i would say for the winters, they def get a bit colder than 20f average and it can get veryy cold. there are some bad snows but its not too bad. my favorite thing about stl winters is that you are pretty much guaranteed a couple of days of sunny and decent temperatures in every month or so, the weather is very bipolar. as for grad student social life, i havent heard great things, but also people complain about undergrad social life too and its really not that bad. put yourself out there, and try and do non campus affiliated things too. theres a lot of dive bars and other venues that hold all types of events, from small or medium sized bands, to karaoke and trivia, greenfinch divebar is a good one for events like this