Thank you. I kinda thought they'd do that. But was hoping someone that lived through it would chime in and say what they went through!
How do they designate a building as a a safe shelter? Location, obviously. But what else?
Shelters are designated by the local or state government. Location, building design, building structural design/safety, space to house people, ability to provide shelter services (sleeping space, washrooms, food service, showers, etc). They are often schools and government buildings, sometimes churches/synagogues/mosques/etc.
Most jurisdictions have pre-designated shelters [i.e. they have a list of buildings suitable for being shelters] and may or may not already have some supplies prestocked at them. It is part of the emergency planning a community does. These shelters can be used for local emergencies that affect only part of a community, or in the case of large emergencies like hurricanes they will be used to shelter people who have evacuated from elsewhere [or, in some cases, more locally]. Think, for example, of the large shelters San Antonio, Austin, and Dallas set up for evacuees from Houston. Man shelters will be "run" by the Red Cross as part of standing agreements.
There will also sometimes be shelters designated as "shelter of last resort" which is a shelter that will not have the normal resources of a normal shelter, is typically in or nearer the impact zone, and is for those who can't get out to a safer location -- the last resort shelter is better than nowhere or riding it out in a definitely unsafe location.
Then, as you saw in Houston, there are impromptu shelters that get set up following some emergencies, where there are immediate needs -- e.g. houses are flooded and people need somewhere safer to go. In Houston all kinds of places have been used that are not normally designated shelters -- churches, mosques, businesses (like that Mattress store that has been in the news).