Tropical Storm Sebastien Forecast Discussion Number 1

By | November 19, 2019

Issued at 1100 AM AST Tue Nov 19 2019


000
WTNT45 KNHC 191501
TCDAT5

Tropical Storm Sebastien Discussion Number   1
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Tue Nov 19 2019

Satellite imagery indicates that the overall cloud pattern of the
disturbance has improved since yesterday, and that the low has
become well-defined. In addition, a late-arriving ASCAT-B
scatterometer pass showed 35-38 kt winds extending 90 n mi from the
center in the northeastern quadrant. This pass also revealed that
the low was nearly closed at the surface, and since the
scatterometer may not have resolved the small scale of the low-level
center it is likely that the surface low is indeed closed. Based on
these data, advisories are being initiated on Tropical Storm
Sebastian with an initial advisory intensity of 40 kt.

Sebastien will not be in an ideal environment for significant
intensification, as it will be battling dry air to its west and
about 20 kt of northwesterly shear for the next couple of days.
However, due to the presence of an upper trough to the west of the
system, a diffluent environment aloft may aid in some slight
strengthening over the next day or so. After that time, the
storm will begin to interact with an approaching cold front,
and some additional intensification may occur due to baroclinic
processes. The cyclone is then expected to become absorbed by the
front in about 48 hours. The various intensity guidance solutions
are in decent agreement, and the official forecast is near the mean
of these forecasts. There are some timing variations among the
models on when the cyclone will become absorbed by the front, and it
is possible that the storm could be absorbed sooner than indicated.

The initial motion is 330/07 kt. Sebastien will be steered to
the northwest in the near term around a deep-layer ridge over the
central Atlantic. By Wednesday, the cyclone should turn north, and
then should accelerate northeastward by Wednesday night as the
cyclone gets caught up in the flow between the retreating ridge and
ahead of the approaching cold front. The model guidance is in good
agreement on this scenario, and on the official forecast track the
cyclone will remain over open waters for the duration of its
existence.


FORECAST POSITIONS AND MAX WINDS

INIT  19/1500Z 20.1N  58.7W   40 KT  45 MPH
 12H  20/0000Z 21.0N  59.2W   40 KT  45 MPH
 24H  20/1200Z 22.2N  60.0W   45 KT  50 MPH
 36H  21/0000Z 23.5N  60.0W   45 KT  50 MPH
 48H  21/1200Z 25.5N  58.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  22/1200Z...DISSIPATED

$$
Forecaster Latto

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