The National Weather Service said they were monitoring the storm system, which made its way between Mt. Olive and Pasadena and Monrovia.
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"This storm may produce strong gusty winds, heavy downpours, lightning and small hail," the NWS said in a tweet.
As for the music festival, it was postponed and attendees were told to evacuate through emergency exits as soon as possible.
"The festival has been postponed until further notice. Please evacuate the festival site immediately. Use the nearest emergency exits. Stay tuned for updates," the festival tweeted, before deleting the post.
Shortly after, festival officials posted a tweet urging festival goers to exit the concert site and go to vehicles or other protected areas for safety.
"Due to severe weather, please exit the event site and move to your vehicles or protected areas outside of the event site for safety. Stay tuned for updates," they tweeted.
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The storm hit during Iggy Pop's performance. Many concert goers were bummed because the evacuation forced them to miss the upcoming performance by Siouxsie Sioux.
"It's disappointing, but it is understandable because I did see lightning," said Ivonne Espinoza, who attended the concert.
Michelle Gee, of Pasadena, said she had been looking forward to the concert for months.
"I've been planning this for months. And, its okay. It worked out though. I'm fine," Gee said.
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Festival organizers later said that Iggy Pop and Siouxsie would perform at the Rose Bowl Sunday night.
Everyone with Saturday passes will be allowed into the venue once doors open at 5 p.m.