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Tropical Storm Sebastien Discussion Number  18
NWS National Hurricane Center Miami FL       AL202019
500 PM AST Sat Nov 23 2019

Sebastien's cloud pattern is beginning to resemble that of an
extratropical cyclone, with a shield of moderately cold cloud tops
extending well to the north of the cyclone's surface center. That
said, Sebastien is still producing enough convection near its center
to be considered a tropical cyclone for now. ASCAT-B data that
arrived shortly after the last advisory was issued had support for
55 kt and that is the basis for the initial intensity. The ASCAT
data also showed that the surface wind circulation of Sebastien is
elongated but still closed.

Sebastien is accelerating northeastward, with an initial motion of
045/26 kt. Most of the 12Z models show a significantly faster
northeastward track for the tropical storm and the official forecast
has been adjusted accordingly. However, the NHC forecast is slower
than the model consensus and additional large changes may be
required in the next advisory. Confidence in the track forecast
remains low due to the continued lack of run-to-run consistency
amongst the models. With regard to intensity, a majority of the
guidance calls for little change in strength since it seems that
baroclinic forcing will at least offset the negative influences of
decreasing SSTs and high shear during the next day or two. The NHC
intensity forecast is very near the consensus. Cyclone phase-space
diagrams derived from the GFS and ECMWF indicate that extratropical
transition could occur by Sunday afternoon, but the NHC forecast is
slightly more conservative. The reality is that Sebastien could
become extratropical or open into a trough at nearly any time
between this evening and early next week.

Sebastien or its remnants are expected to bring gusty winds and
rain to the Azores beginning on Sunday. Please see products issued
by the Portuguese Institute for the Sea and the Atmosphere (IPMA)
for more information.


FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z 32.4N  45.4W   55 KT  65 MPH
 12H  24/0600Z 35.0N  41.5W   55 KT  65 MPH
 24H  24/1800Z 37.8N  35.3W   55 KT  65 MPH
 36H  25/0600Z 40.9N  27.6W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  25/1800Z 44.8N  19.3W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  26/1800Z...DISSIPATED

$$
Forecaster Zelinsky

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