Tropical Storm Sebastien formed northeast of the Leeward Islands on Tuesday morning.  At 10:00 a.m. EST on Tuesday the center of Tropical Storm Sebastien was located at latitude 20.1°N and longitude 58.7°W which put it about 275 miles (445 km) northeast of the Leeward Islands.  Sebastien was moving toward the north-northwest at 8 m.p.h. (13 km/h).  The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1008 mb.

More thunderstorms developed closer to the center of a low pressure system northeast of the Leeward Islands on Tuesday morning and the National Hurricane Center designated the system as Tropical Storm Sebastien.  There was a distinct low level center of circulation evident on visible satellite imagery.  However, the distribution of thunderstorms was asymmetrical.  The stronger thunderstorms were occurring in bands in the eastern half of the circulation.  Bands in the western half of the circulation consisted primarily of showers and lower clouds.  Storms near the center were generating upper level divergence which was pumping mass away to the east of the tropical storm.  Winds to tropical storm force were occurring primarily in the northeastern quadrant of Sebastien.  Those winds extended out 100 miles (160 km) from the center of circulation.

Tropical Storm Sebastien will move through an environment marginally favorable for intensification during the next day or two.  Sebastien will move over water where the Sea Surface Temperature is near 28°C.  So, there will be sufficient energy in the upper ocean to support intensification.  An upper level trough near the east coast of the U.S. will produce southwesterly winds which will blow toward the top of the circulation.  Those winds will create moderate vertical wind shear and they are probably contributing to the asymmetrical distributions of thunderstorms.  Tropical Storm Sebastien could strengthen if the shear does not increase.  However, if the upper level winds get stronger, then wind shear will cause Sebastien to weaken.

The upper level trough near the east coast of the U.S. and a subtropical high pressure system over the Atlantic Ocean will interact to steer Tropical Storm Sebastien toward the north during the next 24 to 36 hours.  On its anticipated track Sebastien will move away from the Leeward Islands and it is forecast to stay southeast of Bermuda.