In addition to Sullivan County, the significant weather advisory is in effect for Hawkins and Hancock counties in Tennessee and Scott and Lee counties in Southwest Virginia.
According to the NWS, Doppler radar was tracking a strong thunderstrom around 4:45 p.m. 11 miles north of Surgoinsville, Tenn. and 13 miles east of Jonesville, Va. The storm was reportedly moving northeast at 25 mph.
Forecasters said the storm could bring hail up to a half-inch in diameter and wind gusts of 40 mph.
Locally heavy rains in excess of an inch per hour are also expected with the storm. Motorists are warned to be alert to ponding water on roadways and possible minor flooding in low-lying areas.
Locations in the storms path include Kingsport, Bloomingdale, Church Hill, Mount Carmel and Okolona in Tennessee and Gate City, Weber City, Clinchport and Hiltons in Virginia.