One pr. of lightweight long trousers for leaving home, + a windbreaker. STL just gets those weird cold snaps sometimes, and we might be having another when you come home; my DS was born on Mem. Day weekend, and it was 48 the day we brought him home.
While Florida is very seldom is cool this time of year, it *is* rainy, so a windbreaker can be useful. (We use ponchos in the parks, but windbreakers everywhere else.)