549 WTNT45 KNHC 302050 TCDAT5 Tropical Storm Idalia Discussion Number 17 NWS National Hurricane Center Miami FL AL102023 500 PM EDT Wed Aug 30 2023 Idalia's radar and satellite presentation has become increasingly asymmetric. A coastal front appears to have developed just inland of the coasts of Georgia and South Carolina, with heavy rainfall being enhanced along and to the northwest of this boundary. The highest WSR-88D velocities observed are about 60-65 kt at an elevation of 5000 feet, indicating that Idalia is no longer a hurricane. The advisory intensity is set at 60 kt, which is probably a bit generous. The initial motion is now northeastward, or 035/18 kt, with Idalia moving between the northwestern periphery of a mid-level ridge located over the Greater Antilles and a deep-layer trough moving across the northeastern U.S. The storm is expected to turn toward the east and east-southeast during the next 48 hours, but then slow down considerably as the trough lifts out and leaves Idalia behind. There is fairly high confidence that Idalia will move slowly eastward over the western Atlantic, in the vicinity of Bermuda, during the latter part of the forecast period, although there is less confidence in the details of that motion due to large model spread. Additional weakening is expected while Idalia's center continues moving over land through tonight. However, once the center moves back offshore on Thursday, model fields suggest that Idalia could interact with the aforementioned coastal front, with an area of stronger winds developing along the boundary near eastern North Carolina during the day. Based on this scenario, the NHC intensity forecast closely follows the GFS solution at 24 and 36 hours. After 36 hours, there is more uncertainty on Idalia's intensity and structure. Some guidance suggests that Idalia will continue interacting with the front and possibly become post-tropical, particularly if it struggles to maintain organized deep convection. For now, the NHC forecast keeps Idalia as a tropical storm for the entire forecast period. KEY MESSAGES: 1. Storm surge is expected along the southeastern U.S. coast within the Storm Surge Warning area through tonight. Coastal flooding is also expected within the Storm Surge Watch area in North Carolina on Thursday. Residents in these areas should follow any advice given by local officials. 2. Tropical-storm-force winds will affect portions of the southeastern U.S. coast through Thursday. 3. Areas of flash, urban, and moderate river flooding, with considerable impacts, are expected from east central Georgia, through eastern South Carolina and eastern North Carolina into Thursday. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 32.2N 81.7W 60 KT 70 MPH...INLAND 12H 31/0600Z 33.4N 79.3W 50 KT 60 MPH...INLAND 24H 31/1800Z 34.1N 75.7W 50 KT 60 MPH...OVER WATER 36H 01/0600Z 33.7N 72.2W 45 KT 50 MPH 48H 01/1800Z 32.8N 69.7W 45 KT 50 MPH 60H 02/0600Z 32.1N 68.2W 45 KT 50 MPH 72H 02/1800Z 31.7N 67.3W 45 KT 50 MPH 96H 03/1800Z 32.3N 65.1W 45 KT 50 MPH 120H 04/1800Z 34.4N 61.3W 45 KT 50 MPH $$ Forecaster Berg