Willamette Aviation, just south of Portland.
http://www.willametteair.com/
I didn't do my PPL training there, I just hired an aircraft quite a few times whilst on holiday from the UK (where I did my training).
Great small flying school, good rates, and very friendly and helpful.
As for the weather, I was there in the summer, and happened to be at a time of a heatwave! There were 2 weeks without a cloud in the sky. But the week after was 100ft cloud base and rain! Basically the weather around Oregon can change very drastically very quickly, so watch out.