Joey Dempster

Provo » Mt Nebo

Mt Nebo, North Couloir

11,800'

North

40°

Skier

Intentionally Triggered

Soft Slab

Wind Drifted Snow

Facets

8"

125'

1,500'

We climbed Nebo with the intention of skiing the North Couloir. From above, the surface looked as if sluffs from the cliffs above had deposited the surface snow without triggering anything larger, which we took as a good sign. However, when we sent a skier down the cornice and into the upper couloir on belay, his weight immediately triggered a slab avalanche which propagated below him about 20 feet, and about 125 feet to the skier's right. It did not propagate left, or above. The avalanche wasn't very large since the slab was low density and only about 8 inches thick, and it did not break out very far below the crown, and it did not step down. It did entrain a good deal of surface snow and ran the entire length of the couloir. Since there was a lot of hangfire left, we opted to not ski the couloir and the slope tester climbed back up the rope. It is my opinion that there were many slopes today at high elevation that could have produced similar results. I also feel that they will remain sensitive tomorrow. Additional snowfall will make them more active, and impossible to see. S winds were high in the morning, but by the time we were on the summit, they shifted around to the N, then died almost completely except for the odd gust. There was, however, ample evidence of wind transport well down off the high ridges. We found a way out lower down the ridge and got to see A LOT of steep snow. Wind loaded snow was ubiquitous below the ridge lines, and in many gullies further down, although the ridges in between were scoured (cross loading) and that is what we used to exit safely. Down lower, there was plenty of weak snow in the snowpack (old facets). We had no choice but to ski cut many such slopes between 10,600 and 9600 on the way out, but found no significant slab overlying or stored energy so there were no signs of avalanche hazard in the non-wind affected terrain. The bottom line is that avalanche hazard was present primarily in wind loaded snow today. It was relatively easy to deal with because you could see it, but new snowfall could obscure it and make tomorrow tricky. There is video of the avalanche, but it isn't available tonight. Hopefully tomorrow.