The Big Valley Jamboree in Camrose, Alberta was struck by a sudden storm on August 1, 2009 resulting in the partial collapse of the stage killing 1 and injuring 75, 15 of whom were transported to hospital. The event which attracts up to 20,000 people per day had paramedics on site who immediately started attending to the injured.
Large event organizers should always complete a HIRA and put plans in place to deal with the highest potential risks. High winds and rain, although not having the same impact, blew through the Big Valley Jamboree in 1999 providing some history to the risks faced at this event.
Local media coverage can be found at the Edmonton Journal:
http://www.edmontonjournal.com/news/Valley+Jamboree+cancelled+after+with+Alberta+storm/1853266/story.html