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Hurricane Sam Forecast Discussion Number 9
2021-09-24 22:57:16| National Hurricane Center (Atlantic)
Issued at 500 PM AST Fri Sep 24 2021 000 WTNT43 KNHC 242057 TCDAT3 Hurricane Sam Discussion Number 9 NWS National Hurricane Center Miami FL AL182021 500 PM AST Fri Sep 24 2021 Sam's structure on satellite has improved this afternoon, with overshooting cloud tops rotating quickly around the small central dense overcast, providing the appearance that the hurricane is mixing out the dry air that affected its core structure this morning. In fact, the last few visible satellite images suggest that a small eye is clearing out within the cirrus canopy. Interestingly, a recent 1930 UTC SSMIS pass suggests concentric bands are also forming in Sam's core structure and it remains to be seen how that structure will affect the wind field evolution of the hurricane. The latest subjective Dvorak intensities estimates form TAFB and SAB were a consensus T4.5/75 kt, while the latest SATCON and ADT estimates from UW-CIMSS have also increased this afternoon. The current advisory intensity has been set to 75-kt. Given the recent trends on satellite, this may be conservative. The motion of Sam has remained on the same heading, though the storm is beginning to slow down with the estimate now at 280/10 kt. An additional slowdown in forward motion is anticipated in the short-term as the mid-level ridging shuffles to the northwest of the tropical cyclone, impeding its forward motion. However, after 72 hours, this same ridge is expected to shift back to the northeast of Sam as a deep-layer trough off the Eastern United States coastline becomes established. Track guidance spread increases towards the end of the forecast, with the ECMWF and GFS remaining at odds, with the former on the equatorward side, and the latter on the poleward side of the track envelope. The consensus aids, however, have changed little from the previous advisory and the latest NHC track forecast is quite similar to the previous one. Now that Sam appears to have mixed out the dry air seen this morning, intensification, likely rapid, is resuming. The only fly in the ointment is the current concentric banding structure on microwave imagery, which hints at the possibility of an eyewall replacement cycle. Assuming the smaller eye will not collapse in the short term, rapid intensification appears likely over the next 12-24 hours and the intensity has been raised in the short term, taking Sam to major hurricane intensity by tomorrow. The latest NHC intensity forecast now shows a slightly higher peak at 120-kt in 48 hours followed by very gradual decay, due to a subtle increase in vertical wind shear, potentially some upwelling effects due to the slow forward notion, and eyewall replacement cycles that could lead to a broadening of the wind field. The latest NHC intensity forecast starts out on the high side of the intensity guidance but ends up very close to the HCCA consensus aid by the end of the forecast period. Regardless of the details, Sam is expected to be a significant hurricane through the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 12.1N 44.8W 75 KT 85 MPH 12H 25/0600Z 12.3N 46.5W 90 KT 105 MPH 24H 25/1800Z 12.7N 48.1W 105 KT 120 MPH 36H 26/0600Z 13.1N 49.3W 115 KT 130 MPH 48H 26/1800Z 13.5N 50.5W 120 KT 140 MPH 60H 27/0600Z 14.3N 51.7W 115 KT 130 MPH 72H 27/1800Z 15.1N 52.9W 115 KT 130 MPH 96H 28/1800Z 16.9N 55.1W 110 KT 125 MPH 120H 29/1800Z 18.9N 58.1W 110 KT 125 MPH $$ Forecaster Papin
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