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Post-Tropical Cyclone Idalia Discussion Number  27
NWS National Hurricane Center Miami FL       AL102023
500 AM AST Sat Sep 02 2023
 
Although there has been an increase in convective activity 
associated with Idalia overnight, the cyclone remains attached to
a frontal boundary, and is therefore still analyzed as an
extratropical low.  The cyclone appears to have deepened slightly 
with the minimum pressure estimated to be around 999 mb, based on 
surface observations from Bermuda.  With the slight deepening, 
the initial wind speed has been increased to 50 kt.   Winds across 
Bermuda have increased overnight with the official observing site 
at the LF Wade International Airport reporting wind gusts as high 
as 43 kt.  Some of the elevated weather stations on the island 
have reported sustained tropical-storm-force winds.  

The global models indicate that little change in strength will 
occur during the next day or so.  The guidance also suggests that 
Idalia may not completely shed its frontal structure despite the 
presence of deep convection to the northeast of its center today. 
As a result, the official forecast now shows Idalia potentially 
becoming a subtropical cyclone for a fairly short period of time 
this weekend, but confidence in that is low.  Regardless of 
Idalia's status, tropical-storm-force winds will continue over 
Bermuda through tonight.  Later in the period, the cyclone should 
gradually weaken while it moves northward over the central 
Atlantic. 

The forward speed of Idalia has slowed considerably overnight, with 
the initial motion estimate of eastward or 090/5 kt.  A slow 
eastward motion is expected today, but a shortwave trough moving 
off the coast of the northeastern United States tonight should 
cause Idalia to begin moving northeastward on Sunday.  By late 
Monday the cyclone is expected to turn northward around the eastern 
side of a developing cut-off low south of Nova Scotia. The track 
guidance is in somewhat better agreement than it was a day ago, and 
the latest NHC track prediction is near the center of the model 
envelope. 

KEY MESSAGES:
 
1. Tropical storm conditions will continue in Bermuda into tonight. 
Heavy rainfall may cause areas of flash and urban flooding on the 
island.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  02/0900Z 31.7N  64.6W   50 KT  60 MPH...POST-TROPICAL
 12H  02/1800Z 31.5N  63.9W   50 KT  60 MPH...SUBTROPICAL CYCLONE
 24H  03/0600Z 32.1N  62.2W   50 KT  60 MPH
 36H  03/1800Z 33.6N  60.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 48H  04/0600Z 35.7N  58.6W   50 KT  60 MPH...POST-TROP/EXTRATROP
 60H  04/1800Z 37.7N  57.6W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  05/0600Z 40.0N  57.8W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  06/0600Z 41.6N  58.6W   35 KT  40 MPH...POST-TROP/EXTRATROP
120H  07/0600Z 43.5N  59.0W   30 KT  35 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Brown
 
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