Tropical Storm Sebastien Forecast Discussion Number 20

By | November 24, 2019

Issued at 500 AM AST Sun Nov 24 2019

000
WTNT45 KNHC 240847
TCDAT5
Tropical Storm Sebastien Discussion Number  20
NWS National Hurricane Center Miami FL       AL202019
500 AM AST Sun Nov 24 2019
Sebastien is on the slow boat to becoming fully extratropical.
Just within the past few hours, the cyclone has taken on an
appearance more resembling a subtropical cyclone, with all of its
deep convection located in a band to the north of the center and it
being co-located with an upper-level shortwave trough.  Based on
Sebastien's evolving structure, TAFB provided a subtropical
classification of ST3.5.  Combining this estimate with the most
recent ADT and SATCON estimates, the initial intensity remains 55
kt.
Since Sebastien's extratropical transition has been so drawn out,
it's difficult to pinpoint exactly when the process will be
complete.  Phase-space diagrams from the GFS and ECMWF models
indicate that the cyclone will be deep cold core within the next
6-12 hours.  However, these models keep the low-level vortex
separate from a second upstream upper-level trough for the next 30
hours, and they also keep deep convection going (in simulated
satellite imagery) near but to the north of the center for that same
amount of time.  Because of deep-layer shear around 40 kt and sea
surface temperatures near 20C, the official forecast continues to
show extratropical transition complete by 24 hours, but that
forecast should be considered with a margin of error of plus or
minus 12 hours.  Regardless of its status, Sebastien is forecast to
only gradually lose strength over the next 48 hours and is likely to
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.
The northeastward acceleration continues, with Sebastian moving
050/29 kt.  Additional acceleration is expected during the next 24
hours, with the new NHC track forecast a little faster than the
previous iteration and generally a blend of the TVCN and HCCA
consensus aids.  No forecast points are provided starting at 72
hours since by that time Sebastien is expected to have merged with
another weather system southwest of Ireland and the United Kingdom.
This larger complex system could bring gusty winds and heavy rains
to portions of western Europe within the next few days.
FORECAST POSITIONS AND MAX WINDS
INIT  24/0900Z 36.0N  40.0W   55 KT  65 MPH
12H  24/1800Z 38.5N  34.8W   55 KT  65 MPH
24H  25/0600Z 42.1N  26.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
36H  25/1800Z 46.2N  17.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
48H  26/0600Z 50.0N  11.4W   45 KT  50 MPH...POST-TROP/EXTRATROP
72H  27/0600Z...MERGED WITH ANOTHER WEATHER SYSTEM
$$
Forecaster Berg

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