The main factor in Portland weather is the gorge cutting through two mountain ranges, creating an airflow corridor between the ocean and the hot inland plains

So after the sun goes down it stays hot and a little more stifling even until 2AM or so as the hot inland air passes by on the way out to sea

The flip side is the morning and afternoon aren’t as hot as they might be otherwise, because they’re moderated by the cool sea air coming in. Which is to say if it’s this hot already at 9:45, woo boy is it gonna get bad