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| #1246321 (Received by flhurricane at: 10:57 PM 27.Sep.2025) TCDAT4 Tropical Depression Nine Discussion Number 6 NWS National Hurricane Center Miami FL AL092025 1100 PM EDT Sat Sep 27 2025 The depression this evening remains fairly disorganized, with convection mainly oriented in a north-south fashion with only a hint of banding on the northern side. A NOAA-P3 aircraft that has been flying through the system only found a peak 700 mb flight level wind of 34 kt about 60 n mi away from the center. Tail Doppler Radar (TDR) analysis from the aircraft also shows the center is quite broad at the 1 km level with the mid-level center tilted to the south with height. Based on the plane data, the initial intensity remains 30 kt this advisory, which is a little under the various satellite-based estimates, but close to a recently received scatterometer pass which had a peak wind of 31 kt. Aircraft fixes both at 700 mb and from the TDR analysis shows the cyclone is meandering right now, with a best guess at a motion of 310/2 kt. Over the next couple of days, the system should turn north-northwestward with a somewhat faster forward motion, moving within the flow between an upper trough over the southeast U.S. and a subtropical ridge over the Atlantic. This forecast brings the depression over the central and northwestern Bahamas through early Monday, then toward the southeast U.S. coast by Tuesday. It is notable to point out the system has been moving slower than anticipated over the last 12-24 hours, and this could have significant implications on its ultimate track. Even in the first 48 hours of the track, there is quite a bit of spread in the north-south direction in both the deterministic and ensemble model guidance. Faster tracks appear to interact more with the upper-level trough to the west, resulting in a track closer to the Carolina coastline, while slower tracks largely miss a direct interaction with this feature, and turn sharply east faster as Hurricane Humberto approaches from the east, eroding the mid-level ridging steering TD9 northward. Because it has been moving slower than expected, its becoming more likely its ultimate track will fall in the slower and sharper east camp. In response, the overall track guidance this cycle shifted slower and further east, and the NHC track forecast was shifted in that direction. The latest track roughly splits the difference between the ECMWF and GFS tracks, and lies quite close to the HCCA consensus aid, but still not as far south and east as the Google DeepMind ensemble mean (GDMI). The depression is now showing signs of intensification yet, with a broad diffuse structure on aircraft observations persisting. However, the intensity guidance is insistent that deep central convection will soon form near the center, helping to tighten up the surface wind field. While the cyclone will have some southerly shear to deal with, it does not appear to be prohibitively strong, only 15-20 kt for the next 48 hours in both the GFS and ECMWF-based SHIPS guidance. That, combined with warm 29-30 C sea-surface temperatures should promote steady intensification once a central core develops, and the NHC intensity forecast was nudged up a little higher showing a 70 kt peak in 48 hours. After this period is where the intensity forecast gets tricky. The large spread in north-south tracks also has a significant impact on the amount of shear the system will face in the 48-96 hour time-frame. Solutions that are further south have less shear to contend with and are generally stronger. For now, the latest intensity forecast will cap the peak intensity at 70 kt through the end of the forecast, close to the HCCA intensity guidance, but this could be conservative if the track shifts further south and east. The system could also interact with a baroclinic zone towards the end of the forecast, but because the forecast track remains over warm waters at day 5, extratropical transition won't be shown quite yet. Even if this further offshore track pans out, the wind field is expected to expand quite a bit, and given the expected storm structure, a risk of significant heavy rainfall, storm surge, and wind impacts is still possible for the southeast U.S. coast even if the storm center remains offshore. KEY MESSAGES: 1. The depression is expected to strengthen and bring tropical storm conditions to portions of the central and northwestern Bahamas through Sunday night. Tropical storm conditions are also possible along portions of the east coast of central Florida beginning Monday, where a Tropical Storm Watch is in effect. 2. Rainfall will continue to impact eastern Cuba and the Bahamas through the weekend, which will likely produce flash and urban flooding. Mudslides are possible in the higher terrain. 3. There is a significant threat of heavy rainfall early next week from portions of the east coast of Florida northward into the Carolinas, which could cause flash, urban, and river flooding. 4. The depression is expected to be at or near hurricane intensity when it is closest to the southeast U.S. coast early next week, where there is a risk of storm surge and wind impacts. Although it is too soon to specify the exact location and magnitude of impacts, residents should closely monitor the latest forecast updates and ensure that they have their hurricane plan in place. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 22.4N 76.7W 30 KT 35 MPH 12H 28/1200Z 23.6N 77.0W 35 KT 40 MPH 24H 29/0000Z 25.5N 77.5W 50 KT 60 MPH 36H 29/1200Z 27.3N 77.9W 60 KT 70 MPH 48H 30/0000Z 28.7N 78.2W 70 KT 80 MPH 60H 30/1200Z 29.9N 78.1W 70 KT 80 MPH 72H 01/0000Z 30.4N 77.3W 70 KT 80 MPH 96H 02/0000Z 31.0N 75.0W 70 KT 80 MPH 120H 03/0000Z 31.5N 71.4W 70 KT 80 MPH $$ Forecaster Papin |
