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Post-Tropical Cyclone Idalia Discussion Number  28
NWS National Hurricane Center Miami FL       AL102023
1100 AM AST Sat Sep 02 2023
 
Although the center of Idalia has moved to the southeast of 
Bermuda, the island is experiencing the cyclone's strongest winds, 
which are located on the northwestern side of the frontal boundary.  
Deep convection persists to the northeast of the center, but the 
system is still extratropical due to the presence of attached 
fronts.  The initial wind speed remains 50 kt based on continuity.

Idalia's initial motion is eastward, or 100/8 kt, and a relatively 
slow eastward motion is expected to continue today.  A shortwave 
trough is forecast to move off the coast of New England and cut off 
from the mid-latitude flow, which should cause Idalia to accelerate 
toward the northeast and then north during the next few days.  
Another slow down is likely on days 4 and 5 a couple of hundred 
miles southeast of Nova Scotia when Idalia becomes collocated with 
the cut-off low.  The NHC track forecast is a bit east of the 
previous forecast, but not as far east as the HCCA aid.

Even though Idalia is producing what could be considered organized 
deep convection, global models fields are fairly unanimous that the 
cyclone will not shed its associated frontal boundaries.  Since the 
most likely scenario is that Idalia will remain an extratropical 
cyclone, a transition to a tropical or subtropical cyclone is no 
longer indicated in the official forecast during the short term 
(although it still can't be completely ruled out).  Little change 
in strength is anticipated during the next day or two, but gradual 
weakening is forecast toward the end of the forecast period when 
Idalia goes through another occlusion and fills south of Nova 
Scotia.
 
Even though Idalia is no longer explicitly forecast to transition 
to a tropical or subtropical cyclone, NHC advisories will continue 
until tropical storm conditions diminish on Bermuda and the 
Tropical Storm Warning is discontinued (most likely some time 
Sunday morning). 

 
KEY MESSAGES:
 
1. Tropical storm conditions will continue in Bermuda through early 
Sunday.

2. Swells from Idalia are forecast to cause life-threatening surf 
and rip current conditions along portions of the east coast of the 
United States through Labor Day weekend.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  02/1500Z 31.6N  63.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 12H  03/0000Z 31.7N  62.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 24H  03/1200Z 32.9N  60.6W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  04/0000Z 34.7N  58.8W   50 KT  60 MPH...POST-TROP/EXTRATROP
 48H  04/1200Z 36.8N  57.4W   45 KT  50 MPH...POST-TROP/EXTRATROP
 60H  05/0000Z 39.0N  56.9W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  05/1200Z 40.8N  57.4W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  06/1200Z 42.3N  58.7W   35 KT  40 MPH...POST-TROP/EXTRATROP
120H  07/1200Z 44.3N  58.6W   30 KT  35 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Berg
 
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