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Hurricane Sam Forecast Discussion Number 13
2021-09-25 22:40:07| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Sep 25 2021 000 WTNT43 KNHC 252039 TCDAT3 Hurricane Sam Discussion Number 13 NWS National Hurricane Center Miami FL AL182021 500 PM AST Sat Sep 25 2021 After a brief hiccup, Sam has resumed rapid strengthening and now shows a 12-15 n mi wide well-defined eye embedded in a central dense overcast with cloud tops colder than -70C. Subjective satellite intensity estimates from TAFB and SAB were 115 kt at 18Z, and objective intensity estimates have also been near 115 kt. Since the organization has increased some since 18Z, the initial intensity is increased to 120 kt. A NOAA Hurricane Hunter aircraft is currently enroute to Sam to provide a better estimate of the intensity. The hurricane has good cirrus outflow in all quadrants. However, a large arc cloud to the northwest of the central core suggests dry air is present in that area. The initial motion is still west-northwestward or 290/9. There are again no changes to the forecast track reasoning. The weak subtropical ridge currently to the north of Sam is forecast to shift eastward as a large deep-layer mid-latitude trough develops over the western Atlantic. This evolution should cause Sam to continue west-northwestward with some decrease in forward speed during the next couple of days, followed by a turn toward the northwest. Some increase in forward speed is likely by 120 h as Sam encounters stronger steering flow. The track guidance has shifted a little to the north and east since the previous advisory, and the new forecast track is also nudged in that direction, with the new track lying between the HCCA corrected consensus model and the other consensus models. Based on the NHC forecast, and all of the other guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5. The current strengthening is expected to continue for the next 6-12 h, and based on this the new intensity forecast now calls for a peak intensity of 130 kt. After the peak, Sam is expected to be in an environment of warm sea surface temperatures and light wind shear for at least the next 3-4 days. This suggests the intensity will be controlled mainly by eyewall replacement cycles and possible intrusions of dry air, which would cause some short-term fluctuations in intensity. In the bigger picture, the intensity guidance calls for a slow weakening during the 24-120 h forecast period, and the new intensity forecast does the same. However, it is likely that Sam will remain a major hurricane through 120 h. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 13.3N 48.5W 120 KT 140 MPH 12H 26/0600Z 13.7N 49.4W 130 KT 150 MPH 24H 26/1800Z 14.3N 50.5W 125 KT 145 MPH 36H 27/0600Z 14.9N 51.6W 120 KT 140 MPH 48H 27/1800Z 15.7N 52.7W 120 KT 140 MPH 60H 28/0600Z 16.6N 53.9W 115 KT 130 MPH 72H 28/1800Z 17.4N 55.0W 115 KT 130 MPH 96H 29/1800Z 19.5N 58.0W 110 KT 125 MPH 120H 30/1800Z 22.5N 61.0W 110 KT 125 MPH $$ Forecaster Beven
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