Tropical Storm Sebastien Forecast Discussion

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WTNT45 KNHC 210831

Tropical Storm Sebastien Discussion Number   8
NWS National Hurricane Center Miami FL       AL202019
500 AM AST Thu Nov 21 2019

Sebastien continues to produce a large area of showers and
thunderstorms over its southeastern quadrant with more disorganized
convection to the north and west of the center.  Despite the fair
amount of convection, the cloud pattern lacks banding features and
the center of the storm has been challenging to locate.  The
initial intensity is held at 50 kt based on ASCAT data from several
hours ago.  This data also indicate that Sebastien's wind field is
quite lopsided, with all of its tropical-storm-force winds confined
to the eastern side of the circulation.

The initial motion of the storm is tough to assess given that there
is more than the usual amount of uncertainty in the current
position, but my best guess is 015/7 kt.  A deep-layer trough over
the western Atlantic is approaching Sebastien, and that feature
should cause the storm to accelerate to the northeast during the
next few days.  Although the models agree on the overall scenario,
there are major along-track or speed differences among the models,
with the GFS/HWRF/HMON being the fastest solutions and the ECMWF
being the slowest.  In fact, at 48 hours, the spread between the GFS
and ECMWF models is more than 700 n mi.  The NHC forecast lies
roughly near the middle of the guidance envelope and ends up near
the UKMET model, but this prediction is of low confidence.

Upper-level diffluence, relatively warm SSTs, and a moist
environment should allow for Sebastien to strengthen during the next
24 hours or so.  In fact, all of the hurricane regional models show
significant or even rapid intensification during that time period.
Given the asymmetric structure of Sebastien currently, rapid
intensification seems unlikely, and the NHC intensity prediction
lies closer to the lower end of the guidance in the short term.
Beyond 24 hours, Sebastien will be moving into progressively more
hostile conditions of increasing shear, drier air, and cooler
waters.  These conditions should end the opportunity for
strengthening and begin the process of extratropical transition.
Sebastien is expected to become fully extratropical in about 48
hours, but this could occur sooner if the GFS is correct or later if
the ECMWF is correct.  The extratropical low is forecast to slowly
weaken and dissipate in about 4 days.


INIT  21/0900Z 23.0N  61.2W   50 KT  60 MPH
 12H  21/1800Z 24.5N  59.3W   60 KT  70 MPH
 24H  22/0600Z 26.6N  56.4W   70 KT  80 MPH
 36H  22/1800Z 29.4N  52.0W   70 KT  80 MPH
 48H  23/0600Z 32.4N  47.1W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  24/0600Z 38.6N  34.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  25/0600Z...DISSIPATED

Forecaster Cangialosi

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