The public television station in Seattle has a special on Mt Rainer and it has been shown a number of times here in Oregon. I don't know their station's name but you might check with them. The Oregon station is Oregon Public Broadcasting, OPB. Part of their presentation was outlining the past eruptions and the extent and breath of the lahars. If I remember correctly, they said that thirty minutes was the time estimated between eruption and lahars hitting populated areas.

When Mt St Helens went off, I lost all my rain gutters on my home in Portland, Oregon. The fallout was very heavy and could only be moved so far with high pressure water hoses then you had to resort to machanical means to move it. It is hard to imagine how much energy and force that stuff has when it is flowing especially if it also at elevated temperatures. Now I live slightly east and between Mt Hood and Mt Adams. So I watch the mountains!