Post-Tropical Cyclone Idalia Discussion Number  23
NWS National Hurricane Center Miami FL       AL102023
500 AM AST Fri Sep 01 2023

Idalia remains an extratropical cyclone with a front extending to
the northeast of the center. The system also continues to be devoid 
of deep convection. An earlier ASCAT-B scatterometer overpass 
caught the western portion of the circulation and it revealed peak
winds of around 45 kt.  Based on that data, the initial intensity 
has been lowered to 50 kt for this advisory.

The cyclone appears to be turning east-southeast and slowing down 
as forecast. An even slower east-southeast to eastward motion is 
expected during the next 24 to 36 hours as a mid-latitude trough 
passes to the north of the system.  Another shortwave trough 
exiting the Mid-Atlantic coast of the United States on Sunday is 
expected to cause Idalia to begin moving faster toward the northeast 
later in the weekend.  There is tremendous spread in the track 
guidance after 72 hours. The ECMWF takes Idalia northward ahead of 
the aforementioned trough, while the GFS keeps a much weaker 
cyclone meandering over the central subtropical Atlantic.  The 
latest NHC track forecast lies between these extremes and is close 
to the multi-model consensus aids and the GFS ensemble mean. 

Additional weakening is expected during the next 12-24 hours.  
After that time, simulated satellite imagery from the GFS and ECMWF 
indicate that deep convection is likely to return, potentially 
transitioning Idalia back to a tropical cyclone when it is near 
Bermuda.  Some slight re-strengthening is predicted during that 
time.  Later in the forecast period, little change in strength is 
forecast as vertical wind shear increases and Idalia is likely to  
transition to an extratropical cyclone once again. The 
extratropical transition has been moved up to 96 hours, but the 
latest ECMWF run suggests that the transition could occur much 
sooner.  Given the differences in the global models regarding the 
structure and track of Idalia after 72 hours, there is significant 
uncertainty in the long range track and intensity forecast for 
this system.  

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.


FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 32.6N  69.8W   50 KT  60 MPH...POST-TROPICAL
12H  01/1800Z 31.8N  67.9W   45 KT  50 MPH...POST-TROP/EXTRATROP
24H  02/0600Z 31.0N  66.3W   40 KT  45 MPH...POST-TROP/EXTRATROP
36H  02/1800Z 31.0N  65.2W   40 KT  45 MPH...TROPICAL CYCLONE
48H  03/0600Z 31.7N  63.6W   45 KT  50 MPH
60H  03/1800Z 33.2N  61.8W   50 KT  60 MPH
72H  04/0600Z 34.6N  60.2W   50 KT  60 MPH
96H  05/0600Z 37.0N  58.7W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  06/0600Z 38.5N  59.3W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown