Mt. Alyeska Avalanche
From "The Snowy Torrents"

February 21, 1969. Accident Summary:  On the morning of February 21, avalanche control was carried out with the lower 75-mm recoilless rifle before Mt. Alyeska Ski Area was opened. This resulted in three size 4 avalanches and one size 5. At the end of the ski day, more control work was done from the upper gun above the chairlift. Snow Ranger Chuck O'Leary began firing at 16:00. The first shell on Center Ridge caused no release. Convinced that the ridge should release, O'Leary aimed at another fracture zone on Center Ridge and brought down a large part of the ridge (SS-AA-3). 

The next shot was fired at 16:12 in the Palisades area above and behind Center Ridge. In the past when this area released, the avalanche ran behind Center Ridge stopping in a transition zone, or if large, ran down onto a second transition in the Silvertip area. When the 75-mm shell hit in the Upper Left Palisades, a fracture line shot across three other areas - Alyeska Chute, Sunspots, and Alyeska Peak - for a combined width of nearly a mile.

The avalanche that resulted traveled down the mountain at terrific speed. It over ran the first transition behind Center Ridge and then zoomed across Silvertip, the second transition. Here it turned and dropped down and across the Main Bowl trail and into the Canyon. The dust cloud that developed was 400 to 500 feet high.

When the avalanche dropped into the Canyon, the effects of either a devastating air blast or an air-borne powder avalanche were seen. Although the canyon wall on the Racing Trail side is over 200 feet high, the powder avalanche tore out 100 trees from 8 to 20 feet tall that screened the trail from the Canyon. Skiing down the Racing Trail at this time were four patrolmen on sweep and one recreational skier. The airblast struck them, knocking them off their skis and rolling and tumbling them about 50 feet. One patrolman suffered a cut nose and another a sprained knee.

The avalanche continued down the canyon and spilled out onto the Racing Trail leaving 3 feet of debris on the trail. The slide came to a stop 500 feet short of the base area, but the dust cloud completely covered and a strong wind buffeted the base area, sending relaxing skiers running for cover.

Avalanche Data:  This frightening avalanche was classified as SS-AA-S-G-J; the G indicates that the avalanche ran to the ground in the starting zone and the J indicates airblast. The fracture line averaged 4 feet in depth and was a mile long. The avalanche had a vertical drop of 3400 feet and traveled a distance of 2-1/4 miles.

Comments:  This was the first time that an artificially released avalanche in the upper area had overrun the transitions and reached the lower ski area. A complete revision of safety procedures was begun following this avalanche. Snow Ranger O'Leary said this was the largest and most violent avalanche that he had ever witnessed at the ski area. Even the two large avalanches released during the 1964 Good Friday Earthquake were dwarfed by comparison.

The destructive airblast that accompanied this avalanche occurs only with very large avalanches and can reach amazingly forceful proportions. In a documented case near Berthoud Pass, Colorado in 1964, a 7000-pound dump truck and two heavy attachments for a tractor were carried 65 feet horizontally and 50 feet downhill by what was assumed to be the airblast from a fast-moving avalanche. Amazingly, the truck landed on its wheels and not so