After receiving over a foot of snow in the past few days, skiers in the Bridgers were able to trigger a wind-loaded test slope near the Playground. With more snow in the forecast, storm slabs and wind slabs will remain sensitive over the next few days. Photo: G. Antonioli
On any slope that had old slabs of wind drifted snow we experienced large collapses and shooting cracks.
<p>Yesterday, my partners and I toured in the mountains above Hebgen Lake near West Yellowstone and found a shallow-weak snowpack (<strong><a href="https://www.youtube.com/watch?v=MeTYd9mOyGk&list=PLXu5151nmAvRNl9ku…;, <strong><a href="https://www.mtavalanche.com/images/18/shallow-weak-unstable-hebgen">pho…;). On slopes that had thicker slabs of wind drifted snow, we experienced large collapses and shooting cracks. A similar observation came from the Lionhead area, where a snowmobiler experienced cracking and collapsing (<strong><a href="https://www.mtavalanche.com/images/18/collapsing-and-cracking2">photo</…;). These are clear warning signs the snowpack lacks structural integrity and can’t support much weight.</p>
<p>As snow and wind impact the area the next few days, conditions will become increasingly unstable. The best approach to safe riding will be to stay off steep slopes and avoid avalanche run out zones.</p>
<p>Today, human triggered avalanches are possible and the avalanche danger is rated <strong>MODERATE</strong>. The danger could rise to <strong>CONSIDERABLE</strong> by this afternoon with the arrival of new snow and wind.</p>
<p>The snowpack around Cooke City contains multiple weak layers capable of producing avalanches. North of town, a layer of facets 1-1.5’ below the surface is the main avalanche concern (<u><strong><a href="https://www.youtube.com/watch?v=W6iS9ManzPo&index=2&list=PLXu51…;, <u><strong><a href="https://www.mtavalanche.com/images/18/instability-near-lulu-pass">photo…;). Yesterday, a snowmobiler triggered a slide on Mt Abundance that most likely failed on this layer (<a href="https://www.mtavalanche.com/images/18/snowmobiler-triggered-mt-abundanc…;)</a>. The slide broke 1-1.5’ feet deep, a 100’ wide and funneled into a terrain trap. Fortunately, nobody was caught. However, this slide was easily large enough to injure or bury a skier or rider. Similar avalanches are possible today. The only way to assess the strength and distribution of this weak layer is dig a snowpit and do a stability test. If test results point to instability, stick to lower angle terrain or find a slope with a stronger structure. </p>
<p>South of town, the snowpack is shallower and weaker. On Thursday, Doug toured up Republic Creek and found a faceted snowpack (<strong><a href="https://www.youtube.com/watch?v=4Vb2Y4UZJ1Q&index=2&t=0s&li…;). This structure will become increasingly unstable as snow and wind impact the area the next few days. Avoiding steep slopes and avalanche run out zones will be the best way to stay safe with an increasing avalanche danger.</p>
<p>Today, the avalanche danger will start out <strong>MODERATE</strong>, but could rise to <strong>CONSIDERABLE</strong> by this afternoon.</p>
<p>The Bridger Cloud worked its magic yesterday delivering 8” of cold smoke powder. Hyalite and Big Sky weren’t as lucky, but still picked up a few inches. This round of low density snow did not put significant stress on a generally stable snowpack, but did provide plenty of ammunition for the development of fresh wind slabs (<u><strong><a href="https://www.youtube.com/watch?v=2lO7K1Y-gOs&index=4&list=PLXu51…;
<p>Today, the primary avalanche concern will be wind loaded slopes. West-northwest winds increased significantly overnight and will remain strong through the day. The combination of strong winds and fresh snow available for transport will create wind slabs that will be easy to trigger. Wind slabs will grow in size and distribution as the day progresses. Watch for signs of instability such as shooting cracks and avoid all steep slopes that have wind drifted snow.</p>
<p>Today, dangerous avalanche conditions exist on wind loaded slopes which have a <strong>CONSIDERABLE</strong> avalanche danger. Non-wind loaded slopes have a <strong>MODERATE</strong> avalanche danger.</p>
After last season's two snowmobiler avalanche fatalities on Reas Peak in the Centennial Range we created a webpage so riders could get good, relevant information regarding snowpack and avalanches.
Skiers intentionally triggered small wind slabs that were 2-3" deep and up to 20 ft wide that did not run very far. They also reported low-volume dry loose activity.
The avalanche on the south face of Mt Abundance was set off at about 2:00 pm on Thursday. Nobody was caught and it was triggered right behind a rider as he was climbing. It was estimated to be 1 foot deep, 100 feet wide, and ran a 100 feet slope distance. It was big enough to bury a person. Photo: J. Boyko
Collapsing and cracking in Watkins Creek in Lionhead. The snow is weak and collapses like this indicate instability. Photo: B. Rasmussen
Collapsing and cracking in Watkins Creek in Lionhead. The snow is weak and collapses like this indicate instability. Photo: B. Rasmussen
From an email from Bret Rasmussen:
(Collapsing) "This occurred as I was driving through the snow. I had observed this evidence of collapsing in a few areas, but in the picture I actually witnessed the snow settle. Above and South of Watkins Creek. East Southeast aspect. 8000 feet."
His assessment (we concur): " The weak layer does not seem to be healing... It's going to take a deeper snow pack to promote stability but at the same time this will stress the existing weak layer. I'm afraid we are headed for a very troubled avalanche season!"
The attached video and snowpit is from the same day, not very far to the north in Trapper Creek.
