Acording to a placue in Mt. Tabor Park, Portland has more volcanos with in it's city limits, than any other city in the world (it also have the largest park, Forrest Park, and the largest book store, Powells). Mt Hood, approxamitly 40 miles east of Portland is a volcano long over due for eruption. The the plate techtonic movements moving the subduction zone (and magma chamber) west, when will these little in town volcanos blow? The after effect would be staggering. As the airport and most of down town are build on fill, they would slide into the Columbia and Wilamette Rivers. The Oregon sides of the bridges along the columbia and most of the bridges on the Wilamette are like wise built on fill (the on and off ramps, that is. The supports are built into the bed rock), which would leave the bridges unaccessable. This would trap all of North and North East Portland trapped between rivers and lava. Or perhaps it would be a pyrotechnic blast like Mt. St. Helens, flattening all of Portland, and then flooding it, the the ash clogging the rivers until they were just huge mud flows.
They say the same thing about Tacoma, with Mt. Rainer also being over due for an eruption. Rainer is poised to have huges mud slides that will flatten everything for hundreds of miles around it.
The real question is, will either mountains eruption be something on the scale of Mt. Mazama? When it blew, it covered almost all of Oregon, some of Northers California, Idaho and Washington with it's lava flows and ash fall. There are still, thousands of years later, huge lava and obsidian fields where nothing will grow. Mt. Mazama is now better knows as Crater Lake.
My prediction? With in my life time, one of those two will erupt.