Threatening weather forced the 20th annual Potomac River Jazz Club Picnic inside to an open-sided pavilion at Blob's Park on Saturday afternoon, but the overcast skies failed to dampen the spirits of the overflowing crowd. The club is devoted to pre-World War II jazz -- especially the pioneering New Orleans and Chicago music of the 1920s -- and clarinets, tubas, cornets and banjos were there in abundance as nine different bands from the Baltimore-Washington area played the vintage repertoire from 11 a.m. to 8 p.m.

Most of the music performed was composed before 1930, but there was something refreshingly modern about the way the bands made jazz improvisation danceable -- much as modern groups like the Dirty Dozen Brass Band have been trying to do. The concrete floor between the folding chairs was often bustling with white-haired, hot-stepping dancers. When the Pontchartrain Causeway New Orleans Jazz Band did the slow blues stomp "She Rocked Me," somebody's grandmother stood up at a picnic table and did an incendiary bump-and-grind. By 4:30, when sufficient quantities of beer and bratwurst had been consumed, dozens of couples were jitterbugging to the Buck Creek Jazz Band's version of Jelly Roll Morton's "Chicago Breakdown."

The most impressive bands of the day were Baltimore's Peabody Ragtime Ensemble and Washington's Hot Jazz. Trumpeter John Mathis introduced Duke Ellington's "The Mooch" with a growling plunger solo, and the Peabody septet followed with a jumping rendition of Cab Calloway's "Minnie the Moocher," which featured pianist Larry Cione leading the picnickers in call-and-response scat phrases. Better yet was Hot Jazz, which specializes in small-combo swing from the '30s. Pushed by leader-drummer Brooks Tegler and guitarist Tom Sullivan, the septet lived up to its moniker with very fast, very heated performances of Ellington's "Cottontail" and Count Basie's "The King."