Tropical Storm Sebastien Forecast Discussion

WTNT45 KNHC 201447

Tropical Storm Sebastien Discussion Number   5
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Wed Nov 20 2019

Sebastien continues to struggle with dry air and northwesterly
shear, with satellite imagery showing an exposed low-level
center to the west of the deep convection. The current structure of
the storm should prevent significant intensification during the next
12 hours. After that time, Sebastien will begin to interact with a
mid- to upper-level trough that will produce upper-level divergence
over the cyclone. This will provide a window for Sebastien to
intensify before making the transition to an extratropical cyclone
in about 48 hours. All available guidance intensifies the storm, and
the dynamical models continue to strengthen it more than the
statistical models. In fact, the mesoscale models unanimously make
Sebastian a hurricane within 36 hours. However, these models are
likely intensifying the cyclone too quickly over the next 12 hours,
and therefore may have a slight high bias. The official forecast
takes this into account and keeps the storm just below hurricane
strength before extratropical transition by 48 hours, which is a
blend of the ECMWF and GFS solutions.

Sebastien appears to be approaching its westernmost point and a turn
to the north should begin later today, followed by acceleration to
the northeast starting tonight due to the influence of the
approaching trough. The track guidance is in agreement on this
overall scenario. The NHC forecast track is just slightly slower
than the previous one in the first 24 hours, and is close to the
model consensus.


INIT  20/1500Z 21.1N  61.7W   45 KT  50 MPH
 12H  21/0000Z 22.0N  61.7W   45 KT  50 MPH
 24H  21/1200Z 23.7N  60.2W   55 KT  65 MPH
 36H  22/0000Z 26.3N  57.2W   60 KT  70 MPH
 48H  22/1200Z 29.8N  52.5W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  23/1200Z...DISSIPATED

Forecaster Latto/Mello


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