Warren County is set for prime-time TV coverage tonight.PBS stations across the nation are scheduled to air a one-hour Bluegrass Underground special featuring some of the finest performances from the Emmy-winning TV series filmed in Warren County at Cumberland Caverns. Local PBS affiliates are scheduled to air the special Friday at 9 p.m.Called “The Best of Bluegrass Underground,” the show features a mix of Americana’s hottest young performers (Jason Isbell, Old Crow Medicine Show), iconic artists (Lucinda Williams, Vince Gill, Doyle Lawson, Widespread Panic) and rising stars (Sarah Jarosz, Davina & The Vagabonds).“There are a lot of people around the country who are going to see McMinnville, Tenn., on Friday night and that’s a good thing,” said Bluegrass Underground founder Todd Mayo. “This is only the second time in its history PBS has done something like this.