Avalanche in Lionhead. Broke on wind slab then triggered persistent slab underneath.
Skied Ellis yesterday, good snow but quite busy. Dug a pit at ~8000’, ENE aspect. No failure in ECT with snowpack from 130-140cm. Bit of a crust/layer ~ 30cm below surface on open, exposed slopes but not present in forest. Minor wind impact on top slopes, zero wind impact below ~7800’.
<p>Human-triggered avalanches are likely on steep slopes with fresh drifts of wind-loaded snow in the mountains around Bozeman and Big Sky.</p>
<p><span>Today's primary concern is </span><strong>wind slab avalanches </strong><span>breaking 1-3 feet deep</span>. The Big Sky Ski Patrol reported rapid wind loading yesterday, triggering many wind slab avalanches during mitigation work. Expect similar conditions in the backcountry today. Recent backcountry avalanche observations include a snowmobiler-triggered slide on Buck Ridge yesterday (<a href="https://mtavalanche.com/node/34193">details and photo</a>), thin wind slabs triggered by a group up Storm Castle Creek on Friday (<a href="https://www.mtavalanche.com/node/34169"><span>observation</span></a>) and several small wind slabs reported on Thursday in North Bridger Range, near the ice climbs in the Main Fork of Hyalite Creek and on Mt. Blackmore (<a href="https://www.mtavalanche.com/node/34160"><span>N. Bridger’s observation,</span></a> <a href="https://www.mtavalanche.com/node/34144"><span>main fork of Hyalite observation</span></a>, <a href="https://www.mtavalanche.com/node/34137"><span>Mt. Blackmore observation</span></a>).</p>
<p><strong>Storm slab avalanches </strong>are a secondary problem. They will break within the layers of new snow as precipitation rates peak. While these will not be as large as wind slab avalanches, both may be large enough to injure or bury skiers and riders. Loose snow avalanches (sluffs) will be dangerous where they can tumble backcountry travels through technical terrain.</p>
<p>Seek out slopes sheltered from the effects of the wind and recognize recent avalanche activity and shooting cracks as critical information about a slope’s instability. Avoid slopes steeper than 30 degrees below cornices and where you observe signs of significant wind loading.</p>
<p>The danger is CONSIDERABLE on wind-loaded slopes and MODERATE on all others.</p>
<p>Three-day storm totals nearing a foot of snow with an inch of <a href="https://avalanche.org/avalanche-encyclopedia/#snow-water-equivalent-swe… water equivalent</span></a> and fluffy snow drifting into unstable slabs create dangerous avalanche conditions on wind-loaded slopes in the mountains around West Yellowstone, Island Park, and Cooke City.</p>
<p><strong>W</strong><strong>ind slab avalanches</strong> breaking 1-2 feet deep are the primary problem. Yesterday, ski guides working south of Cooke City reported numerous avalanches (<a href="https://mtavalanche.com/node/34189"><strong><span>photos and observation</span></strong></a>). With heavier precipitation rates, instability will spread from wind-loaded slopes to all steep terrain in the form of storm slab and loose snow avalanches.</p>
<p><strong>Persistent slab avalanches</strong> failing on buried weak layers 2-3 feet deep remain an area of uncertainty. They are most likely on wind-loaded slopes where the snow layer resting on weak layers is thicker and more cohesive. Last week, Alex and I saw four avalanches that broke on layers of sugary facets and feathery surface hoar buried in late January. Avalanche activity on these layers hasn’t been widespread, but activity in the <a href="https://www.mtavalanche.com/node/34156"><span>Lionhead area,</span></a> <a href="https://www.mtavalanche.com/images/25/cornice-triggered-avalanche-miner… City</span></a>, and <a href="https://www.youtube.com/watch?v=wZ3k35z5Ej8&embeds_referring_euri=h… Fork</span></a> keeps it on the radar.</p>
<p>Avoid wind-loaded slopes where avalanches are more likely and would be larger. Seek out non-wind-loaded terrain, stay alert for signs of instability (recent avalanches, shooting cracks, and collapsing) and dig down three feet to test for instability before committing to steep slopes. The danger is CONSIDERABLE on wind-loaded slopes and MODERATE on all others.</p>
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I could visibly see some layer distinction about 8-12” deep. This is the same weakish layer we found in our column test, where we got ect16 just below robs knob. This was on a north facing aspect. Photo: I Tuttle
After taking a heavy landing on top of a pillow just off the highway past lolo pass road, I looked back up and could visibly see some layer distinction about 8-12” deep. This is the same weakish layer we found in our column test, where we got ect16 just below robs knob. This was on a north facing aspect.
Buck ridge, snowmobile triggered avalanche. Propagated storm slab.
Buck Ridge, snowmobile triggered avalanche. Propagated in the storm slab. Photo: C Erhard
From IG message: “Buck ridge, snowmobile triggered avalanche. Propagated storm slab.”
