I just realized that I've seen four outdoor shows in my concert-going career, and only one, Nanci Griffith at the Minnesota Zoo amphitheater in June, didn't involve lightning and torrential downpours.
Aug. 21, 1999 -- R.E.M. w/Wilco at Midway Stadium in St. Paul. An online reviewer posted: As it got dark, it began to rain. It rained lightly for several songs (prompting "Have You Ever Seen the Rain"), but it wasn't bad. When Michael came on stage to do "Hope", for the encore though, it started to get ugly. He went off on Nike, Wal-Mart, etc. for a while, then there was a VERY bright lightning strike nearby, which made him apologise and retract everything bad he'd said! He got through "Hope" perfectly, making no mistakes on the guitar. Due to the lightning, they skipped a big chunk of the encore (no "Why Not Smile") and finished with old favorites. Occasionally, it seemed like the lightning and rain were part of the show! It turned into a torrential downpour during "It's The End of the World As We Know It" and the band signed off. In the mad dash to the exits, lightning struck nearby, which caused the stadium lights to go out. Everybody got soaked blindly groping for the exits in the cold, hard rain. It was a really good time.
June 12, 2003 -- Pearl Jam w/Idlewild at the Verizon Wireless Amphitheatre in Bonner Springs, Kan. "A lovely day turned to dark skies, lightning and a torrential downpour starting during Idlewild's set. During the set change, the first row of seats is flooded with water pouring from the upper sections. Mike comes out and tosses several shirts out to the crowd. The stage is mopped, equipment is covered and the band prepares to play as the downpour turns to steady rain and the lightning becomes less threatening." While singing the line "oh dear dad, can you see me now?" line from show-opening song "Release", a bolt of lightning cracked behind the stage, to which Eddie responded "Hi, Dad." At the end of the song, Eddie says, "Welcome to the wettest show on Earth."
Saturday, July 23, 2001 -- U2 w/Interpol at TCF Bank Stadium, Minneapolis. It started raining about three songs into U2's set and toured to downpours at least twice, but the band never stopped and the people never left. Bono did snippets of "Singin' in the Rain," "Rain" by The Beatles and "Purple Rain" by Minneapolis' own Prince.
In each case, I think the rains made the shows even more memorable, and at least I was smart enough to bring a rain jacket to the Pearl Jam and U2 shows.