18-19

<p>Since yesterday morning the mountains near West Yellowstone received more than 20” of snow equal to 2.2” of <u><a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a></u> (SWE). Strong southwest wind drifted this snow into thick unstable slabs, which will continue to grow with snow and wind today. Large avalanches of new and wind-drifted snow are likely on all steep slopes and will break naturally. This rapid, heavy load will test the strength of weak layers buried 4-6’ deep at the base of the snowpack, and very large avalanches are possible (<u><strong><a href="https://www.youtube.com/watch?v=PW2xrOl76-Q&amp;t=0s&amp;index=2&amp;li…;, <u><strong><a href="https://www.youtube.com/watch?v=SFA1kKs6wsw&amp;t=0s&amp;index=2&amp;li…;). Avoid all steep slopes and runout zones below. Avalanche danger is <strong>HIGH</strong> on all slopes.</p>

<p>The southern Madison and Gallatin ranges and mountains near Cooke City received more than a foot of new snow equal to 1.2”-1.4” of SWE. Overnight, strong southwest wind formed 2-3’ thick drifts that are easy to trigger and can avalanche naturally. Avoid travel on and underneath steep slopes adjacent to scoured ridgelines or below large cornices. Though less likely, avalanches breaking on sugary persistent weak layers are possible, similar to the large slides ten days ago on Henderson Mountain (<strong><a href="https://www.mtavalanche.com/node/20085">details</a></strong&gt;) and in the Taylor Fork (<strong><a href="https://www.mtavalanche.com/node/20107">details</a></strong&gt;). On non-wind loaded slopes, storm slabs 1-2’ thick and loose snow avalanches are easy to trigger and large enough to bury a person. Stability will decrease with more snow and wind today. Avalanche danger is <strong>HIGH</strong> on wind loaded slopes and <strong>CONSIDERABLE</strong> on all other slopes.</p>

<p>Yesterday Doug and I toured north of Bridger Bowl. There was no wind loading or cracking of the snow surface to indicate unstable soft slabs, and the main concern was loose snow avalanches (<strong><a href="https://www.youtube.com/watch?v=i5VuoKAZuAQ&amp;index=3&amp;list=PLXu51…;). Ian found similar conditions in Beehive (<strong><a href="https://www.youtube.com/watch?v=fqaOjRpv09g&amp;list=PLXu5151nmAvRNl9ku…;). Since yesterday the mountains near Bozeman and Big Sky received 4-6” of snow equal to .2-.6” of SWE. Strong wind formed fresh drifts that are possible to trigger and will grow larger and more unstable with snow and wind today. Watch for blowing and drifting snow and cracking of the snow surface as signs to avoid steep, wind-loaded terrain. There is an isolated chance you could trigger a deep avalanche on buried weak layers (<u><strong><a href="https://www.mtavalanche.com/images/19/snowmobile-triggered-avalanche-fi…;, <u><strong><a href="https://www.mtavalanche.com/images/19/avalanche-crown-butte-2">photo</a…;, <u><a href="https://www.youtube.com/watch?v=_5mSOkh9iqE&amp;list=PLXu5151nmAvRNl9ku…;). The most suspect slopes are heavily wind loaded, or where the snowpack is relatively shallow (less than 3-5 feet). Today, new low density snow and strong wind make avalanches possible and avalanche danger is <strong>MODERATE</strong>.</p>

<p>If you get out and have any avalanche or snowpack observations to share, contact us via our <u><a href="https://www.mtavalanche.com/node/add/snow_observation">website</a></u&gt;, email (<u><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></u&gt;), phone (406-587-6984), or Instagram (#gnfacobs).</p>

<p>Near West Yellowstone and in the southern Madison and Gallatin Ranges weak layers of sugary snow at the base of the snowpack have had minimal stress from new snow over the past week (<strong><a href="https://www.mtavalanche.com/weather/wx-avalanche-log">weather log</a></strong>), and they will hardly notice today’s lightweight snow. Yesterday Eric and I toured near Hebgen Lake outside of West Yellowstone. We found that a lot of force is required to break the weak snow at the foundation of the snowpack, and avalanches on this layer are difficult to trigger (<strong><a href="https://www.youtube.com/watch?v=PW2xrOl76-Q&amp;t=0s&amp;index=2&amp;li…;). However, these avalanches will be large and there are no warning signs such as collapses in flat terrain or reactive test slopes (<u><strong><a href="https://www.youtube.com/watch?v=SFA1kKs6wsw&amp;t=0s&amp;index=2&amp;li…;, <u><strong><a href="https://www.mtavalanche.com/node/20160">details and photos</a></strong></u>). Tracks on a slope do not indicate stability. Due to difficult stability assessment, the best strategy is to avoid steep slopes, especially those beneath cornices or in steep rocky terrain.</p>

<p>This morning the mountains have 3-6” of new snow equal to 0.3-0.5” of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a> (SWE). Moderate southwest wind drifted this snow into small soft slabs that can be triggered by a skier or rider. These slabs are most hazardous if they catch you in higher consequence terrain where they could push you into trees or over cliffs. Today, buried weak layers and new snow make avalanches possible and avalanche danger is <strong>MODERATE</strong>. With more snow later today expect slabs of new and wind-drifted snow to grow, and overall stability to decrease.</p>

<p>In the mountains near Bozeman, Big Sky and Cooke City recent moderate wind formed drifts 1-2’ thick that are possible to trigger (<u><strong><a href="https://www.mtavalanche.com/images/19/skier-triggered-avalanche-sheep-m…;). Blowing and drifting snow or cracking of the snow surface are signs to avoid steep, wind-loaded terrain. With more snow and wind today expect drifts to grow and become more unstable. In the Bridger Range east wind will form drifts in unusual locations.</p>

<p>Avalanches breaking deeper than new and wind-drifted snow are unlikely. On specific slopes there is an isolated chance you could trigger a deep avalanche similar to a couple that were triggered last week near Big Sky (<u><strong><a href="https://www.mtavalanche.com/images/19/snowmobile-triggered-avalanche-fi…;, <u><strong><a href="https://www.mtavalanche.com/images/19/avalanche-crown-butte-2">photo</a…;, <u><a href="https://www.youtube.com/watch?v=_5mSOkh9iqE&amp;list=PLXu5151nmAvRNl9ku…;). The most suspect slopes for this problem are heavily wind loaded, or where the snowpack is relatively shallow (less than 3-5 feet). Today, avalanche danger is <strong>MODERATE</strong> on wind loaded slopes and <strong>LOW</strong> on non-wind loaded slopes. This afternoon danger will increase if snowfall is heavy.</p>

<p>If you get out and have any avalanche or snowpack observations to share, contact us via our <u><a href="https://www.mtavalanche.com/node/add/snow_observation">website</a></u&gt;, email (<u><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></u&gt;), phone (406-587-6984), or Instagram (#gnfacobs).</p>