The storm that broke at Wolf Trap on Saturday afternoon sent lawn partons of the Royal Ballet's "Swan Lake scurrying for cover. At first the thunder, lighting and rain seemed right for the arrival of the two uninvited guests at Prince Siegfried's betrothal party on stage.

Derek Rencher, insinuating and sinister, loomed like someone who could indeed command the elements. Marguerite Porter as his daughter, with her Black Swan allure, engaged Siegfried -- Derek Deane -- in the famous pas de deux. As she began the 32 fouettes, the electrical power failed. Porter, to the audience's delight, kept on turning and finished the duet with Deane. It was one of those occasions that can make a leading dancer into a star.

The British Company's founder, Ninette de Valois, has been preaching the gospel that ballet stars must emerge naturally from the ensemble. That happened in Porter's case. Not long ago she was among the Royal's corps dancers, who make the troupe's traditional productions as velvety and fresh as a classic English lawn. They are natured slowly rather than constantly drilled for homogeneity.

On Saturday, Porter's decision that the show must go on only capped her sensually elegant embodiment of the dual "Swan" role. Her body unfurls low on the spine, and the dancing isn't hurried even when tempi are fast. Under the circumstances, it wouldn't be fair to judge whether her fouettes were brilliant.

Deane's Siegfried was restrained, except when dancing with Odette or Odile, the swans. He's one of the Royal's tall, streamlined dancers, combining torso articulation and flow in his first pensive solo. He stiffened, through, for the vigorous Black Swan duet. The Ballet's fourth act was danced with a few auxiliary lights, and the scenery was manually controlled.

Saturday night's "Swan Lake" was without external incident. Exuberance showed because the company had ridden out the afternoon storm. With Monica Mason in the Swan roles, typecasting was avoided. She's such a fine actress and stylist that one forgets the bold features and athletic body with its unhoned strength.

Mason's White Swan is melancholy personified. She revels in assurance as the Black Swan and shrugs her shoulders at the definite problems she has with turns. David Wall overacted Siegfried. He's not one of the Royal's lianer model danseurs, but he creates a neat cubic space as he moves.

A sure case of typecasting was Wendy Ellis in Sunday afternoon's "Remeo and Juliet." She stays a child in the role, but with sparkling footwork and eager carriage of head and arms, keeps her characterization fresh. Her torso is not very palint. She's more prefected at present than Porter, whose potential may be greater. Wayne Eagling, her Romeo, is a very casual performer for the Royal Ballet. On occasion he lets the fans know that those long legs of his can flash as straight as swords. For Key moments -- his glimpses of Juilet, Mercutio's and Tybalt's deaths -- his acting becomes eloquent as his blood boils.

On Sunday night David Wall as Romeo was hard - pressed keeping the spotlight from his two friends, Anthony Dowell's Mercutio and Derek Deane's Benvolio. Leslie Collier, a wiser Juliet than Wendy Ellis and more studied than either Ellis or Merle Park on opening night, proved to be the Royal's richest dancer because of her fullbodied Russian fluidity.

The Royal Ballet's brief week here was a reminder that the classical idea in academic dancing and stagecraft can be realized at all levels of a company.