Sure, that is pretty standard

I am not sure how you think that disproves some of the comments in and around this thread saying people were "not allowed to call in" or "why was the facility open" or other things like that

Warnings happen minutes before, and last 15-30 mins, maybe an hour. These are not long lasting events.

Notice you did not say that your workplace "shut down" or "told employees to go home" or anything like that

They are to shelter in place, Arriving employee go to the shelter, employee getting off shift are prohibited from leaving. That is normal procedure at most companies