769 WTNT41 KNHC 220252 TCDAT1 Potential Tropical Cyclone Sixteen Discussion Number 3 NWS National Hurricane Center Miami FL AL162023 1100 PM EDT Thu Sep 21 2023 Nighttime proxy-visible satellite imagery suggests that the center of the low pressure system off the southeastern U.S. coast has become better defined this evening. Nearly all of the associated deep convection remains oriented in a curved band extending around the northern and eastern sides of the circulation, although a few convective elements are beginning to fill in on the back side of the system as well. Based on 1-minute wind observations from NOAA buoy 41002 and ship reports, the initial intensity is raised to 35 kt, with all tropical-storm-force winds currently north of the center on the north side of an attached warm front. The center does not appear to have moved much since the time it became better defined. Based on previous fixes, the current motion is estimated to be north-northeastward (015 degrees) at 3 kt. The dynamical guidance indicates that the center should begin moving northward--or re-form to the north--overnight, with a northward or north-northwestward motion continuing through Sunday morning. This forecast track brings the center of the low inland across eastern North Carolina on Saturday, and then across the southern Chesapeake Bay and the Delmarva Peninsula Saturday night and Sunday. Except for the errant HWRF (which appears too far west), the rest of the guidance is tightly clustered, and the NHC forecast is near the middle of the envelope. This new forecast is a bit east of the previous prediction, mainly for the portion of the track over land. Baroclinic processes associated with a sharp upper-level trough to the west of the low are expected induce strengthening, and the NHC intensity forecast is close to the GFS, ECMWF, and HCCA solutions, bringing the maximum winds to 50 kt in 24-36 hours before the center reaches the North Carolina coast. In addition, simulated satellite imagery suggests there could be an increase of deep convection near the low's center by Friday evening, and phase-space diagrams show the system just barely entering the deep warm core regime. Therefore, transition to a tropical cyclone is shown by 24 hours, although there still is some uncertainty on that occurrence due to frontal boundaries still in the area. Weakening is forecast after the center moves onshore, and the system is expected to become extratropical again by 60 hours. An official forecast is only provided out through 72 hours since global model fields show the low becoming absorbed by the associated frontal boundary off the New Jersey coast by 96 hours. Key Messages: 1. Low pressure off the southeastern U.S. coast is producing tropical-storm-force winds and is forecast to strengthen further before it reaches the coast of North Carolina on Saturday. Tropical storm conditions are expected along portions of the southeastern and mid-Atlantic U.S. coasts within the Tropical Storm Warning area beginning on Friday and continuing into Saturday night. 2. There is a danger of life-threatening storm surge inundation over portions of eastern North Carolina and southeastern Virginia, including Pamlico and Albemarle Sounds, and the lower Chesapeake Bay, where Storm Surge Warnings are in place. Residents in these areas should follow advice given by local officials. 3. Heavy rainfall from this system could produce localized urban and small stream flooding impacts across the eastern mid-Atlantic states from North Carolina to New Jersey Friday through Sunday. 4. Swells generated by this system will affect much of the U.S. east coast through the weekend, likely causing life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 29.5N 75.3W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 22/1200Z 31.2N 75.5W 45 KT 50 MPH...POTENTIAL TROP CYCLONE 24H 23/0000Z 32.5N 76.1W 50 KT 60 MPH...TROPICAL CYCLONE 36H 23/1200Z 34.4N 76.5W 50 KT 60 MPH 48H 24/0000Z 36.4N 76.6W 40 KT 45 MPH...INLAND 60H 24/1200Z 37.9N 76.2W 30 KT 35 MPH...POST-TROP/EXTRATROP 72H 25/0000Z 39.0N 75.2W 30 KT 35 MPH...POST-TROP/EXTRATROP 96H 26/0000Z...DISSIPATED $$ Forecaster Berg