Evacuation announcements were broadcast over speakers, posted on signs and tweeted on official Twitter account.
"ATTENTION: Serious weather is about to hit Grant Park. ALL festival goers must evacuate IMMEDIATELY. Head to our 3 evacuation locations NOW," the official Lollapalooza Twitter account tweeted about the same time.
Thunder and lightning accompanied the rains that poured down shortly after everyone was evacuated. The skies finally cleared around 5:30 p.m. and officials worked with the city to reopen Grant Park for Lollapalooza.
-- Reported by NBCChicago.com