In May you will usually have a sunny day, in the 60's and 70's.
Figure on being ready with an umbrella or a jacket, though. Many days we have thunder storms that move across the sky, and the rain dumps down for a few minutes. Those of us who live here, usually find a covered spot to wait it out, because it rarely lasts for long, then it's back to sunshine.
However, don't be totally shocked to look out your window and see snow flying in May. Anything is possible this month.
I will say, that we see the Crab Apple trees burst out in bright pink blossoms, and huge old lilac bushes burst forth in lavender fragrant blooms. Trees are getting their leaves this month.
If you plan on going up to the mountains, any day of the year, plan on taking a jacket with you, as the weather can turn cool in a matter of minutes.