Tropical Storm Sebastien Forecast Discussion

WTNT45 KNHC 212032

Tropical Storm Sebastien Discussion Number  10
NWS National Hurricane Center Miami FL       AL202019
500 PM AST Thu Nov 21 2019

Convection associated with Sebastien has become better organized
during the past several hours, as a band has formed near and over
the center in the eastern semicircle.  Overall, the cloud pattern
remains elongated from north-to-south as the storm is interacting
with a cold front not far to its northwest.  Scatterometer data
received just after the last advisory indicated 45-50 kt winds, and
based on this and current satellite intensity estimates the initial
intensity remains 50 kt.

The cyclone has turned northeastward with the initial motion now
050/8.  Southwesterly mid-latitude flow should steer Sebastien or
its remnants quickly northeastward, and the guidance is in good
agreement on the direction of movement during the next several
days.  However, there is a significant spread in the forward speed,
with the new ECMWF run being much slower than the other guidance.
The new track forecast will be only slightly slower than the
previous forecast, but adjustments to the forecast forward speed
may be necessary later.

Sebastien is in an apparently unfavorable environment for
development due to strong shear, decreasing sea surface temperatures
along the forecast track, and an expected extratropical transition
as it merges with the front.  The intensity guidance is rather
divergent during the first 24 h, with the SHIPS and LGEM models
showing weakening while the dynamical guidance continues to forecast
strengthening despite the environment.  After that time, all
guidance indicates that Sebastien should weaken.  The intensity
forecast continues the trend of the previous forecast in showing
some strengthening during the first 24 h in agreement with the
dynamical models, followed by weakening and extratropical transition
by 48 h. Given how close the cold front is to the storm at this
time, it is possible that extratropical transition could occur
earlier than currently forecast.


INIT  21/2100Z 23.8N  59.7W   50 KT  60 MPH
 12H  22/0600Z 25.5N  57.1W   60 KT  70 MPH
 24H  22/1800Z 27.9N  53.2W   65 KT  75 MPH
 36H  23/0600Z 30.6N  48.4W   55 KT  65 MPH
 48H  23/1800Z 33.5N  43.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  24/1800Z 39.0N  31.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
 96H  25/1800Z...DISSIPATED

Forecaster Beven


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