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| #1150873 (Received by flhurricane at: 4:51 PM 01.Sep.2023) TCDAT5 Post-Tropical Cyclone Idalia Discussion Number 25 NWS National Hurricane Center Miami FL AL102023 500 PM AST Fri Sep 01 2023 Idalia is still an extratropical cyclone lacking deep convection. ASCAT data from a few hours ago showed winds of 40-45 kt to the northwest of the center, so the initial intensity is set at 45 kt. The initial motion is still toward the east-southeast (105 degrees), but the forward speed has slowed down to about 13 kt. Idalia is becoming collocated with a developing mid-level low, and it is therefore expected to slow down further and turn eastward during the next 24 hours as it moves near Bermuda. After 24 hours, a shortwave trough is forecast to move off the coast of New England and kick Idalia faster toward the northeast through day 3. But then that shortwave trough is expected to cut off and cause Idalia to slow down again on days 4 and 5 while it moves generally northward. The NHC track forecast is shifted a bit to the left of the previous forecast based on the latest track guidance, and to account for a slight adjustment to the initial position. ECMWF- and GFS-based simulated satellite imagery insist that deep convection will redevelop with Idalia tonight or on Saturday as the system moves over warmer waters and while vertical shear is relatively low. However, the associated frontal zone does not appear to completely dissipate in the global model fields, and FSU phase-space diagrams indicate that the cyclone is likely to have a symmetric but shallow warm core. The NHC forecast continues to show Idalia becoming a tropical cyclone on Saturday, however it is possible that it could be closer to the subtropical side of the spectrum. Some strengthening is possible during the tropical/subtropical phase, and the NHC intensity forecast lies close to the IVCN intensity consensus. Idalia is then expected to become fully extratropical again by day 3. 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 through the weekend. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 32.0N 66.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 12H 02/0600Z 31.7N 65.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 24H 02/1800Z 31.5N 64.1W 50 KT 60 MPH...TROPICAL CYCLONE 36H 03/0600Z 32.2N 62.5W 50 KT 60 MPH 48H 03/1800Z 33.7N 60.6W 50 KT 60 MPH 60H 04/0600Z 35.6N 59.0W 50 KT 60 MPH 72H 04/1800Z 37.8N 58.2W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 05/1800Z 40.9N 58.4W 35 KT 40 MPH...POST-TROP/EXTRATROP 120H 06/1800Z 42.0N 58.6W 30 KT 35 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg |
