Also a threat from a Mt. Rainier eruption are lahars (massive mudslides). These will be triggered upon eruption by the melting of the massive snow pack on Rainier that instantaneously becomes billions of gallons of water that will destabilize mountainsides and mix with dirt creating huge mudslides traveling at the speed of freight trains. Communities at the foot of the mountain could be hit within an hour of the eruption by lahars that scour the ground and leave nothing in their wake. Historically, some of these lahars from Mt. Rainier eruptions have reached the Puget Sound.
A worst case scenario of a major eruption of Mt. Rainier could make the 1980 eruption of Mt. St. Helens look like a grade school science project in comparison.
Lahar and Pyroclastic Flow are synonyms