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Tropical Storm Sebastien Discussion Number  14
NWS National Hurricane Center Miami FL       AL202019
500 PM AST Fri Nov 22 2019

Sebastien's structure has not changed since this morning. ASCAT data
that arrived just after the issuance of the previous advisory showed
that the system was producing winds slightly above 45 kt, a little
higher than the previous estimate. Since the structure of the
tropical storm has not changed since the time of the ASCAT, the
intensity has been set at 50 kt. It should be noted that this is
merely an adjustment of the intensity assessment and is not an
indication of strengthening.

The agreement between the intensity models is remarkably poor. In
just the first 24 h of the forecast, the solutions range from
dissipation (HMON), to hurricane-strength (HWRF), to steady-state
(most of the global models). Since the shear is high and SSTs ahead
of the cyclone will be fairly cold, the solution that makes the
most sense to me is that of the global and statistical-dynamical
models, which generally show slow weakening during the next couple
of days. Dissipation is still expected within 72 h.

The Jekyll and Hyde behavior of the models isn't limited to the
intensity forecast. While it seems clear that Sebastien will move
east-northeastward or northeastward along the southern end of a
frontal boundary during the next couple of days, the speed at which
it will move is very unclear. The deeper the cyclone remains, the
more influence strong upper-level southwesterly winds will have on
its track, and the faster it will likely move. The latest track
guidance unanimously shows a dramatically faster forward motion of
Sebastien. In fact, the consensus now shows a forward speed nearly
twice as fast as the previous NHC advisory. The official forecast
continues to chase the models but has not been moved nearly as far
as HCCA or TVCN. Confidence in the track forecast is quite low.

FORECAST POSITIONS AND MAX WINDS

INIT  22/2100Z 26.2N  53.7W   50 KT  60 MPH
 12H  23/0600Z 27.3N  51.8W   50 KT  60 MPH
 24H  23/1800Z 28.9N  49.2W   45 KT  50 MPH
 36H  24/0600Z 31.0N  45.8W   45 KT  50 MPH
 48H  24/1800Z 33.1N  41.5W   40 KT  45 MPH
 72H  25/1800Z...DISSIPATED

$$
Forecaster Zelinsky

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