I basically taught myself photography by shooting waterfalls in the Gorge when I was growing up there. May is really a great time to shoot the waterfalls if you know the tricks. To start with, cloudy weather is your friend, it reduces the overwhelming contrast that sunlight creates between the dark rocks and the white water. It may be rainy, but it doesn't rain all the time, there will be breaks as well as completely dry days in May.
May is the time when the water flow is at its maximum, which is usually, but not always a good thing.
Be sure to stop at Latourelle Falls, walk up the short path and take a shot at the little viewpoint. You will see a big patch of colorful lichen on the cliff next to the falls.
Wahkeena falls has a nice trail that goes up the path of the stream and there are several spots where you can get closeups of the flowing water.
Oneonta Gorge is impressive, but in May the water will be too high to walk into it for photos.
Multnomah Falls is the big one, but there are usually so many tourists swarming around in May that you can't get a nice shot.
If you don't mind driving about a half hour North of the Gorge, look up Panther Creek Falls. It's a little tricky to find, but I think with GPS, you should be able to drive right to it. Don't expect to see a sign, it keeps disappearing. Follow the link to a video I shot there in May of 2013 when I was testing my 5D3. This was using a 28mm 1.8 lens. http://www.beyondboudoirphoto.com/blog/2014/9/rainy-season-in-the-pacific-northwest-video
Oh, and don't worry about snow in May, Mt. Spokane has probably been sampling too much of that Walla Walla wine tonight.