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Tropical Storm Sebastien Forecast Discussion


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970 
WTNT45 KNHC 220834
TCDAT5

Tropical Storm Sebastien Discussion Number  12
NWS National Hurricane Center Miami FL       AL202019
500 AM AST Fri Nov 22 2019

Sebastien has become less organized this morning.  Strong
southwesterly vertical wind shear of about 40 kt and dry air have
caused the low-level center of the storm to become exposed to the
southwest of the main area of deep convection.  The initial
intensity remains 50 kt, based on the earlier ASCAT data, but this
is a little above the latest Dvorak estimates and could be generous.

The center of Sebastien has turned more to the right than expected,
with the initial motion estimated to be 065/13 kt.  The models have
changed significantly this cycle, and now show a much slower forward
motion to the northeast during the next few days.  This change
appears to be connected to a more vertically shallow system that
moves in the lower-level flow.  The NHC track forecast has been
adjusted considerably slower and a little to the right of the
previous one, but it still is faster than nearly all of the
typically reliable models.  Future adjustments will likely be made
if the model trend continues.

It no longer appears that Sebastien will strengthen.  In fact, all
of the intensity guidance now suggests that the storm will gradually
weaken during the next few days as Sebastien remains in strong wind
shear conditions and moves over progressively cooler waters.  In
addition, the model guidance has finally come into agreement that
Sebastien will not merge with the nearby cold front. Instead, the
cyclone is expected to move ahead of the weakening front and become
a non-convective post-tropical cyclone in about 2 days, or less,
when it moves over SSTs below 22 C.  As mentioned in the previous
discussion, NHC and the models have had a challenging time figuring
out when Sebastien would lose its tropical characteristics, and we
have held the transition timing steady at 48 hours for a while.
Although there is still uncertainty, the models are in better
agreement in the timing of this transition.  This intensity forecast
is a little lower than the previous one to come into better
agreement with the latest models.


FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 24.8N  57.0W   50 KT  60 MPH
 12H  22/1800Z 26.0N  54.3W   50 KT  60 MPH
 24H  23/0600Z 27.2N  51.7W   45 KT  50 MPH
 36H  23/1800Z 28.7N  49.1W   40 KT  45 MPH
 48H  24/0600Z 30.0N  46.2W   35 KT  40 MPH...POST-TROPICAL
 72H  25/0600Z 33.3N  38.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  26/0600Z...DISSIPATED

$$
Forecaster Cangialosi