Tropical Storm Sebastien Forecast Discussion Number 4

By | November 20, 2019

Issued at 500 AM AST Wed Nov 20 2019

WTNT45 KNHC 200837

Tropical Storm Sebastien Discussion Number   4
NWS National Hurricane Center Miami FL       AL202019
500 AM AST Wed Nov 20 2019

Deep convection developed closer to the center of Sebastien
around the time of the previous advisory, but northwesterly shear
and dry air have caused it to wane within the past hour or two.  A
late arriving ASCAT-C overpass revealed peak winds a little above
40 kt. Given the typical undersampling of that instrument, the
initial intensity has been raised to 45 kt. The shear and dry air
are expected to keep Sebastien's intensity in check today, but by
tonight the upper-level divergence is forecast to increase while the
storm motion and shear vector line up.  This is expected to allow
for some strengthening and nearly all of the intensity guidance and
global models predict deepening.  The new intensity forecast is
again similar to the previous advisory and lies between the lower
statistical guidance and the more aggressive dynamical models.

Sebastien appears to have turned northwestward with an initial
motion of 305/7 kt. A turn to the north should occur later today,
and then a northeastward motion is predicted to begin by Wednesday
night as the storm is picked up by a deep layer trough moving
eastward across the western Atlantic.  The track guidance is in
agreement on this overall scenario but there are some significant
differences in how quickly Sebastien accelerates northeastward.  The
GFS, HWRF, and HMON show the storm recurving quickly ahead of
the trough while the ECMWF is quite a bit slower.  The NHC track is
a little slower than the previous forecast, but remains close to the
various consensus aids and the latest UKMET ensemble mean.


INIT  20/0900Z 21.0N  61.0W   45 KT  50 MPH
 12H  20/1800Z 21.7N  61.5W   45 KT  50 MPH
 24H  21/0600Z 23.2N  60.8W   50 KT  60 MPH
 36H  21/1800Z 25.3N  58.6W   55 KT  65 MPH
 48H  22/0600Z 28.7N  54.4W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  23/0600Z...DISSIPATED

Forecaster Brown

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