I undertook a September 23 day hike three or four years ago. The weather was ideal from start to finish, and the sky was cloudless.
Finishing the hike, I took a shower at the WP Store, then went to Lone Pine for a meal. When I stepped out of the eatery and looked west, it was snowing hard all the way down to the Portal.
I read the next day that people who were dressed only in shorts and t-shirts stumbled down from the summit and begged use of tents at Trail Camp. They couldn't go further because the trail was obliterated, snow was blowing, and they didn't have enough clothing. A bit surprisingly, no one seems to have died from exposure.