is forecast to increase over Sam, which should induce more weakening by this weekend. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 15.2N 51.4W 115 KT 130 MPH 12H 27/1800Z 15.9N 52.2W 115 KT 130 MPH 24H 28/0600Z 16.8N 53.3W 115 KT 130 MPH 36H 28/1800Z 17.7N 54.4W 115 KT 130 MPH 48H 29/0600Z 18.6N 55.6W 110 KT 125 MPH 60H 29/1800Z 19.6N 57.1W 110 KT 125 MPH 72H 30/0600Z 21.0N 58.8W 110 KT 125 MPH 96H 01/0600Z 24.9N 61.6W 110 KT 125 MPH 120H 02/0600Z 30.5N 62.0W 105 KT 120 MPH $$ Forecaster Reinhart
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