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Post-Tropical Cyclone Idalia Discussion Number  26
NWS National Hurricane Center Miami FL       AL102023
1100 PM AST Fri Sep 01 2023
 
Satellite images and surface observations indicate that Idalia
remains an extratropical cyclone with frontal features extending
through the center.  However, deep convection has recently formed
just to the northeast of the center, and that could signal the
beginning of a transition to a subtropical or tropical cyclone.
The initial intensity is held at 45 kt.
 
The cyclone is still moving eastward, or 100 degrees, at about 14
kt.  A continued eastward motion, but at a much slower pace, is
expected on Saturday as the system separates from a deep-layer
trough.  After that time, a turn to the northeast and then the
north is forecast as the cyclone cuts off from the progressive flow
over the north Atlantic.  The NHC track forecast is a little
slower than the previous one to be in better agreement with the
latest consensus models.
 
GFS and ECMWF simulated satellite images suggest that deep
convection will continue to increase near the center overnight, and
that could influence Idalia's transition to a tropical or
subtropical cyclone on Saturday.  There could also be some increase
in strength during that time.  However, by late in the weekend,
very strong southwesterly shear is expected to affect the system
and that should end the opportunity for strengthening and cause the
system to transition back to an extratropical cyclone.  The NHC
intensity forecast is similar to the previous one and near the
middle of the guidance envelope.
 
KEY MESSAGES:
 
1. Tropical storm conditions are expected in Bermuda on Saturday.
Heavy rainfall will likely cause areas of flash and urban flooding
on the island through the weekend.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  02/0300Z 31.8N  65.3W   45 KT  50 MPH...POST-TROPICAL
 12H  02/1200Z 31.6N  64.3W   50 KT  60 MPH...TROPICAL CYCLONE
 24H  03/0000Z 31.8N  62.9W   50 KT  60 MPH
 36H  03/1200Z 32.8N  61.2W   50 KT  60 MPH
 48H  04/0000Z 34.5N  59.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  04/1200Z 36.7N  58.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  05/0000Z 38.8N  58.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  06/0000Z 41.1N  58.8W   35 KT  40 MPH...POST-TROP/EXTRATROP
120H  07/0000Z 42.6N  58.9W   30 KT  35 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Cangialosi
 
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