Start spreading the news. Broadway has the kind of all-American musical that audiences have been waiting for in "The Will Rogers Follies," a Tommy Tune production that puts an exclamation mark ending on a humdrum season.

Summer visitors to the Big Apple will want to start ordering their tickets for the show that opened Wednesday at the handsomely refurbished Palace Theater, the last show of the 1990-91 Broadway season coming in just under the wire of eligibility for Tony awards.It's the best musical theater since "Grand Hotel" and "City of Angels," and composer Cy Coleman's best score since "Barnum."

It cost $6.25 million and every cent shows. Florenz Ziegfeld's "stairway to the stars" splendor is perfectly recaptured by the wizard designing team of Tony Walton, sets, Willa Kim, costumes, and Jules Fisher, lighting.

Inspired by Rogers' long association with Ziegfeld as a starliner in the showman's perennial Broadway "Follies," the book by Peter Stone provides vignettes from the "Poet Lariat's" life - from his departure from the family ranch in Oklahoma for the vaudeville circuit at the turn of the century to his death in an airplane crash in 1934.

The vignettes are interspersed with dance numbers in the glitzy Tommy Tune style and other divertissements, including a trained dog act that warrants the loudest applause of the evening.

It is high time that the greatest American humorist of the century and one of the first to have his own radio show and syndicated newspaper column is given recognition in the theater where he was most at home. Literally roping us into this belated tribute with laconic charm and versatile talent is Keith Carradine, an actor-singer destined for the role.

Carradine does not resemble Rogers, but his lopsided smile, innate warmth and casually graceful demeanor create an acceptable illusion.

What Carradine gives the show is heart, an organ not likely to receive much nourishment from a text that skips around so much that there are moments - especially in the slightly tedious first act - when it doesn't seem to be going anyplace.

Gregory Peck's voice is recorded for the role of Ziegfeld, an unseen presence who gives stage directions from the top balcony.

Tune, the actual director-choreographer, takes an almost tangible role in the show, infusing it with his own verve and originality as a dancer. He is particularly successful in recreating the dance essence of the 1930s, such as a powder puff ballet, but his best choreography is performed without an actual dance step by a seated chorus line whose arms-and-legs routine includes straw hats fitted up as tambourines.

Rogers' aridly witty comments of current affairs are nicely selected for the show, but no longer pack the wisecracking punch they had for the humorist's contemporaries.