This week, Lucy Shelton is staying at the Watergate--a historic first, and totally unexpected.

The Watergate is where the National Symphony Orchestra lodges its star guest soloists--people with names like Perlman, Serkin or Rampal. The brilliant young soprano sings frequently in Washington, but usually she has the more humble accommodations of a chamber musician, a specialist in contemporary music. Often, because she loves to sing low-budget music, she stays in a friend's house.

Shelton's unexpected rise to the luxury bracket began a few weeks ago in Hong Kong, when Galina Vishnevskaya caught a flu shortly before coming to Washington for her engagement with the NSO. Vishnevskaya had enough strength for four performances of Shostakovich's 14th Symphony last week, but not enough to prepare a new score for a world premiere this week. So--on a week's notice--in stepped Shelton, who has done more than two dozen world premieres in the past five years. And tonight, if all goes well, a new audience will learn what contemporary music audiences already know--that Lucy Shelton is one of the most promising singers in the United States.

Shelton was born about 30 years ago in California, and she has been a star for several years in the specialized world of contemporary music. She is a winner of the prestigious Walter W. Naumberg Award, which has launched careers for many young musicians. In fact, Shelton is the only musician who has won the Naumberg twice: in the late 1970s as a member of the Jubal Trio and again in 1980 as a soloist.

Although tonight marks her debut with the NSO, she has already won intense applause for performances with major orchestras in St. Louis, St. Paul, Denver, Houston, Baltimore and Pittsburgh, and she will make her New York Philharmonic debut in a contemporary music festival later this year. Until now, her work in Washington has been in chamber music--at the Library of Congress, the Wolf Trap Barns and particularly at the Hirshhorn Museum, where she sings regularly with the Twentieth Century Consort.

Talk to composers like Ezra Laderman, whose Fifth Symphony she will premiere tonight, or David Del Tredici and Joseph Schwantner--Pulitzer Prize winners whose music she sings regularly--and you get nothing but superlatives. "She is a remarkable artist," says Laderman, whose "Mass for Cain" she premiered on CBS television a few years ago. "She not only knows the notes, she knows the meaning of the notes. She is a consummate musician, as well as a lovely person to look at, and she performs with a very special temperament and intensity. It was a stroke of luck for all of us that she was available on such short notice and able to come to terms with the music so quickly."

Quickly, yes. But Shelton's had practice. When she was a senior at Pomona College in California, still trying to decide whether she would become a singer or a flutist, Shelton stepped out of the student orchestra to take a singing role in the "Christmas Story" of Heinrich Schutz with only an hour and a half to prepare. "I had just finished playing piccolo with a brass quintet," she recalls, "and they told me the Evangelist was ill and they wanted me to substitute. I was playing flute in the orchestra and filling in on the harpsichord. My family came expecting to hear me play the flute, and they were surprised when they saw me up on the stage singing."

That year Shelton finally decided to be a singer. She grew up in a musical family (her parents met at a music camp, and a sister plays viola in the Denver Symphony), began studying piano when she was 3 and flute when she was 11, but didn't begin to study voice until she was a sophomore in college. "I sort of always knew I was going to do something in music," she says, "and it was a matter of choosing what area. I think the deciding factor was repertoire. Every period of music has wonderful vocal repertoire, but that can't be said for the flute. I will never run dry; the voice has an edge on all other instruments that way."

Shelton had already planned to be in Washington this week; on Saturday, a day after her gig with the NSO ends, she will be singing with the Twentieth Century Consort at the Hirshhorn. The week is proving a bit busier than she had expected.

After Vishnevskaya decided to cancel, there were frantic moments at the NSO; not many sopranos can master a tricky new score and give an NSO-level world premiere on a week's notice. But NSO pianist Lambert Orkis (a member of the Twentieth Century Consort and a frequent partner in Shelton's voice-and-piano recitals) had given a tape of Shelton's singing to Mstislav Rostropovich several months ago, and the conductor had been impressed. The orchestra was in New York March 2 for a performance in Carnegie Hall, so Rostropovich visited Shelton at her apartment in Manhattan. She was in Washington the next day for her first rehearsal.

Last Friday, she rehearsed with the Consort in the morning and attended the NSO rehearsal in the afternoon. Since then, she has been rehearsing two programs steadily, except for Sunday when she dashed off to New York to sing Schubert.

Shelton's repertoire extends from the Middle Ages up to music that is still being written--with some emphasis this year on Brahms and Webern, who are having anniversaries. She enjoys music of all periods, but finds something special in new music, particularly if it is written specifically for her. "It's always an adventure commissioning a new piece of music," she says. "You don't know what the composer is hearing in his mind or what he will imagine for you to do. It's like having your hair or your clothes done by someone new; they may emphasize something that you don't want emphasized, at first. But if you try it out, sometimes you can learn new things about yourself."