GNFAC Avalanche Forecast for Thu Dec 8, 2022
<p>In the mountains south of Big Sky and around West Yellowstone and Cooke City, strong winds the first half of the week drifted recent snow into thick slabs sitting on top of weak, faceted snow. On Tuesday, a snowmobiler triggered a large avalanche that broke on weak snow near the ground at Chimney Rock near Cooke City. We have limited details but thankfully, everybody made it out (<a href="https://www.mtavalanche.com/node/27114"><strong><span>limited details</span></strong></a>). Recent natural avalanches in the Taylor Fork in the Southern Madison Range and near Henderson Bench outside Cooke City demonstrated the snowpack was teetering on the edge of failure (<a href="https://www.mtavalanche.com/node/27117"><strong><span>TF details</span></strong></a><strong>, </strong><a href="https://www.mtavalanche.com/node/27113"><strong><span>Henderson photo and details</span></strong></a>).</p>
<p>Today, natural avalanches are unlikely; however, human-triggered avalanches breaking 2-3 feet deep remain a deadly possibility. Yesterday, Ian rode into the Taylor Fork and found a thick layer of weak facets buried 18-24” deep (<a href="https://www.youtube.com/watch?v=O2iI_MqPIqE"><strong><span>video</span>…;). It isn’t everywhere, but it exists on many slopes in the Southern Madison and Southern Gallatin Ranges and near West Yellowstone and Cooke City.</p>
<p>Dig down to assess for the presence of this persistent weak layer. If you find it, stay in terrain less than 30 degrees steep. The avalanche danger is MODERATE because human-triggered avalanches are possible.</p>
<p>In the mountains around Bozeman and Big Sky, the primary concern is avalanches failing on steep slopes with recent drifts of wind-loaded snow. Indicators of these areas of concern include cracking, a stiff snow surface or a hollow “drum” like feel, cornices, and visible drifts.</p>
<p>Yesterday, the Big Sky Ski Patrol triggered 12-18” deep wind-slab avalanches in their alpine terrain. The Bridger Bowl Ski Patrol performed their first round of avalanche mitigation, noting that most slides involved the recent layers of wind-loaded snow. Three days ago, two skiers ascending a steep couloir triggered an avalanche on a wind-loaded slope. At times they were completely submerged in the avalanche, but thankfully they didn’t hit any trees or rocks and came to rest on the surface (<a href="https://www.mtavalanche.com/node/27091"><strong><span>details and photos</span></strong></a>). Watch my <a href="https://youtu.be/u5SmVqnzMgg"><strong><span>video</span></strong></a> from Divide Peak earlier this week to hear our concerns about snow transported by the wind.</p>
<p>Today, find higher quality <em>and safer </em>skiing and riding on non-wind-loaded terrain. Dig and test for any deeper instabilities before committing to a slope. Human-triggered avalanches are possible, and the danger is rated MODERATE. <span> </span></p>
<p>If you get out, please share avalanche, snowpack or weather observations via our<a href="https://www.mtavalanche.com/node/add/snow_obs"><span> </span></a><a href="https://www.mtavalanche.com/node/add/snow_obs"><strong><span>website</s…;, email (<strong>mtavalanche@gmail.com</strong>), phone (406-587-6984), or Instagram (#gnfacobs).</p>
<p>In the mountains around Island Park, strong winds the first half of the week drifted recent snow into thick slabs. In the nearby Lionhead area, these slabs are sitting on top of weak, faceted snow and we are concerned that the same conditions exist in the Centennial Range. While natural avalanches are unlikely today, dangerous human-triggered avalanches breaking 2-3 feet deep remain a deadly possibility. If you intend to ride or ski in avalanche terrain, avoid wind-loaded terrain, and dig down to assess for the presence of this persistent weak layer. If you find it, stay off steep slopes, and <a href="https://www.mtavalanche.com/node/add/snow_obs"><span>send us an observation to let us know what you discovered</span></a>.</p>
<p><span>Ian and I will be in Island Park hunting for weak layers tomorrow. Tomorrow evening, visit the <a href="https://www.mtavalanche.com/forecast/centennials"><span>Island Park Trip Planning page</span></a> to learn what we find and watch our field video.</span></p>
Upcoming Avalanche Education and Events
Our education calendar is full of awareness lectures and field courses. Check it out: Events and Education Calendar.
Natural Avalanche, Sunlight Basin
We saw an R1-D1 across the bowl of Sunlight Basin that looked recent after digging a pit near the weather station. While sledding around the area, we noticed faceting snow at the bottom of the snowpack in multiple locations.
Deep slabs at lava lake
Snowshoeing around the lava lake area I observed a crust on the snow 4-6” thick Laying on top of huge drifts consisting of really fine/powdery snow. This made travel on even the biggest of snowshoes impossible in those areas if the crust was broken. This started around the 7000 foot mark and carried into the higher elevations.
Rider Triggered Avalanche, Chimney Rock
There was a human-triggered avalanche near Chimney Rock on Henderson Mountain. The avalanche failed on weak snow near the ground. We believe a snowmobiler triggered the slope.
From GNFAC visit to crown 12/8: "We rode to Daisy Pass to look at the avalanche that was triggered by a snowmobiler two days ago (12/6/22). It was on the slope east of the pass that people often climb up and out, steeper than the normal route out. It broke 2-3 feet deep and 250-300’ wide, R2-D2. At the crown it broke on a weak layer about 100cm above the ground, but it broke closer to the ground in many places lower on the slope where the snowpack was shallower and weaker. I would guess one of those areas is where it was triggered from."
Natural Avalanche, Henderson Bench
From IG: “Avalanche spotted on Henderson bench today, appears to be cornice triggered yesterday or the day before. 2-5 feet deep, 600’ wide, ran 800 vertical.”
From IG: “Avalanche spotted on Henderson bench today, appears to be cornice triggered yesterday or the day before. 2-5 feet deep, 600’ wide, ran 800 vertical.” Photo: M. Cohen
Forecast link: GNFAC Avalanche Forecast for Thu Dec 8, 2022
Natural Avalanche, Henderson Bench
From IG: “Avalanche spotted on Henderson bench today, appears to be cornice triggered yesterday or the day before. 2-5 feet deep, 600’ wide, ran 800 vertical.”
Faceting in Taylor Fork - Video
We rode from Sage Creek trailhead to the weather station. After we got everything installed, we headed to Sunlight Basin to dig. There was about 12-15 cm of new and wind drifted snow. On the walk to the pit location we noticed the snow was punchy. HS was 94 cm on the E aspect. We got an ECTP 13 on a layer of 2 mm facets 44 cm down from the surface. New and wind drifted snow put a substantial slab on top of this weak layer. After we dug our pit visibility improved, and we saw a R1-D1 across the bowl that looked recent. While sledding around the area we noticed faceted snow in the middle of the snowpack in multiple locations.