Tropical Storm Sebastien Forecast Discussion

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016 
WTNT45 KNHC 221449
TCDAT5

Tropical Storm Sebastien Discussion Number  13
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Fri Nov 22 2019

Sebastien is strongly sheared with deep convection limited to the
northeast quadrant of the cyclone. The intensity has been set at 45
kt, based primarily on the latest TAFB Dvorak classification and
recent ADT fixes. A partial ASCAT overpass at 1239 UTC may not have
captured the strongest winds, but showed a peak value of only 40 kt.
Recent visible imagery and the ASCAT data also indicate that
Sebastien's circulation is not quite as well defined as it was
yesterday, perhaps due to its close proximity to a nearby frontal
boundary.

A significant change was made to the intensity forecast earlier
this morning, and the latest forecast is in line with that new
thinking. Strong shear is expected to prevent Sebastien from getting
better organized, so gradual weakening is anticipated. The HWRF
is once again a notable outlier, as the 06Z run stubbornly forecasts
Sebastien to become a hurricane. While not impossible, that scenario
appears unlikely and has been discounted. Aside from the HWRF, the
dynamical guidance otherwise dissipates Sebastien within about 3
days, and this is reflected in the official forecast. Given
Sebastien's shallow and disorganized structure, it is certainly
possible that its wind field may become poorly-defined and the
system could dissipate sooner than currently forecast.

The tropical storm continues to move at a slower pace and to the
right of previous forecasts. The initial motion estimate is 075/13
kt. The latest NHC track forecast is slower and to the right of the
previous advisory, closer to the global model consensus. The cyclone
is expected to continue generally northeastward or
east-northeastward near the southern end of an eastward-moving
frontal boundary through the weekend or as long as it remains a
tropical cyclone.


FORECAST POSITIONS AND MAX WINDS

INIT  22/1500Z 25.2N  55.3W   45 KT  50 MPH
 12H  23/0000Z 25.8N  53.5W   45 KT  50 MPH
 24H  23/1200Z 27.0N  51.3W   45 KT  50 MPH
 36H  24/0000Z 28.2N  49.0W   40 KT  45 MPH
 48H  24/1200Z 29.3N  46.3W   35 KT  40 MPH
 72H  25/1200Z...DISSIPATED

$$
Forecaster Zelinsky




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