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Tropical Storm Sebastien Discussion Number  21
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Sun Nov 24 2019

It is not clear if Sebastien is a closed cyclone at the surface. The
fast forward motion of the cyclone (28 kt) may have caused it to
open into a trough of low pressure. The ship MSC Beijing (DFDE2)
recently reported northwest winds about 70 n mi north-northeast of
the cyclone's estimated center, and it has been difficult to
identify easterly winds in visible imagery this morning. That said,
ASCAT data was inconclusive regarding the state of Sebastien's
circulation and the system is still producing some deep convection
near its center. AMSU data at 1135 UTC also indicated that Sebastien
still has a weak warm core. For those reasons, it appears prudent to
maintain advisories at this time.

As mentioned above, Sebastien is moving very quickly northeastward
and it should accelerate in that direction during the next 24 h.
This is shown by all of the dynamical models which are finally in
decent agreement. Based on simulated satellite imagery, it appears
that the cyclone will continue to produce central deep convection
for the next 12 h or so, and extratropical transition is forecast to
finish within 24 h. Although an extratropical point is shown at 36 h
for continuity purposes, in reality Sebastien will more likely open
into a trough of low pressure, given the forecast forward motion of
nearly 40 kt relative to its 50 kt forecast intensity at this time
tomorrow. This could happen at any time.

Regardless of whether it is a tropical cyclone, extratropical
cyclone, or a remnant trough, Sebastien or its remnants will likely
bring gusty winds and heavy rains to portions of the Azores later
today and tonight. Please see products issued by the Portuguese
Institute for the Sea and the Atmosphere (IPMA) for more
information.


FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 37.4N  37.0W   55 KT  65 MPH
 12H  25/0000Z 39.8N  31.0W   55 KT  65 MPH
 24H  25/1200Z 43.8N  22.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  26/0000Z 47.7N  14.8W   45 KT  50 MPH...POST-TROP/EXTRATROP
 48H  26/1200Z...DISSIPATED

$$
Forecaster Zelinsky

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