Croatia/Slovenia in May?

As late in May as possible is ideal. Earlier in May is typically good weather- and crowd-wise, but the sea take time to warm up. I am a fellow southerner (SC) and like my sea water like bath water, so I always found the sea pretty chilly on three visits in late May. However, there were enough hot days that I could take a dip. The most popular towns have become only more so since my last visit, but I would not hesitate to visit in May. Only September would be better.
You can get the actual stats on website like wunderground.

If you ever have any questions about weather--highs, lows, precipitation--look up the city on Wikipedia. All the information's there for any visit to any place virtually on earth.

Wikipedia does have more detailed information than you'll find in the typical travel article or guidebook, but especially for travel during shoulder seasons, I want to see day-by-day stats. And I find it helpful to know what happened in recent years, not just averages that I assume were taken over a longer period of time (pre-global warming?).

great information, thanks

We spent the second half of May in Slovenia and Croatia this year. Air temps were fantastic the whole time and we had very little rain. We river kayaked in mid-May on the Soca river and we were in full wetsuits, paddle jackets, booties, helmets and were quite comfortable. No need for any gloves. We kayaked near Dubrovnik in late May and I waded into the water up to my waist and it was chilly for a southerner, but OK. I didn't want to get fully wet for the paddle back just in case I might get chilled. Crowds were fine, but I did go onto the websites to search which days had cruise ships in Dubrovnik and Kotor and arrange my schedule to be in those cities on days of very, very low cruise traffic. I highly recommend doing that.

The water in May is too cold for swimming. We usually go to Croatia in March or April and have a fab time. We don't miss the swimming.