Life for people living in the shadow of a stratovolcano would be much simpler if eruptions were easily predicted. Earthquakes and steam-venting often signal an impending eruption, but the activity may go on for weeks or months before the eruption actually happens. The May 18, 1980, eruption of Mount St. Helens in southern Washington’s Cascade Range was preceded by almost two months of seismic activity and steam-venting. Many residents had begun to think the agencies issuing the warnings were “crying wolf,” and refused to leave homes located in the flow paths. Many others were in the scenic area hiking and camping. But the mountain did blow — and it blew with a vengeance, spawning pyroclastic flows that traveled at 50 to 80 miles per hour and sending about 520 million tons of ash eastward across the United States.

For more: http://www.popsci.com/environment/article/2009-02/shadow-redoubt