[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 010844
TCDAT5

Post-Tropical Cyclone Idalia Discussion Number  23
NWS National Hurricane Center Miami FL       AL102023
500 AM AST Fri Sep 01 2023

Idalia remains an extratropical cyclone with a front extending to
the northeast of the center. The system also continues to be devoid
of deep convection. An earlier ASCAT-B scatterometer overpass
caught the western portion of the circulation and it revealed peak
winds of around 45 kt.  Based on that data, the initial intensity
has been lowered to 50 kt for this advisory.

The cyclone appears to be turning east-southeast and slowing down
as forecast. An even slower east-southeast to eastward motion is
expected during the next 24 to 36 hours as a mid-latitude trough
passes to the north of the system.  Another shortwave trough
exiting the Mid-Atlantic coast of the United States on Sunday is
expected to cause Idalia to begin moving faster toward the northeast
later in the weekend.  There is tremendous spread in the track
guidance after 72 hours. The ECMWF takes Idalia northward ahead of
the aforementioned trough, while the GFS keeps a much weaker
cyclone meandering over the central subtropical Atlantic.  The
latest NHC track forecast lies between these extremes and is close
to the multi-model consensus aids and the GFS ensemble mean.

Additional weakening is expected during the next 12-24 hours.
After that time, simulated satellite imagery from the GFS and ECMWF
indicate that deep convection is likely to return, potentially
transitioning Idalia back to a tropical cyclone when it is near
Bermuda.  Some slight re-strengthening is predicted during that
time.  Later in the forecast period, little change in strength is
forecast as vertical wind shear increases and Idalia is likely to
transition to an extratropical cyclone once again. The
extratropical transition has been moved up to 96 hours, but the
latest ECMWF run suggests that the transition could occur much
sooner.  Given the differences in the global models regarding the
structure and track of Idalia after 72 hours, there is significant
uncertainty in the long range track and intensity forecast for
this system.

KEY MESSAGES:

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


FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 32.6N  69.8W   50 KT  60 MPH...POST-TROPICAL
 12H  01/1800Z 31.8N  67.9W   45 KT  50 MPH...POST-TROP/EXTRATROP
 24H  02/0600Z 31.0N  66.3W   40 KT  45 MPH...POST-TROP/EXTRATROP
 36H  02/1800Z 31.0N  65.2W   40 KT  45 MPH...TROPICAL CYCLONE
 48H  03/0600Z 31.7N  63.6W   45 KT  50 MPH
 60H  03/1800Z 33.2N  61.8W   50 KT  60 MPH
 72H  04/0600Z 34.6N  60.2W   50 KT  60 MPH
 96H  05/0600Z 37.0N  58.7W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  06/0600Z 38.5N  59.3W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown
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