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Tropical Storm Sebastien Discussion Number  16
NWS National Hurricane Center Miami FL       AL202019
500 AM AST Sat Nov 23 2019

The cloud pattern of Sebastien has changed significantly overnight.
Deep convection has decreased substantially during the past several
hours and the remaining thunderstorms are organized around a
mid-level eye feature seen in satellite images.  Model vertical
cross sections indicate that Sebastien is extremely titled with the
low-level center, although I can't see it, likely well displaced to
the southwest of the mid-level eye.  The initial intensity is held
at 55 kt based on the ASCAT data that came in a little after 00 Z,
but it looks like Sebastien is on a weakening trend.

Strong southwesterly vertically wind shear of about 40 kt is
responsible for the titled structure of the cyclone.  Since the
shear is not expected to let up and because Sebastien is headed for
steadily cooler waters, it seems reasonable to assume that weakening
is likely.  Although the models agree that Sebastien should slowly
lose its strength, it remains unclear if Sebastien will transition
to a post-tropical cyclone before it opens up into a trough during
the next few days.  The NHC forecast continues to show dissipation
by day 3, but it is certainty possible that it dissipates before
then.

The tropical storm is moving east-northeastward at 15 kt ahead of a
cold front and trough.  This motion with an increase in forward
speed is expected during the next few days as Sebastien becomes more
embedded in the faster mid-latitude flow.  The models are in
relatively good agreement, and only small changes were made to the
previous NHC forecast.

The initial wind radii were modified based on the aforementioned
ASCAT passes, and the 12-ft sea radii were adjusted based on ship
and altimeter data.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 27.8N  50.6W   55 KT  65 MPH
 12H  23/1800Z 29.4N  48.2W   50 KT  60 MPH
 24H  24/0600Z 31.6N  44.2W   45 KT  50 MPH
 36H  24/1800Z 34.0N  38.8W   40 KT  45 MPH
 48H  25/0600Z 37.0N  31.6W   35 KT  40 MPH
 72H  26/0600Z...DISSIPATED

$$
Forecaster Cangialosi

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