[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 012035
TCDAT5

Post-Tropical Cyclone Idalia Discussion Number  25
NWS National Hurricane Center Miami FL       AL102023
500 PM AST Fri Sep 01 2023

Idalia is still an extratropical cyclone lacking deep convection.
ASCAT data from a few hours ago showed winds of 40-45 kt to the
northwest of the center, so the initial intensity is set at 45 kt.

The initial motion is still toward the east-southeast (105 degrees),
but the forward speed has slowed down to about 13 kt.  Idalia is
becoming collocated with a developing mid-level low, and it is
therefore expected to slow down further and turn eastward during
the next 24 hours as it moves near Bermuda.  After 24 hours, a
shortwave trough is forecast to move off the coast of New England
and kick Idalia faster toward the northeast through day 3.  But
then that shortwave trough is expected to cut off and cause Idalia
to slow down again on days 4 and 5 while it moves generally
northward.  The NHC track forecast is shifted a bit to the left of
the previous forecast based on the latest track guidance, and to
account for a slight adjustment to the initial position.

ECMWF- and GFS-based simulated satellite imagery insist that deep
convection will redevelop with Idalia tonight or on Saturday as the
system moves over warmer waters and while vertical shear is
relatively low.  However, the associated frontal zone does not
appear to completely dissipate in the global model fields, and FSU
phase-space diagrams indicate that the cyclone is likely to have a
symmetric but shallow warm core.  The NHC forecast continues to show
Idalia becoming a tropical cyclone on Saturday, however it is
possible that it could be closer to the subtropical side of the
spectrum.  Some strengthening is possible during the
tropical/subtropical phase, and the NHC intensity forecast lies
close to the IVCN intensity consensus.  Idalia is then expected to
become fully extratropical again by day 3.


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 through the weekend.


FORECAST POSITIONS AND MAX WINDS

INIT  01/2100Z 32.0N  66.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
 12H  02/0600Z 31.7N  65.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
 24H  02/1800Z 31.5N  64.1W   50 KT  60 MPH...TROPICAL CYCLONE
 36H  03/0600Z 32.2N  62.5W   50 KT  60 MPH
 48H  03/1800Z 33.7N  60.6W   50 KT  60 MPH
 60H  04/0600Z 35.6N  59.0W   50 KT  60 MPH
 72H  04/1800Z 37.8N  58.2W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  05/1800Z 40.9N  58.4W   35 KT  40 MPH...POST-TROP/EXTRATROP
120H  06/1800Z 42.0N  58.6W   30 KT  35 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg
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