Double ash tracks from Mount St. Helens



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.’