Joan Tower is among the select circle of women composers increasingly favored by the musical establishment. The favoritism is due in part to concert-world politics and to the enlightened embrace of minorities, but Tower and her colleagues in the circle--Ellen Taaffe Zwilich, Shulamit Ran, and several others--deftly craft works that are at once accessible and personal. Tower specializes in weaving gorgeous instrumental textures; her biggest challenge, she readily admits, is in coming up with a structure or context that sustains her coloristic explorations. In her half-hour Concerto for Orchestra, commissioned for the CSO in 1988 and her longest composition to date, "visceral energy" provides the momentum. The first section juxtaposes slow and fast episodes featuring solos, duos, trios, and other instrumental clusters; the flutes, Tower's favorite instrument, then usher in the whirlwind motives that pit high trumpets against the rest of the orchestra. Similarly, in the second section, the buildup of energy--which begins with a series of lingering melodies--ends in frenzied cries. At the concerto's local premiere this weekend, the guest maestro is Saint Louis Symphony's Leonard Slatkin, a longtime supporter of Tower's works. Also to be performed for the first time by the CSO is Ralph Vaughan Williams's Five Variants of "Dives and Lazarus". Written in 1939, this paean to the British folk tradition is refreshingly rustic even for today's listeners. A lifelong collector and arranger of folk tunes, Vaughan Williams often transmuted folk rhythms and colors into an idiom of his own. In Five Variants, he embellishes several versions of the medieval cautionary song about miserly sins. Prokofiev's Fifth Symphony rounds out the program. Friday and Saturday, 8 PM, Orchestra Hall, 220 S. Michigan; 435-6666 or 435-8122.