Plitvice in late May?

I was there in late May, and the weather was nice enough - mostly sunny, reasonably warm but by no means hot (not swimming weather). It felt like spring.

I agree with the above, by all means, stay in the hotel in the park itself (not the nicest hotel you will ever sleep in, but perfectly OK if a little weird, in a 1970s Soviet Bloc resort style); you should plan your trip so you you arrive mid-afternoon, check in, get out and walk the park for a few hours before dinner, sleep, get up early and be out there when the park opens, enjoy a few more hours, then when the crowds roll in late morning, check out and get on the road to your next destination.

This is my go-to website for historical weather data: timeanddate.com. You can look at a daily temperature graph for May 2018, May 2017, May 2016--more years than you'll want to fool with. I usually look at the last five years when I'm planning trips.

But Scythian and David are right: Your major concern should be crowding rather than weather. Don't trip to visit the park as a day-trip from Split, Zagreb, etc. I did that in 2015 and it was a nightmare--nothing like the experiences I had years ago, before the park became so popular. I was in the ticket line for about an hour, after which I was in a slow-moving mob of people on the walkways. It's really important to spend the night in or near the park so you can get inside when it opens. A few people have reported that it's not too bad late in the afternoon, either.

Waterfalls were running fast in mid-May 2015 due to the prior rains - definitely more dramatic than in October a few years earlier (but fall colors were prettier). I visited on a beautiful day in May. A perfect time to go - though you could get rain any day of the year. You're always playing the odds with the weather.

Some people have some flexibility in their schedule, and if you do, you might not commit to a day in Plitvice until you see a few days how what the weather will be like. Sometimes people can move things around and go on a different day. On my last visit that May, I didn't plan to go to Plitvice at all (having been there before) until the day before - I had an itinerary where I was just winging it and booking my next hotel each day as I went along. If you stay in one of the park hotels (overpriced but convenient), you can probably book on booking.com with free cancellation up til a few days before check-in.

I'd be more worried about the mobs of people. Get into the park at 7am if you can when it opens so you can have a few hours before the big tour groups descend on Plitvice!

Wheater is warm and hot from 10 AM to 3 PM. The Plitvice region is cold in general, take a jacket with you.