What to expect as far as weather
Typically in the spring in Washington and Oregon
Rain. More rain. Warmer end of April and May - low 40s at night | low 60s during the day, cold March with snow at high elevations
Are we talking western part of the states---Or east of the mountains
I'm not sure about all of Oregon but for the southwest corner I'll second the rain gear because it can flat out pour. Dress in layers and I like a packable puffy jacket.
The weather in those states will vary greatly depending on what region your in. If near the coast in either state prepare for rain, back in the eastern parts of the states the weather is very arid with minimal rainfall. Especially southeast Oregon as it's pretty much the high desert.