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