A late-afternoon thunderstorm seriously dampened the "NASCAR Rocks" festival at Merriweather Post Pavilion Thursday night, essentially turning the night into a long jam by the Allman Brothers Band, a good deal for the large crowd of "Peach-heads" in attendance.

It's fitting that the ABB is celebrating its 30th anniversary (slogan: "Never trust any band under 30") by headlining the event, since founders Gregg and Duane Allman grew up in the stock-car mecca of Daytona, Fla. Fans are also buzzing about 20-year-old guitar whiz Derek Trucks (nephew of drummer Butch Trucks), who many hope is the spiritual heir to guitarist Duane, killed in a 1971 motorcycle wreck.

It was immediately clear that the immensely talented young man has inspired the band. Guitarist Dickey Betts and keyboardist Gregg Allman improvised with vigor during "I Got to Change My Way of Livin' " and Muddy Waters's "Trouble No More," while Derek chased them with slide licks. Aside from a lethargic acoustic interlude of "Seven Turns" and "Melissa," the band (including second drummer Jaimoe, percussionist Marc Quinones and bassist Oteil Burbridge) exuded fluid jam power. "JJ's Alley" swirled around a hard bop riff, Allman's vocals sounded vivified on "Blue Sky," and Jack Casady and Jorma Kaukonen of Hot Tuna (which opened with a too-brief set) joined a rousing version of Blind Willie McTell's "Statesboro Blues."

Though still trading on themes and riffs from its '70s heyday and clinging to rampant mushroom imagery and "trippy" slide shows, the Allman Brothers Band played three hours that were surprisingly immediate.