What is the weather normally like in mid to late May in Portland and the coastal area

www.weatherbase.com

Nice. It's normally nice mid to late May in those areas

That said it IS the Pacific Northwest so it might rain!

Give us a hint . . . Portland Oregon or Portland Maine?

Portland to Cannon Beach to Willamette Valley is our planned route.

Almost impossible to predict...early May more likely to be "wet" and windy...especially near the coast...Portland however often mild, not
necessarily wet.
In my experience, July, August, Sept.& sometimes Oct. are the warmest months. However, the Oregon coast can be cool and foggy in July and August..

Yes, May can be very hard to predict. It can be cold and rainy, or we can have a hot spell like we did this year. Just bring layers and you will be fine. For the coast you can pretty much count on it being chilly at least, and probably a bit of rain as well.