Looking at the most recent high res forecast models for Monday. I'm kind of speechless. The overnight temperature right now is already like a normal daily high. Tomorrow will be hotter. Monday will be out of this world. In fact, it will be hotter in Portland than about 99.8% of the world.
Many people are asking or talking about the role of global warming. It absolutely does contribute, but there's more to it than that. Global warming makes high temperature events more likely (and it especially increases the overnight lows), but the most extreme temperature events require a perfect combination of factors.
In this case, what we're experiencing could be more accurately called a Foehn wind. Normally the temperatures along the west coast are moderated by westerly winds off of the Pacific. But soon we'll be getting southeasterly winds passing over the Cascades. You can see this from the direction of the windbarbs in the above image. Not only does this bring in the hot airmass that was already in place from the more desert-like interior, but as the air descends the west side of the mountains it heats up even further. At the dry adiabatic lapse rate, descending the Cascades can heat the air by an additional 10F or more. That's how we get temps over 110F in western Washington and Oregon. This kind of airmass movement on top of such a prolonged period of dry and sunny weather is a perfect storm.