Last week, backcountry skiers triggered a massive avalanche on Bald mountain near Breckenridge, CO, that was half a mile wide and ten feet deep in places. Thankfully nobody was caught.
Avalanche was triggered on our second run, after skiing an adjacent run with a more easterly aspect our first run (ENE aspect for the first run). Same elevation both runs. We saw one very small crack stretch a few feet from our skis in shallow terrain over scree our first run, but that was the only sign of instability we observed until the avalanche.
– Report on CAIC
Cutting at the top of the slope, the first skier intentionally caused a windslab slide about 50-feet wide and 3-4 feet deep. This then sympathetically triggered another slide in the chute, which triggered the entire basin wall to release. The avalanche was rated as R4/D3.5.
First skier intentionally cut the top of the slope, triggering a windslab beneath the cornice about 50 feet wide. Skier was able to get off the slab after about 3 seconds by skiing to a predetermined safe zone to the right, away from the gut. He was not caught. That slab stepped down 3-4 feet to a deeper weak layer roughly 200 vertical feet below our entry. The gut of the chute ran fully from there but still was isolated to the gut. When it neared the bottom, about 1,000 feet below us, the entire left side of the chute released – roughly 850 feet above where the debris from the first slide was at the time. Once that sympathetic slab had reached the flat bottom but was still moving, we watched the much larger basin to our left rip wall to wall, to ground, starting at least 1,000 feet above where the other debris was churning at the time. It sent a very large avalanche racing down the bowl. We estimated the crown to be as deep as 10 feet in places. Most of it appeared to be 4-8 feet deep. The debris pile from that avalanche overran much of the debris pile from the original chute. We estimated the distance from the first wind slab to the northern edge of the third, sympathetic avalanche to be almost a half mile wide. We very much underestimated the potential for deep slab avalanches today. We also underestimated the wind slab’s potential to step down into deeper layers, though we talked about it at length. Of note: After our first run, while skinning back up, we observed two small soft slab avalanches from either yesterday or the day before that ran about 800 feet but only entrained surface snow. Both released from a similar spot in the bowl that ultimately fractured 8-10 feet deep today (the skier’s right side of the basin that you see in the photo of the fracture line). Also of note: there was no direct visible propagation connection between the chute and the bigger basin that slid last. They are separated by a rocky ridge for almost the length of the chute. Our biggest takeaway was that this snowpack is nowhere near stable in a lot more places than one might expect. The chute that released had been exposed to sun for weeks; it was not a super high sheltered northerly slope. In fact it is probably in the sun from 8am to 5pm every day right now when the sun is out.
– Report on CAIC
Most slopes are safe from avalanches this weekend. This does not mean avalanches are impossible. There are a few isolated slopes in rocky, steep, northerly terrain in the alpine where you can trigger a very large avalanche breaking on weak layers near the ground. You can greatly reduce your chance of triggering this kind of avalanche by avoiding wind-loaded features, especially in thin rocky areas with highly variable snow depths. If we get more sunshine than expected, look for wet, slushy surface conditions. You might be able to trigger small wet avalanches later in the day in steep rocky terrain.