A small but significant explosive event occurred yesterday at 5:25 p.m. PST. Pilot reports indicated that the resulting steam-and-ash plume reached an altitude of 36,000 feet above sea level within minutes. The main eruption pulse lasted about 10 minutes, but lower levels of activity persisted for at least another 15 to 45 minutes. Within minutes of the onset of this event, we lost communication with 7 monitoring stations in the crater, but not with any stations outside the crater. The event followed a few hours of slightly increased seismicity that was noted but not interpreted as precursory activity. There were no other indications of an imminent change in activity. Still images from a camera at the northeast end of the crater mouth show a clear component of explosive vertical jetting associated with the event and evidence of ballistics extending at least as far as the north side of the old dome. Aerial photos in of the waning phases of the activity in conjunction with these still images show evidence of small ash flows having moved north and onto to old lava dome. There were reports of fine dustings of ash falling in Ellensberg, Yakima, and Toppenish, Washington between 7pm and 9pm yesterday. As of 2am today, the leading edge of the plume had been tracked to western Montana as a faint and diffuse cloud.