Tropical Storm Sebastien Forecast Discussion Number 17

By | November 23, 2019

Issued at 1100 AM AST Sat Nov 23 2019

WTNT45 KNHC 231454

Tropical Storm Sebastien Discussion Number  17
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Sat Nov 23 2019

First-light visible imagery and a few late-arriving microwave
overpasses revealed that Sebastien's elongated center is located
well northeast of previous estimates. It isn't clear if the center
reformed or if it simply accelerated more than expected overnight,
but the initial motion estimate is now 055/23 kt. A significant
change to the track forecast was made based on the updated
initial position, and the NHC forecast now shows a much faster
motion for Sebastien.

The cyclone's cloud pattern is somewhat ragged and does not look
entirely tropical. That said, Sebastien is still producing enough
deep convection to be classified as a tropical storm. Unfortunately,
no ASCAT data was available this morning to help assess the strength
of the winds or how well-defined the circulation is at the surface.
The initial intensity is conservatively maintained at 55 kt based
primarily on continuity from previous advisories, but this could be
generous since Dvorak intensity estimates are lower.

Every aspect of the forecast remains highly uncertain. The models
have been extremely inconsistent in forecasting Sebastien, with the
guidance varying drastically from run to run and with unusually
large spread in each forecast. The latest indications are that
Sebastien will move quickly northeastward with little change in
strength for the next few days before it eventually opens into a
trough around the time it nears the Azores in a couple of days.
Simulated satellite imagery suggests that the cyclone will still
produce enough deep convection to be classified as a tropical storm
through Sunday, though it could become post-tropical before it
reaches the Azores. Given the elongated nature of Sebastien's
circulation and its fast forward speed, it could also open into a
trough and dissipate at any time.

It is not currently clear if Sebastien will reach the Azores as a
tropical storm. Regardless of its status, Sebastien or its remnants
could bring gusty winds and rain to the Azores beginning Sunday.
Please see products issued by the Portuguese Institute for the Sea
and the Atmosphere (IPMA) for more information.


INIT  23/1500Z 30.4N  47.7W   55 KT  65 MPH
 12H  24/0000Z 32.1N  44.5W   55 KT  65 MPH
 24H  24/1200Z 34.4N  40.0W   55 KT  65 MPH
 36H  25/0000Z 36.8N  34.0W   55 KT  65 MPH
 48H  25/1200Z 39.3N  26.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  26/1200Z...DISSIPATED

Forecaster Zelinsky

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