Major Cascades volcanoes include, from south to north:
[ul][li]Lassen Peak, Shasta Co., CA[/li][li]Mt. Shasta, Siskiyou Co., CA[/li][li]Mt. Mazama, Klamath Co., OR[/li][li]Mt. Jefferson, Linn & Jefferson Cos., OR[/li][li]Mt. Hood, Hood River Co., OR[/li][li]Mt. St. Helens, Skamania Co., WA[/li][li]Mt. Rainier, Pierce Co., WA[/li][li]Mt. Baker, Whatcom Co., WA[/ul][/li]
I believe there are a couple in B.C. as well, but my atlases don’t do western Canada well. There are undoubtedly other volcanoes in the Cascades, but these seem to be the largest and most significant mountains.
The idea is that an eruption of any major size would be destructive to a city within a short distance, regardless of “which side of the mountain blows” – a side-collapse eruption such as St. Helens had not being the norm though not uncommon.
The problem with Seattle (and even more its southern suburbs and Tacoma) in a Rainier eruption would not be so much the pyroclastic effects, which would probably die down within a ten-mile range, but the fact that it’s a Flippin’ High Mountain right above the Seattle-Tacoma-Olympia Metro. Area, and the two main channels downhill from it are the Pullayup and White Rivers, which descend to Tacoma and the Kent Valley respectively. Both gas and lava would follow these two paths downhill.
IMHO, all of the Cascades volcanoes are a threat to some degree, with Shasta (and Mazama) perhaps the least likely to blow. The danger falls into two categories – threat to life from those near the volcano who do not evacuate in time, and threats to property in the path of eruption products (solid, liquid, and gaseous). The violence of an eruption influences what is “too near” and what “the path of eruption products” might be. There are buried ash beds from Pliocene and Pleistocene eruptions extending across the inland Northwest (south Idaho, southwest Montana, western Wyoming) from eruptions orders of magnitude above those known to history. Such violent eruptions are unlikely, of course – but not at all impossible. IIRC, St. Helens ranks with the most violent eruptions in history, somewhere about sixth or seventh.