Sarah McLachlan, the Chieftains Steal the Clouds' Thunder

Blue skies held off gray clouds as the evening sky took hold and the acoustic sounds of Sarah McLachlan and the Chieftains filled the Great Woods amphitheater.

The ''Acoustic Tour'' began just three concerts ago, but already McLachlan and the Chieftains play like old friends, and the Chieftains' music is as timeless as ever.

But tonight began with McLachlan. Resplendent in a stunning silver silk dress, McLachlan greeted the three-quarter capacity crowd with a cute grin and a wink. The audience welcomed her with thunderous applause.

McLachlan's recent popularity has surged with the release of her third album, ''Fumbling Towards Ecstasy.'' The album went platinum, selling more than 1.5 million copies and receiving a Grammy nomination. Her first album was decidedly acoustic, but her latest attempt grips the listener with a pulsing rhythm and haunting vocals. Widely played on the radio, McLachlan entertained a knowledgeable crowd July 18.

Looking like an angel, and singing like one too, McLachlan opened her 45-minute set with ''Possession.'' From the first faint chords on the piano, the audience recognized the song, and cheers drowned her first words. But that didn't take away anything from the incredible soprano voice and complex arrangements that this twenty-something Canadian has so masterfully designed.

Her self-directed video for ''Possession'' won a Canadian Music Video Award last year.

After ''Possession,'' three members of McLachlan's band joined her for a rousing rendition of ''Plenty.'' Like most of her songs, ''Plenty'' is about a failed relationship. This one involved a man who feigned sensitivity to attract women.

McLachlan's relaxed style gave the Great Woods setting a small-club atmosphere. She indulged herself and her fans by spending a few moments before each song, explaining their origin.

She followed with ''Path of Thorns,'' which came from her breakthrough second album, ''Solace.'' Most of the set, however, consisted of songs from ''Fumbling Towards Ecstasy.'' Like many folk-rock singers, McLachlan's music is composed during times of anguish, so its beauty usually belies the sorrowful themes.

McLachlan's cheerful and often goofy stage presence more than compensated for the depressing songs. Her band, especially the play-all master, David Kershaw, had a great stage presence on its own. Kershaw, the biggest ham ever to play the bass, stole the show when he attempted to steal McLachlan's spotlight during a bass rift.

Kershaw played bass, keyboard, keyed flute, and backup vocals. He, along with drummer Ashwin Sood, harmonized beautifully with McLachlan.

Paddy Moloney of the Chieftains joined McLachlan for the song ''Elsewhere.'' When her set ended, the crowd of 7,600 brought her back for an encore.

After a short break, the largely Irish audience was treated to the quintessential Irish experience, the Chieftains. They have been performing for 33 years; their recent release of ''A Long Black Veil'' has attracted a more popular audience.

Their mannerisms make it clear that the Chieftains love their Celtic folk music. As always, they brought out a number of guests, including Irish dancers Donny Golden and Carla Butler, and a Spanish bagpipe wizard named Carlos Nunez.

But the greatest moment of all was when Moloney introduced a young violinist from Cape Breton, Nova Scotia: Ashley MacIsaac not only played a mean fiddle, but he also step-danced while playing. The Chieftains appeared stunned (they claimed they'd just met him outside the arena), and the crowd leapt to its feet.

On that warm summer evening, McLachlan and the Chieftains showed that they have formed a perfect partnership. To top it off, the gray clouds that had held off for the entire concert let loose after the crowd funneled out of the arena. Everything went right that night.

* The ''Acoustic Tour'' continues with the following US shows: Dallas, July 31; Austin, Texas, Aug. 1; and Denver, Colo., Aug. 3.