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| #1150797 (Received by flhurricane at: 10:45 AM 01.Sep.2023) TCDAT5 Post-Tropical Cyclone Idalia Discussion Number 24 NWS National Hurricane Center Miami FL AL102023 1100 AM AST Fri Sep 01 2023 Idalia is not producing deep convection and remains an extratropical cyclone, with the strongest winds located along the northwestern side of the attached frontal boundary. The initial intensity remains 50 kt based on continuity from the previous advisory. Scatterometer should hit the area of strongest winds later this morning and give us a better idea on the intensity. The cyclone may be finally slowing down a bit, and the initial motion estimate is east-southeastward (110 degrees) at 15 kt. Additional deceleration is expected during the next 36 hours while the center turns eastward and moves near or to the south of Bermuda. A shortwave trough is forecast to move off the coast of New England by early Sunday (about 48 hours), and that feature should turn Idalia toward the northeast at a faster speed by day 3. Even then, there is disagreement among the models whether this trough will pull Idalia quickly northward (e.g., the ECMWF) or if it will leave Idalia behind (e.g, the GFS). The NHC track forecast therefore shows a slower north or north-northeast motion on days 4 and 5, leaning toward the various consensus aids. There remains significant uncertainty on what type of cyclone Idalia will be during the next 3 days. On one hand, simulated satellite imagery from the GFS and ECMWF models shows deep convection redeveloping near Idalia`s center on Saturday once the cyclone reaches warmer waters. On the other hand, global model fields show the thickness gradient near the front weakening, but they don`t necessarily show Idalia shedding all of its frontal features. To maintain continuity from previous forecasts, the NHC forecast shows Idalia regaining tropical storm status in about 24 hours while it is near Bermuda, and some reintensification is possible after that time. Model fields show the temperature gradient tightening again as Idalia goes through a possible occlusion in 3-4 days, and the forecast therefore shows an extratropical cyclone again on day 4. Based on the updated forecast, the Bermuda Weather Service has issued a Tropical Storm Warning for the island. KEY MESSAGES: 1. Tropical storm conditions are expected in Bermuda by Saturday. Heavy rainfall will likely cause areas of flash and urban flooding on the island. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 31.9N 67.9W 50 KT 60 MPH...POST-TROP/EXTRATROP 12H 02/0000Z 31.3N 66.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 24H 02/1200Z 30.9N 64.9W 40 KT 45 MPH...TROPICAL CYCLONE 36H 03/0000Z 31.1N 63.7W 45 KT 50 MPH 48H 03/1200Z 32.0N 61.8W 50 KT 60 MPH 60H 04/0000Z 33.5N 59.9W 50 KT 60 MPH 72H 04/1200Z 35.1N 58.2W 45 KT 50 MPH 96H 05/1200Z 38.3N 57.2W 40 KT 45 MPH...POST-TROP/EXTRATROP 120H 06/1200Z 40.1N 57.1W 35 KT 40 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg |
