Looking for the title and author of this short story: A man is walking down the street when he comes to a young woman in a sort of fugue state taking off her clothes; a crowd is beginning to surround her. She sort of wakes up and is confused and dazed -- doesn't know how she came to be there or why she's undressing, etc. The man puts his coat around her, helps her, talks to her.
He is some sort of scientist - he's been tracking cultural trends and has observed patterns where odd or strange societal change comes right before major cataclysms. Silly, weird trends, fads, people doing unexplained things, etc, and then something awful. He's tracking something similar now, and links her behavior to his studies. They keep in touch, and one day he calls her and says NOW. The go together into the mountains (I think they start in CA and head east) and live for a while in a cabin, until one day he calls her out onto the porch and they watch the mushroom cloud in the distance.
I thought for a long time the story was called "The Silly Season," but that seems to be a different story entirely.