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| #1150666 (Received by flhurricane at: 4:48 PM 31.Aug.2023) TCDAT5 Post-Tropical Cyclone Idalia Discussion Number 21 NWS National Hurricane Center Miami FL AL102023 500 PM EDT Thu Aug 31 2023 Idalia has become post-tropical. The center has become less defined, and the system is attached to a well-defined frontal boundary that extends to the north and northeast of the center. In addition, the cyclone is no longer producing deep convection. Earlier ASCAT and Air Force Reserve reconnaissance aircraft showed that a band of 50-55 kt winds has developed to the northwest of the center near the front, and the initial intensity is therefore set at 55 kt. The initial motion is eastward, or 090/18 kt. Idalia is embedded within the base of a deep-layer trough moving across the northeastern U.S. and Atlantic Canada, but this trough is expected to leave Idalia behind in the next 24-48 hours. As a result, Idalia is forecast to turn east-southeastward and slow down considerably (less than 5 kt) when it approaches Bermuda by 48 hours. A second trough that moves off the U.S. east coast on Sunday should turn Idalia toward the northeast and north at a faster forward speed on days 4 and 5. The updated NHC track forecast is a little south and east of the previous forecast beyond day 2, leaning toward the model consensus aids. HCCA, in particular, is on the eastern side of the guidance envelope, which may suggest that additional eastward adjustment might be needed in subsequent advisories. Idalia is expected to remain a convection-less extratropical cyclone for the next day or two, particularly since it is expected to move over the cold wake of Hurricane Franklin. Its intensity is also likely to gradually weaken during that time. However, the cyclone is forecast to reach warm sea surface temperatures again by Saturday, and global model fields show the thickness gradient weakening. In addition, the GFS and ECMWF simulated satellite show deep convection redeveloping by Saturday or Saturday night. Therefore, Idalia is expected to transition back to a tropical storm in 48 hours, with some restrengthening possible. Global model fields show a possible occlusion occurring around day 5, and the NHC forecast therefore shows Idalia again becoming extratropical by the end of the forecast period. KEY MESSAGES: 1. Tropical storm conditions are possible in Bermuda by Saturday. Heavy rainfall will likely lead to areas of flash and urban flooding on the island. 2. Water levels are expected to subside and winds are expected to diminish along the coast of North Carolina through tonight. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 33.5N 73.5W 55 KT 65 MPH...POST-TROP/EXTRATROP 12H 01/0600Z 33.0N 70.8W 50 KT 60 MPH...POST-TROP/EXTRATROP 24H 01/1800Z 31.8N 68.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 36H 02/0600Z 31.0N 66.7W 40 KT 45 MPH...POST-TROP/EXTRATROP 48H 02/1800Z 30.9N 65.9W 45 KT 50 MPH...TROPICAL CYCLONE 60H 03/0600Z 31.3N 64.9W 50 KT 60 MPH 72H 03/1800Z 32.1N 63.4W 50 KT 60 MPH 96H 04/1800Z 35.1N 60.5W 50 KT 60 MPH 120H 05/1800Z 38.0N 59.9W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg |
