Rescheduling an interview

I have an interview scheduled for tomorrow during my lunch break. I currently go into the office and I've been planning to take the call in a park nearby. However there are thunderstorms forecasted for the next couple days and it'd be awkward having to explain why I'm "going for a walk" when it's raining outside. If I ask the employer to reschedule, do I mention the weather/situation or should I just keep it at "personal emergency" or something vague like that?

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