If anything May can be early if you aren't prepared for cold weather. While uncommon, it is a good idea to plan for snow. Check the weather often when in the high country.
You can go camping in May, but the likelihood of you getting rained on is probably pretty good. I would always suggest looking at the weather before you take off and go to that area. There will likely still be some snow and areas where it's very cold especially at night.
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