Post-Tropical Cyclone Idalia Discussion Number  24
NWS National Hurricane Center Miami FL       AL102023
1100 AM AST Fri Sep 01 2023

Idalia is not producing deep convection and remains an extratropical 
cyclone, with the strongest winds located along the northwestern 
side of the attached frontal boundary.  The initial intensity 
remains 50 kt based on continuity from the previous advisory.  
Scatterometer should hit the area of strongest winds later this 
morning and give us a better idea on the intensity.

The cyclone may be finally slowing down a bit, and the initial 
motion estimate is east-southeastward (110 degrees) at 15 kt.  
Additional deceleration is expected during the next 36 hours while 
the center turns eastward and moves near or to the south of 
Bermuda.  A shortwave trough is forecast to move off the coast of 
New England by early Sunday (about 48 hours), and that feature 
should turn Idalia toward the northeast at a faster speed by day 3. 
Even then, there is disagreement among the models whether this 
trough will pull Idalia quickly northward (e.g., the ECMWF) or if 
it will leave Idalia behind (e.g, the GFS).  The NHC track forecast 
therefore shows a slower north or north-northeast motion on days 4 
and 5, leaning toward the various consensus aids.

There remains significant uncertainty on what type of cyclone 
Idalia will be during the next 3 days.  On one hand, simulated 
satellite imagery from the GFS and ECMWF models shows deep 
convection redeveloping near Idalia's center on Saturday once the 
cyclone reaches warmer waters.  On the other hand, global model 
fields show the thickness gradient near the front weakening, but 
they don't necessarily show Idalia shedding all of its frontal 
features.  To maintain continuity from previous forecasts, the NHC 
forecast shows Idalia regaining tropical storm status in about 24 
hours while it is near Bermuda, and some reintensification is 
possible after that time.  Model fields show the temperature 
gradient tightening again as Idalia goes through a possible 
occlusion in 3-4 days, and the forecast therefore shows an 
extratropical cyclone again on day 4.  

Based on the updated forecast, the Bermuda Weather Service has 
issued a Tropical Storm Warning for the island.


KEY MESSAGES:

1. Tropical storm conditions are expected in Bermuda by Saturday.
Heavy rainfall will likely cause areas of flash and urban flooding 
on the island.


FORECAST POSITIONS AND MAX WINDS

INIT  01/1500Z 31.9N  67.9W   50 KT  60 MPH...POST-TROP/EXTRATROP
12H  02/0000Z 31.3N  66.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
24H  02/1200Z 30.9N  64.9W   40 KT  45 MPH...TROPICAL CYCLONE
36H  03/0000Z 31.1N  63.7W   45 KT  50 MPH
48H  03/1200Z 32.0N  61.8W   50 KT  60 MPH
60H  04/0000Z 33.5N  59.9W   50 KT  60 MPH
72H  04/1200Z 35.1N  58.2W   45 KT  50 MPH
96H  05/1200Z 38.3N  57.2W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  06/1200Z 40.1N  57.1W   35 KT  40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg