The following images are, for the most part, from the US Geological Survey, primarily from the Cascade Volcano Observatory's Hazards Pages. You can click on any of the images to view larger versions.

In class, I asked what indicators might be used to predict an eruption from one of the Cascade volcanoes. One criteria would be a particular type of earthquake that is characteristic of magma moving under a volcano. This figure, from the Washington State Department of Natural Resources, shows what harmonic tremors look like when compared to typical earthquakes.
This image, provided by the University of Washington for the US Geological Survey, shows the depth and amount of seismic activity that occurred beneath Mount St. Helens from 1980 through 1996. During 1980 there is clear seismic activity indicating magma movement from 12 kilometers depth all the way up to the surface. Between 1988 and 1991, there is relatively well constrained activity from 9 to 4 kilometers depth and at approximately 2 kilometers depth. These depth ranges and seismic activity may indicate that the magma chamber and its conduit, or plumbing system, for Mount St. Helens lies at that depth.
As explained in class, the eruption of Mount St. Helens wasn't the typical central vent type eruption. As the magma chamber below Mount St. Helens was slowly filled, it inflated the mountain and caused a bulge in the flanks of the mountain. Being slightly off-centered also contributed to the devastation of the eruption. Just before the eruption, a small earthquake triggered a landslide centered on the bulge (over the magma chamber), which allowed the chamber to be exposed. This series of figures from the US Geological Survey, shows the events that happened during and after the landslide. Since the bulge and landslide were on the northern face of the mountain, the ensuing directed blast was focused in that direction as well.
The volcanic hazards associated with any eruption aren't just the erupting lava and ash as most people might think. Ash ejected high into the atmosphere can effect temperatures, acidity of rain, as well as the total amount of sunlight reaching the ground. Pyroclastic flows from an eruption may travel hundreds of kilometers from the eruption at hundreds of kilometers per hour to devastate regions that aren't thought to be in danger. Even a relatively quiet volcano can have unseen dangers in the form of fumaroles that vent toxic gases such as carbon dioxide, an odorless, colorless gas that is heavier than air and which will settle in depressions forming death zones, or hydrogen sulfide - that rotten egg-smelling gas.
As seen in this post-eruption devastation map from the US Geological Survey, most of the destruction was focused north of Mount St. Helens. Pyroclastic deposits (ash-flows, pumice flows, etc.) were a minor constituent of the total devastation. The associated mudflows caused more damage.
Other volcanoes in the Cascade Range aren't free of the hazards that were seen at Mount St. Helens. This map from the US Geological Survey, shows the mudflows that have been identified in the last few years as having come from Mount Rainier. Notice that some towns, the red dots, are built right in the path of possibly future mudflows. Even the suburbs of Seattle and Tacoma are built on mudflows and debris flows of Mount Rainier. With all of Mount Rainier's glaciers, a future eruption, even if small, could have devastating effects many miles downstream.
As recently as 500 years ago, the Puyallup River was the site of devastating mudflows that roared down it's drainage and covered the future sites of Electron and Orting. The older Osceola Mudflow, ~5000 years old, was even more extensive. It flowed north from Mount Rainier along the West Fork of the White River and the White River drainages then headed west. As soon as it left the confines of the drainage in the Cascades and entered the Puget Sound Lowlands, it spread laterally. A similar flow today would cover towns almost all the way to Puget Sound.
To the north of Mount Rainier, Glacier Peak posses just as much a hazard. Mudflows from it reached all the way to the northern part of Puget Sound and covered what became the towns of Arlington and Mount Vernon.
Although not in an as populated region as Mount Rainier and Glacier Peak, Mount Shasta has had serious mudflows and debris flows. The area north from Shasta along Interstate 5 almost up to Hornbrook was covered by a large landslide and avalanche. If you have driven along I-5 into California, you may have noticed the flats areas between the hummocky hills north of Mount Shasta - you were driving through an old, ~Pleistocene-aged, landslide deposit!
This last figure, a location map showing the major volcanic peaks in the Cascades and their relationship to population centers, should make you ask the question - what are the future hazards from volcanoes in the Cascades?