She is in at least stage 5 of dementia. Every storm we have is the worst one she's ever seen and things might come in the window and hurt us. When there's wind, she's afraid it's going to blow her down. Once she said it might blow the school bus over and half of them would die. When we went to see bison she told people that the farmer said they would kill you. He didn't. Now when it rains she watches the patio and says the water is coming right up to the house and she's afraid it's going to come in. It's nowhere near. The very edge of the patio is all that's wet. I'm sad that she feels afraid and also it's hard for me to hear and try to soothe her. Who decides when she might need some medication, what are options and how do we decide?
In general with delusions going along with them -- that is, not arguing about them -- is considered best practice. "You are right, Mom, high winds could tip a bus over. I'm so glad there is no tornado predicted for now. The winds are high, but not that bad now." "I see the water starting on the patio. I'm so glad the builder slanted it away from the house, so it won't come in here." "Bad storms really can be destructive, can't they? The weather service says this one is not that bad, but let's move away from the windows, just in case. In fact, let's go into the den and have some ice cream while we wait for the storm to subside."
The idea is to let her know you take her concerns seriously, to reassure her about her safety, and if possible to distract her onto another topic.
I'm not suggesting this approach instead of seeing a doctor. Both can be helpful.