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| #1246427 (Received by flhurricane at: 4:54 PM 28.Sep.2025) TCDAT4 Tropical Storm Imelda Discussion Number 9 NWS National Hurricane Center Miami FL AL092025 500 PM EDT Sun Sep 28 2025 In the last outbound northeast leg, the Air Force reconnaissance flying through the system found 850 mb flight level winds of 43 kt, high enough to support upgrading Tropical Depression 9 to Tropical Storm Imelda at the 18 UTC intermediate advisory. On that last fix, a dropsonde launched in the center had a 1000 mb pressure, but with a 34 kt surface wind gust that could be related to a nearby convective feature, and a 998 mb pressure is used as the minimum pressure. The initial intensity remains at 35 kt this advisory, which is on the low side of the mean of the satellite estimates, which range from 35-45 kt. A more established motion with Imelda appears to finally be occuring this afternoon, due northward at 360/8 kt. A general northward motion is expected to continue over the next day or so as southerly steering persists from both an upper trough over the southeastern United States and the subtropical ridge centered just east of Bermuda. After that period of time, the subtropical ridge's steering influence decreases as Hurricane Humberto's larger outer circulation creates a large weakness. The end result of this pattern change is that Imelda is now forecast to turn sharply east-northeastward into this weakness, starting on Tuesday and continuing through the remainder of the forecast period, following behind Humberto. The track guidance this cycle has finally started to stabilize near the previous forecast track, though is still a tick faster than the previous forecast cycle, and quite a bit of along-track spread remains in the 4-5 day time frame. The NHC track forecast lies in between the GFS and ECMWF track solutions, closest to but not quite as fast as the Google DeepMind ensemble mean (GDMI). Imelda's structure is gradually becoming better organized, with more curvature in the convective bands on its eastern side, though the earlier burst closer to the center has waned. Assuming a better developed core forms soon, low to moderate vertical wind shear between 15-20 kt, warm 28-30 C sea-surface temperatures, and sufficient mid-level moisture should promote steady intensification over the next 2-3 days. In 3-4 days, the cyclone is expected to find itself in the right entrance region of an intensifying upper-level jet, and the enhanced upper-level divergence in that time may help support additional intensification even as the shear begins to increase at this period. After the system passes Bermuda, a strong baroclinic zone is expected to interact with the system, resulting in extratropical transition by day 5 as it gets tangled up with a frontal boundary. However, as stressed previously, this is a low confidence forecast at long range given the recent large track changes. In fact, simulated satellite imagery from the 12 UTC ECMWF still suggests Imelda may produce significant convective bursts while still over warm ocean waters in the day 4-5 forecast period. KEY MESSAGES: 1. Imelda is expected to strengthen and bring tropical storm conditions to portions of the central and northwestern Bahamas through Monday. 2. Rainfall associated with Tropical Storm Imelda will continue to impact eastern Cuba and the Bahamas through Tuesday, which will likely produce flash and urban flooding. Mudslides are possible in the higher terrain. Heavy rainfall across the coastal Carolinas could cause isolated flash, urban, and river flooding through Tuesday. 3. The risk of significant wind impacts along the southeastern United States coast is decreasing, but interests in that area should continue to monitor the latest forecast updates. 4. Swells and high surf from both Humberto and Imelda are expected to produce dangerous marine conditions and rip currents along the east coast of Florida and the Georgia coast through Monday. These conditions are expected to spread northward along much of the east coast of the United States early this week. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 24.2N 77.3W 35 KT 40 MPH 12H 29/0600Z 25.5N 77.4W 45 KT 50 MPH 24H 29/1800Z 27.2N 77.4W 55 KT 65 MPH 36H 30/0600Z 28.4N 77.2W 60 KT 70 MPH 48H 30/1800Z 29.2N 75.8W 65 KT 75 MPH 60H 01/0600Z 29.8N 73.9W 70 KT 80 MPH 72H 01/1800Z 30.7N 70.7W 80 KT 90 MPH 96H 02/1800Z 31.8N 63.2W 80 KT 90 MPH 120H 03/1800Z 33.5N 58.5W 65 KT 75 MPH...POST-TROP/EXTRATROP $$ Forecaster Papin |
