radiation storm, far as I can tell it's a weather event triggered like rain and other RNG weather events.
you need to find shelter with some sort of enclosed protection, doesn't need every window / door closed, just kinda can't be standing next to outer edge. that will protect you from the rad exposure of the radiation storms.
they are really cool I think, adds some flavor to the game prior FO series didn't.
I've only had 2 storms come my way near the starter town / settlement, and each time I just ducked into my house and clicked on my bed to pass time ~4 hours.
So for sure, the storm lasted no more than that and possibly less.