Tropical Storm Sebastien Forecast Discussion Number 3

By | November 20, 2019

Issued at 1100 PM AST Tue Nov 19 2019

WTNT45 KNHC 200237

Tropical Storm Sebastien Discussion Number   3
NWS National Hurricane Center Miami FL       AL202019
1100 PM AST Tue Nov 19 2019

There hasn't much of a change with Sebastien for most of tonight,
with the center still exposed to the northwest of a large area of
deep convection.  Recently, the low-level center appears to be
slowing down and gaining some latitude as convection re-fires near
the center. The initial wind speed will stay 40 kt, pending receipt
of ASCAT data later this evening.

The initial motion has been more to the west during the past few
hours, but a longer-term motion is 290/8.  Sebastien should move
generally northwestward on Wednesday, northward on Wednesday night
and much faster to the northeast on Thursday as the storm moves
along the southwestern periphery of the subtropical ridge and then
ahead of an approaching mid-latitude trough. The forecast has been
adjusted westward in the short-term due mostly to the initial
motion, and then eastward in the longer term due to models
suggesting the storm may stay a little more ahead of the cold front
than before.  These forecast changes, however, are fairly typical
for a disorganized system.

Sebastien has some chance to intensify by late tomorrow as the shear
vector and the storm motion vector line up around the same time as
the upper-level divergence is forecast to increase. This should
cause the cyclone to strengthen, and the new forecast is closely
aligned with the previous one. Around 48 hours, Sebastien is
expected to become an extratropical cyclone as it is overtaken by a
cold front, and then become absorbed by the larger front on Friday.
The latest NHC wind speed forecast is close to the model consensus,
but below the regional hurricane models.


INIT  20/0300Z 20.7N  60.6W   40 KT  45 MPH
 12H  20/1200Z 21.5N  61.5W   40 KT  45 MPH
 24H  21/0000Z 22.9N  61.5W   45 KT  50 MPH
 36H  21/1200Z 24.4N  59.8W   50 KT  60 MPH
 48H  22/0000Z 27.2N  56.5W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  23/0000Z...DISSIPATED

Forecaster Blake

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