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Hurricane Sam Forecast Discussion Number 15
2021-09-26 10:47:43| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Sep 26 2021 000 WTNT43 KNHC 260847 TCDAT3 Hurricane Sam Discussion Number 15 NWS National Hurricane Center Miami FL AL182021 500 AM AST Sun Sep 26 2021 Sam remains a small but intense hurricane this morning, with a well-defined 10 n mi-wide eye. A 0500 UTC AMSR-2 microwave pass reveals Sam has a compact inner core of deep convection surrounding its eye, with curved convective bands primarily extending around the northern and eastern portions of the circulation. Infrared cloud top temperatures briefly warmed overnight in portions of Sam's central dense overcast, perhaps due to some intrusions of drier mid-level air. The 06z objective and subjective satellite estimates ranged from 113-127 kt, but Sam's satellite presentation has improved in recent infrared imagery as cloud top temperatures are cooling around the eye. Thus, the initial intensity is held at 125 kt for this advisory. NOAA aircraft are scheduled to investigate Sam later this afternoon, which should provide helpful information about the structure and intensity of the hurricane. Sam is moving toward the west-northwest, or 295/7 kt. A deep-layer subtropical ridge to the north and northeast of Sam should generally steer the hurricane west-northwestward to northwestward for the next several days. By midweek, an amplifying upper-level trough will move off the east coast of the U.S. and dig southward over the western Atlantic, which will erode the western extent of the steering ridge. The deep-layer southerly flow ahead of the trough should cause Sam to begin recurving, with a somewhat faster north-northwestward motion forecast by day 5. The latest NHC track forecast is very similar to the previous one and generally lies between the HFIP corrected consensus approach (HCCA) and TVCA aids. At days 4-5, there is more spread in the track guidance, with the ECMWF on the far left side of the guidance envelope and the GFS on the far right. The NHC forecast track has been nudged just slightly to the left at 96 and 120 h, but it still lies to the right of HCCA. The hurricane will likely experience some fluctuations in intensity during the next day or two. On the one hand, Sam remains over warm sea-surface temperatures of around 28.5 deg C, with vertical wind shear less than 10 kt as diagnosed from the SHIPS guidance. However, GOES-16 water vapor imagery shows some drier mid-level air in the surrounding environment that may periodically entrain into the inner core of Sam. Additionally, the onset of an eyewall replacement cycle remains a distinct possibility at this stage of Sam's life cycle, although the latest microwave data does not suggest one is imminent. The official NHC intensity forecast lies on the high end of the guidance for the first 36-48 h of the forecast period. Thereafter, the NHC forecast trends closer to the HCCA and IVCN consensus aids. Some gradual weakening is forecast later in the period as the southwesterly vertical wind shear begins to increase, but Sam is still forecast to remain a major hurricane through the 5-day period. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 13.8N 49.7W 125 KT 145 MPH 12H 26/1800Z 14.3N 50.6W 125 KT 145 MPH 24H 27/0600Z 15.0N 51.8W 120 KT 140 MPH 36H 27/1800Z 15.8N 52.9W 120 KT 140 MPH 48H 28/0600Z 16.6N 53.9W 115 KT 130 MPH 60H 28/1800Z 17.5N 55.0W 115 KT 130 MPH 72H 29/0600Z 18.4N 56.3W 115 KT 130 MPH 96H 30/0600Z 20.8N 59.3W 110 KT 125 MPH 120H 01/0600Z 24.5N 62.0W 110 KT 125 MPH $$ Forecaster Reinhart
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