Tropical Storm Sebastien Forecast Discussion

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378 
WTNT45 KNHC 210253
TCDAT5

Tropical Storm Sebastien Discussion Number   7
NWS National Hurricane Center Miami FL       AL202019
1100 PM AST Wed Nov 20 2019

Deep convection has intensified near and to the southeast of the
center of Sebastien this evening, and a recent partial ASCAT
pass suggests the center is still on the edge of the thunderstorms.
Satellite intensity estimates have risen slightly, so the initial
wind speed is set to 50 kt, on the high side of the recent values.

The storm continues to move slowly to the north tonight.
Increasing deep-layer flow ahead of a mid-latitude trough should
turn Sebastien northeastward overnight and cause the cyclone to
further accelerate by late Thursday and Friday.  The models aren't
in great agreement on how quickly the storm will move to the
northeast, however, with the model guidance having a stronger storm
generally moving faster to the northeast.  Since Sebastien
isn't expected to get that intense, it makes physical sense to
avoid the fastest solutions.  Thus, the new forecast is slower than
the last one, closer to the model consensus than the GFS-based
guidance.

The environment is forecast to become more conducive for
strengthening during the next 24 hours as significant upper-level
divergence increases near Sebastien.  Combined with low or
moderate shear and warm waters, these conditions should support
further intensification, and the new NHC forecast now shows
Sebastien as a hurricane for a short period of time.  Notably, this
forecast is still on the conservative side of the guidance, with all
of the regional hurricane models showing Sebastien becoming a fairly
potent hurricane in a day or two.  This doesn't seem likely after
examining the model initial structure of the HWRF/HMON models, which
show a much more vertically aligned cyclone than Sebastien appears
to be now, so the NHC forecast is only a bit higher than the last
one. In a couple of days, the storm will be moving over cooler
waters, with increasing shear and baroclinicity. That should
facilitate Sebastien's transition to a non-tropical cyclone by the
end of the work week, but note that the extratropical transition
has been shifted back about a day due to the stronger-than-forecast
cyclone likely staying a little more separate from an incoming cold
front.


FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 22.2N  61.5W   50 KT  60 MPH
 12H  21/1200Z 23.3N  60.5W   55 KT  65 MPH
 24H  22/0000Z 25.5N  57.7W   65 KT  75 MPH
 36H  22/1200Z 28.0N  53.9W   65 KT  75 MPH
 48H  23/0000Z 31.0N  49.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  24/0000Z 37.0N  38.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  25/0000Z...DISSIPATED

$$
Forecaster Blake




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