Never a good idea to bring an umbrella to an athletic facility. Best off leaving it in your car, it will just get in the way inside. And, in any case, there is no 2nd Amendment right to bear an umbrella.
According to the clear bag policy, umbrellas are considered a prohibited item. Although, like Dhruvil said, USUALLY at the RAC they won't give you a hard time about it, depending on the mood of the person checking bags. Often at the football stadium, you will see large numbers of umbrellas left just outside the entrance. It's kind of the honor system that no one takes your umbrella, and you can pick it up on your way out.
If you're asking that are you allowed to bring an umbrella inside the RAC? then It depends on what kind of event it is. Usually, they don't mind.
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