That means massive pressure will build in the conduit until the plug blows out with explosive force expected to be as great as, if not greater than, the 1980 eruption. When it gets wet, ash becomes two to three times as heavy and it becomes sticky, adhering better to sloped surfaces. Four inches of wet ash is enough to collapse some roofs. They incinerate, asphyxiate, bury or blow apart whatever they hit.
Evacuation plans are pointless.
It's best just to stay way, way back. Because no two eruptions are the same, no one can say how far the eruption will reach or where it will go. Stratovolcanoes like Mount St. Helens are symmetrical, steep-sided peaks that erupt in explosive bursts, rather than seep lava like the shield volcanoes that formed Hawaii. The difference is in the silica content of the molten rock fueling the volcanoes. Magma in stratovolcanoes is viscous due to its high silica content.