Tropical Storm Sebastien Forecast Discussion Number 19

By | November 24, 2019

Issued at 1100 PM AST Sat Nov 23 2019


000
WTNT45 KNHC 240255
TCDAT5

Tropical Storm Sebastien Discussion Number  19
NWS National Hurricane Center Miami FL       AL202019
1100 PM AST Sat Nov 23 2019

Over the large scale, Sebastien appears to be undergoing
extratropical transition, with a large cirrus shield on the
northern side of the storm and unimpressive convection overall.
Near the center, however, the storm still has a small inner core,
with what appears to be a faint eye feature forming during the past
few hours in conventional satellite imagery.  This feature also
shows up in recent microwave data, suggesting that Sebastien still
has more of its tropical character than a cursory look would
indicate.  Thus, Sebastien is held as a tropical cyclone and, since
Dvorak and SATCON estimates remain at 55 kt, the initial wind speed
is held at that value. Unfortunately all 3 scatterometer satellites
missed the cyclone this evening.

Sebastien is quickly moving up on my list of most annoying storms
during the 2019 season, and hopefully this isn't another night of
it making my forecast look silly.  I think (hope?) the intensity
outlook is getting easier since the storm will be moving over
sub-20C waters by tomorrow morning.  There are no signs of
significant upper-level cooling at that time, so Sebastien will
probably be struggling to produce any organized deep convection.
Since baroclinic forcing also appears to be on the wane, a slow
weakening is anticipated, and a more complete extratropical
transition is forecast by this time tomorrow.  This is also
indicated by cyclone phase space diagrams derived from the GFS and
ECMWF models.  While I might have a little more confidence in this
prediction than a couple of days ago, I also wouldn't be surprised
if Sebastien had another trick up its sleeve.

The storm is accelerating northeastward, now estimated at
045/27 kt.  The evening model guidance continues the trend noted
by my predecessor of a faster motion, so the new NHC prediction is
shifted in that direction, and somewhat to the north.   Sebastien or
its extratropical 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  24/0300Z 34.4N  43.1W   55 KT  65 MPH
 12H  24/1200Z 37.0N  38.7W   55 KT  65 MPH
 24H  25/0000Z 40.1N  31.1W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  25/1200Z 43.9N  22.3W   50 KT  60 MPH...POST-TROP/EXTRATROP
 48H  26/0000Z 48.0N  14.5W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  27/0000Z...DISSIPATED

$$
Forecaster Blake

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