Maps that show the volcanic plume track for the May 18 Mount St. Helens eruption indicate that the debris from subsequent eruptions, including the June 12 explosion, may follow a different path. Two major eruptions of the Mount St. Helens volcano, at 8:30 and 10:45 A.M. Pacific Daylight Time on May 18, injected ash into the atmosphere to about 18 km above sea level. During the eruptions, and for several days thereafter, brisk upper-level winds (between 6096 m and 12,192 m) swept volcanic debris over the central states in two to three days, while winds below 3,048 m carried ash over southern Canada for a period of 3 to 4 days.

Meteorologist Roland Draxler of the NOAA Air Resources Laboratories reports that, ‘Based on these surface observations, the ash progressed eastward to about [16 to 32 km/h] across the northern United States and southern Canada.’