Temps topped out at 104F today, and right now, its 10:15pm and still 82F - its going to be a really warm night.
Our heat waves are triggered by compression heating - heat low builds out of california into Western Oregon, starting off the coast, and pull already hot air out of the interior. East of the Cascades, the elevation averages about 4000 ft, so that air comes through the passes and compresses as it sinks into the valleys as it is pulled towards the heat low.
Viola!
Tomorrow, cooler NW air should start to filter in as the heat low begins shifting east of the Cascades. We won't break 100 most likely. But east of the Cascades, they can easily break 110 in this pattern in some areas. By Monday, we should be back close to the normal 82 or so.
Remember, most of Oregon is desert or semi-arid, so we turn into an outpost of the interior sw here in the normally damper and cooler NW corner. We can go 40 to 60 days or more in a row without measurable precip during July-October. Kinda makes up for the fact that it can rain (at .1 to .5 inches a day on average) for 20 days in a row during the winter in my little corner of the state.