may is when spring starts to change to summer
Normally May is a beautiful time to be in the Methow Valley and North Cascades. The days are usually warm(70s to low 80s). And the nights tend to be chilly once the sun goes down(60s to high 30s).
It rarely rains at this time of year but it is possible. Be ready for all possibilities and bring/wear rain gear, warm jackets, and lots of layers, but also don't forget the bathing suits and sun screen!
Another really cool thing about this time of year in this area is that it's prime wildflower season! There will be a rainbow of yellows, reds, purples and greens covering the hillsides and forests and there will be snow still covering the tops of the higher mountains so that makes for a lot of contrasting colors.
NOAA Weather Link for the Stage Area