Mayday, screams the poster. The sun is shining brightly, the traffic into Washington is weekend-morning light. But the White House is no longer. It has just been blown to redioactive smithereens.

"The Last Washington Painting" by Washington's Alan Sonneman is but one of the dark prophecies in "Premonitions of the Corporate Wars," the little nightmare of a group show that goes on public view tomorrow at the Nourse Gallery in Georgetown, 3212 N St. NW.

Missiles fly annd bodies melt in the works of art. John Alexander's "Korporate Klansman" stands guard in the window. His sheet is of gray flannel. Cain does his thing to Abel. From the Technicolor safety of an old movie poster, a youngish Ronald Reagan regards the mayhem sternly. He has a pistol in his hand. The show is filled with angst, rage and hollow laughter. A grinning Nelson Rockefeller, in Particia Kolmer's portrait, dons his Sony Walkman and suggest the walking dead.

Chip Nourse, the artist-dealer who put this show together, first dreamed up its title and then asked fellow artists for works of arrt that fit. A score or more responded. A British fleet is pounding through the South Altantic. In Bill Newman's "Motherhood," a patriotic Errol Flynn wades hip-deep in blood. Fifty thousand warheads are on everybody's mind. "Boom Boom" go the bombs in Big All Carter's little drawing. The show might seem surreal were its timing not just right.

Too many exhibitions in Washington serve the lovely only. But art's old calm is cracking. Something hot and horrid is bubbling again beneath its cool facade. We sense it in the slashings of the new expressionism, in the new photography annd in this creepy show. That ray gun, made by Stuart Lamb, no longer seems a toy. The faces of Arthur Bremer and Sirhan Sirhan stare into the room from drawings by Mark Clark. On Tuesday, from noon to 9 p.m., the gallery will hold a benefit for the D.C. Nuclear Weapons Freeze Campaign. "Premonitions of the Corporate Wars" closes May 23.

You cannot keep your distance from the photographs on view in "Photo '82" at Middendorf/Lane downtown, 404 Seventh St. NW. Their images won't let you. A deformed pig fetus floats pickled in a Mason jar; a barbed hook penetrates the jaw of a flesh-colored fish; soldiers of fortune do their best to fire pistals under water; a cowgirl shows the viewer the brown and purple bruises on her horsekicked thighs. This is not a pallid show.

George Hemphill of the gallery, who chose its 131 pictures, has given little solo shows to 13 gifted artists, none of them yet famous. Each one has a style. And each appears to be an adventurer of sorts. They move in on their subjects. None of them is timid. They photograph whores and urban wreckage, the steaming stones of Iceland, the jungles of Liberia. The best of them have ventured where few of us would tread. And their pictures take us with them.

Don Stuber photographed the fish, the bruised thighs and the wading soldiers. A greenish corpse is seen in a transparent body bag in one of his large color prints. All his pictures jolt. How Bill Burke got his subjects' trust is anybody's guess: A West Virginia father, proud, plump, middle-aged, poses with his baby and his 13-year-old wife. Though strangers are not welcome in the Italian neighborhood of Worcester, Mass.,, where Robert Simone shoots, he is; he grew up there. Washington's Lynn Allen is the only local artist Hemphill has selected; her protraits from Liberia are sensual and warm, yet taut with formal figor. One feels the puulse of paranoia in Ben Davis' strange essay on the deaths of Lee Harvey Oswald, Jack Ruby and John Kennedy. Fred Endsley's gum prints look like photographs of fiction: Flames consume Godzilla. Jane Smith, using two-hour exposures, has caught the eerie, unexpected colors of the night. Martin Stupich's picture of a vaudeville theater being dustily demolished is a work of striking beauty. Gary Lee Super shows Iceland's awesome landscape.

Elin Elisofon has photographed the pickled fetus of a pig. Viewers may well wonder why Rein Pol, a Dutch painter now showing at Meridian House, has painted the same subject. "Photo '82" runs through May. No stronger show of new photography has been seen here in some time.

With its museum loans and 80-page catalogue, "Quiet Places: The American Landscapes of Worthington Whittredge," at the Adams Davidson Galleries, 3233 P St. NW, is an unusually ambitious show for a Washington commerical gallery. Only five of its 26 paintings are for sale. Their prices range from $26,000 to more than $500,000.

Whittredge (1820-1910), who was born in a log cabin near Springfield, Ohio, was not the finest artist of the Hudson River School, but he painted in the thick of it. His fellow artists like him. Asher B. Durand praised his early work; Emanuel Leutze had him pose for "Washington Crossing the Delaware"; he traveled through Italy with Albert Bierstadt and Sanford Gifford, through Colorado and Wyoming with Gifford and John Kensett, and through Mexico with Federic Church. He knew Eastman Johnson, David Johnson and the scout Kit Carson. In 1875, the year William Wilson Corcoran bought his "Trout Brook in the Catskills," Whittredge was elected president of the National Academy of Design.

His views of sunlight-dappled woodlands, Colorado cottonwoods and the shore at Newport are never as dramatic as the operatic landspcaes of Frederic Church and Beirstadt. Instead, they are as friendly as he must have been. Nature in his paintings always welcomes man. There are haystacks in his fields, sails on his seas. In one corner or another of his woodland landscapces one finds a boy, a dog, or a patient fisherman fly-casting for trout.

His art, despite its charm, may be less individual than that of his collegues. Things we now assign to others of hs period--the flaming autumn foliage of, say, Jasper Cropsey, the limunism of Kensett, the moodiness oof George Inness and the pastoralism of Barbison--are blended in his art. One feels Cropsey in his "kaaterskill Falls" (1863-65), Kensett in that painting's skies and Inness' romanticism in "Noon in the Orchard" (1900). When Whittredge is good-as he is in "On the Cache La Poudre River, Colorado (1868) or "Landscape With a Woman Carrying Washing Towards a River" (late 1880s)--he is very good indeed. Cheryl A. Cibulka of the gallery wrote the exhibition's catalogue. "Quiet Places" closes on June 17.