[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 312044
TCDAT5

Post-Tropical Cyclone Idalia Discussion Number  21
NWS National Hurricane Center Miami FL       AL102023
500 PM EDT Thu Aug 31 2023

Idalia has become post-tropical.  The center has become less
defined, and the system is attached to a well-defined frontal
boundary that extends to the north and northeast of the center.  In
addition, the cyclone is no longer producing deep convection.
Earlier ASCAT and Air Force Reserve reconnaissance aircraft showed
that a band of 50-55 kt winds has developed to the northwest of the
center near the front, and the initial intensity is therefore set
at 55 kt.

The initial motion is eastward, or 090/18 kt.  Idalia is embedded
within the base of a deep-layer trough moving across the
northeastern U.S. and Atlantic Canada, but this trough is expected
to leave Idalia behind in the next 24-48 hours.  As a result,
Idalia is forecast to turn east-southeastward and slow down
considerably (less than 5 kt) when it approaches Bermuda by 48
hours.  A second trough that moves off the U.S. east coast on
Sunday should turn Idalia toward the northeast and north at a
faster forward speed on days 4 and 5.  The updated NHC track
forecast is a little south and east of the previous forecast beyond
day 2, leaning toward the model consensus aids.  HCCA, in
particular, is on the eastern side of the guidance envelope, which
may suggest that additional eastward adjustment might be needed in
subsequent advisories.

Idalia is expected to remain a convection-less extratropical
cyclone for the next day or two, particularly since it is expected
to move over the cold wake of Hurricane Franklin.  Its intensity is
also likely to gradually weaken during that time.  However, the
cyclone is forecast to reach warm sea surface temperatures again by
Saturday, and global model fields show the thickness gradient
weakening.  In addition, the GFS and ECMWF simulated satellite show
deep convection redeveloping by Saturday or Saturday night.
Therefore, Idalia is expected to transition back to a tropical storm
in 48 hours, with some restrengthening possible.  Global model
fields show a possible occlusion occurring around day 5, and the
NHC forecast therefore shows Idalia again becoming extratropical by
the end of the forecast period.


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.

2. Water levels are expected to subside and winds are expected to
diminish along the coast of North Carolina through tonight.


FORECAST POSITIONS AND MAX WINDS

INIT  31/2100Z 33.5N  73.5W   55 KT  65 MPH...POST-TROP/EXTRATROP
 12H  01/0600Z 33.0N  70.8W   50 KT  60 MPH...POST-TROP/EXTRATROP
 24H  01/1800Z 31.8N  68.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
 36H  02/0600Z 31.0N  66.7W   40 KT  45 MPH...POST-TROP/EXTRATROP
 48H  02/1800Z 30.9N  65.9W   45 KT  50 MPH...TROPICAL CYCLONE
 60H  03/0600Z 31.3N  64.9W   50 KT  60 MPH
 72H  03/1800Z 32.1N  63.4W   50 KT  60 MPH
 96H  04/1800Z 35.1N  60.5W   50 KT  60 MPH
120H  05/1800Z 38.0N  59.9W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg
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