Tropical Storm Idalia Forecast Discussion Number 17

By | August 30, 2023

WTNT45 KNHC 302050

Tropical Storm Idalia Discussion Number  17
NWS National Hurricane Center Miami FL       AL102023
500 PM EDT Wed Aug 30 2023

Idalia's radar and satellite presentation has become increasingly 
asymmetric.  A coastal front appears to have developed just inland 
of the coasts of Georgia and South Carolina, with heavy rainfall 
being enhanced along and to the northwest of this boundary.  The 
highest WSR-88D velocities observed are about 60-65 kt at an 
elevation of 5000 feet, indicating that Idalia is no longer a 
hurricane.  The advisory intensity is set at 60 kt, which is 
probably a bit generous.

The initial motion is now northeastward, or 035/18 kt, with Idalia 
moving between the northwestern periphery of a mid-level ridge 
located over the Greater Antilles and a deep-layer trough moving 
across the northeastern U.S.  The storm is expected to turn toward 
the east and east-southeast during the next 48 hours, but then slow 
down considerably as the trough lifts out and leaves Idalia behind. 
There is fairly high confidence that Idalia will move slowly 
eastward over the western Atlantic, in the vicinity of Bermuda, 
during the latter part of the forecast period, although there is 
less confidence in the details of that motion due to large model 

Additional weakening is expected while Idalia's center continues 
moving over land through tonight.  However, once the center moves 
back offshore on Thursday, model fields suggest that Idalia could 
interact with the aforementioned coastal front, with an area of 
stronger winds developing along the boundary near eastern North 
Carolina during the day.  Based on this scenario, the NHC intensity 
forecast closely follows the GFS solution at 24 and 36 hours.  
After 36 hours, there is more uncertainty on Idalia's intensity and 
structure.  Some guidance suggests that Idalia will continue 
interacting with the front and possibly become post-tropical, 
particularly if it struggles to maintain organized deep convection. 
For now, the NHC forecast keeps Idalia as a tropical storm for the 
entire forecast period.


1. Storm surge is expected along the southeastern U.S. coast within 
the Storm Surge Warning area through tonight.  Coastal flooding is 
also expected within the Storm Surge Watch area in North Carolina on 
Thursday.  Residents in these areas should follow any advice given 
by local officials.

2. Tropical-storm-force winds will affect portions of the 
southeastern U.S. coast through Thursday.

3. Areas of flash, urban, and moderate river flooding, with 
considerable impacts, are expected from east central Georgia, 
through eastern South Carolina and eastern North Carolina into 


INIT  30/2100Z 32.2N  81.7W   60 KT  70 MPH...INLAND
 12H  31/0600Z 33.4N  79.3W   50 KT  60 MPH...INLAND
 24H  31/1800Z 34.1N  75.7W   50 KT  60 MPH...OVER WATER
 36H  01/0600Z 33.7N  72.2W   45 KT  50 MPH
 48H  01/1800Z 32.8N  69.7W   45 KT  50 MPH
 60H  02/0600Z 32.1N  68.2W   45 KT  50 MPH
 72H  02/1800Z 31.7N  67.3W   45 KT  50 MPH
 96H  03/1800Z 32.3N  65.1W   45 KT  50 MPH
120H  04/1800Z 34.4N  61.3W   45 KT  50 MPH

Forecaster Berg

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