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Tropical Storm Sebastien Forecast Discussion


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000
WTNT45 KNHC 192035
TCDAT5

Tropical Storm Sebastien Discussion Number   2
NWS National Hurricane Center Miami FL       AL202019
500 PM AST Tue Nov 19 2019

The low-level center of Sebastien has been exposed over the past
several hours, as northwesterly shear of 20-30 kt has been pushing
any deep convection off to the southeastern portion of the cyclone.
The initial advisory intensity is being held at 40 kt and is based
on an earlier scatterometer pass that sampled maximum winds of
39 kt.

The initial motion is 305/10 kt. Sebastien is forecast to be steered
to the northwest around a deep-layer ridge over the central Atlantic
through tonight. By Wednesday, the cyclone should turn to the north,
and then accelerate northeastward Wednesday night as the cyclone
gets caught up in the flow between the retreating ridge and ahead of
an approaching cold front. The model guidance remains in good
agreement on this scenario, and the official forecast track keeps
the cyclone over open waters for the duration of its existence.

The shear is not expected to decrease much over the next couple
days while the system remains a tropical cyclone. There may be an
opportunity for some strengthening Wednesday and Wednesday night as
Sebastien turns to the north and northeast and resides under some
favorable upper-level diffluence. By 48 hours, Sebastien is expected
to transition to an extratropical cyclone just ahead of an
approaching cold front. Therefore, additional strengthening
indicated beyond 36 hours should be due to baroclinic processes. The
cyclone should then become absorbed by the cold front by late this
week. The official forecast was adjusted slightly higher from the
previous one at 36 and 48 hours to reflect the anticipated
intensification of the system as it begins its extratropical
transition.


FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 20.6N  59.7W   40 KT  45 MPH
 12H  20/0600Z 21.3N  60.7W   40 KT  45 MPH
 24H  20/1800Z 22.6N  61.9W   45 KT  50 MPH
 36H  21/0600Z 24.2N  61.0W   50 KT  60 MPH
 48H  21/1800Z 26.0N  58.9W   55 KT  65 MPH...POST-TROP/EXTRATROP
 72H  22/1800Z...DISSIPATED

$$
Forecaster Latto