Tropical Storm Sebastien Forecast Discussion

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000
WTNT45 KNHC 211449
TCDAT5

Tropical Storm Sebastien Discussion Number   9
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Thu Nov 21 2019

Sebastien continues to produce a large area of deep convection over
the eastern semicircle of the circulation, but the banding features
are not very well defined.  The intensity estimate remains at 50 kt
based on a blend of subjective Dvorak estimates from TAFB and SAB.
A diffluent upper-level wind environment and warm waters are
expected to bring about some strengthening through tonight, however
the window of opportunity for strengthening will probably close
tomorrow morning.  After that time, drier air and strong shear
should cause gradual weakening.  The official intensity forecast is
a little above the model consensus.  In a couple of days, the
GFS shows Sebastien becoming embedded in a frontal zone, signifying
the transition to an extratropical cyclone as shown in the NHC
forecast.  The ECMWF model suggests that this event could occur
later, but the current close proximity of a front to Sebastian
argues for extratropical transition to occur within 48 hours.

The circulation is a bit elongated from south to north, and the
center remains difficult to locate.  This results in a greater than
normal uncertainty in the initial motion, and my best guess is 030/7
kt.  A turn toward the northeast, with acceleration, is expected
during the next couple of days due to the influence of a mid-level
trough.  There is considerable along-track model spread, with the
ECMWF forecast being much slower than the majority of the guidance.
The official forecast is somewhat slower than the dynamical model
consensus out of respect for that typically reliable model.


FORECAST POSITIONS AND MAX WINDS

INIT  21/1500Z 23.5N  60.5W   50 KT  60 MPH
 12H  22/0000Z 25.0N  58.3W   65 KT  75 MPH
 24H  22/1200Z 27.2N  55.0W   70 KT  80 MPH
 36H  23/0000Z 30.0N  50.4W   65 KT  75 MPH
 48H  23/1200Z 33.2N  45.2W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  24/1200Z 39.7N  31.5W   35 KT  40 MPH...POST-TROP/EXTRATROP
 96H  25/1200Z...DISSIPATED

$$
Forecaster Pasch



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