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Tropical Storm Sebastien Forecast Discussion


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000
WTNT45 KNHC 220247
TCDAT5

Tropical Storm Sebastien Discussion Number  11
NWS National Hurricane Center Miami FL       AL202019
1100 PM AST Thu Nov 21 2019

Sebastien continues to produce a central dense overcast and a large
band on the southeastern side of the circulation. Recent
scatterometer data indicate that the maximum winds are at least 45
kt and, since there has been little overall change in the satellite
presentation within the past several hours, the initial wind speed
will stay 50 kt.

Two notable changes have been made on this forecast.   As
foreshadowed in the previous forecast, the models are no longer
rapidly accelerating the storm to the northeast ahead of a
mid-latitude trough. Instead, there is better agreement tonight on a
steadier motion to the northeast due to the trough in the short
term.  Sebastien will then continue moving northeastward embedded in
the mid-latitude southwesterly flow over the weekend. The new
forecast is substantially slower than the last one, but is still on
the faster side of the guidance due to continuity concerns.

The second significant change is that Sebastien is no longer
expected to become a hurricane.  The cyclone will soon be moving
over sub-26C waters with strong shear, and it seems unlikely to
intensify much in these conditions.  Model intensity guidance
continues to decrease, and the NHC forecast follows that trend.
Weakening should start by the weekend over even cooler waters in a
high-shear environment, and these factors are forecast to cause
Sebastien to transition into a non-convective low or extratropical
cyclone in 36-48 h. Careful watchers of the forecast will note that
NHC has predicted extratropical transition at 48 h since Sebastien
was born over 48 h ago, and this timing has been pushed back
primarily due to the slower-than-expected motion.  Thus, this timing
can't be considered a particularly confident prediction until the
track forecast becomes more consistent.


FORECAST POSITIONS AND MAX WINDS

INIT  22/0300Z 24.7N  57.8W   50 KT  60 MPH
 12H  22/1200Z 26.0N  55.5W   55 KT  65 MPH
 24H  23/0000Z 28.0N  51.7W   55 KT  65 MPH
 36H  23/1200Z 30.3N  47.7W   45 KT  50 MPH
 48H  24/0000Z 32.7N  43.4W   40 KT  45 MPH...POST-TROP/EXTRATROP
 72H  25/0000Z 37.5N  34.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
 96H  26/0000Z...DISSIPATED

$$
Forecaster Blake