I'm in AL. We get tornados, not like OK, but we get severe weather events where it's unsafe to go and the stores are probably shut down too.
I have, when I was unable to reschedule a shop, either sent a link to the NWS or copied and pasted the forecast or the current conditions in the email asking for my shop to be rescheduled. That being said, the Five Guys MSC lets you cancel your shop, and asks you why. When I've needed to, I've picked "other" and explained about the severe weather.