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Tropical Storm Sebastien Discussion Number  22
NWS National Hurricane Center Miami FL       AL202019
500 PM AST Sun Nov 24 2019

Sebastien is gradually losing tropical characteristics.  Satellite
images show a large shield of relatively cold cloud tops on the
north side of the system and in a band that extends from east of the
center south-southwestward.  This cloud pattern resembles an
extratropical cyclone.  However, satellite images and microwave data
also show a small area of deep convection lingering over the center,
which is why the system is still designated a tropical cyclone for
now.  That being said, it is still not clear that the surface
circulation of the storm is well defined. Hopefully, ASCAT data
later this evening can help assess the structure of the circulation.
The initial intensity is held at 55 kt, which is in agreement with
the most recent SATCON estimate from CIMSS at the University of
Wisconsin.

Sebastien is expected to slowly weaken as it remains in very strong
wind shear conditions while moving over progressively colder
waters. FSU phase-analysis diagrams from the global models all show
Sebastien becoming fully extratropical overnight, and the NHC
forecast follows that scenario.  The extratropical low is expected
to merge with another cyclone in 24 to 36 hours.  It is also
possible, as alluded to above, that Sebastien could open into a
trough before then.

Sebastien is racing toward the east-northeast ahead of a cold front,
and the initial motion estimate is 060/34 kt.  This continued
heading at an even faster rate is expected within the fast
mid-latitude flow until the cyclone dissipates.

Regardless of whether it is a tropical cyclone, extratropical
cyclone, or a remnant trough, Sebastien or its remnants will likely
bring gusty winds and heavy rains to portions of the Azores through
Monday morning.  Please see products issued by the Portuguese
Institute for the Sea and the Atmosphere (IPMA) for more
information.


FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 39.2N  32.5W   55 KT  65 MPH
 12H  25/0600Z 42.2N  25.7W   55 KT  65 MPH...POST-TROP/EXTRATROP
 24H  25/1800Z 46.3N  17.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  26/0600Z...MERGED WITH ANOTHER EXTRATROPICAL LOW

$$
Forecaster Cangialosi

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