22-23

<p>Yesterday, the mountains across the advisory area received 2-4” of heavy snow equal to 0.2 to 0.4” of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/null/?cid=nrcseprd1314… water equivalent</span></a> (SWE). Consider three avalanche-related concerns you could encounter in the mountains today.</p>

<ol start="1" type="1">
<li>Most likely are avalanches breaking 3-8” deep in the new and wind-drifted snow. These may be relatively small, but terrain traps like trees, cliffs or rocks amplify the danger.</li>
<li>A recently buried weak layer about a foot deep yielded unstable results in Doug and Alex’s snowpits at Lionhead (<a href="https://www.youtube.com/watch?v=F8HKebJ_OJA&amp;list=PLXu5151nmAvTi1DBS…;, <a href="https://www.mtavalanche.com/node/27323"><strong><span>observation</span…;) and Buck Ridge, where a group of riders reported the same weak layer (<a href="https://www.mtavalanche.com/node/27324"><strong><span>observation</span…;). This thin layer of weak snow creates the potential for avalanches to propagate, or spread, across wider areas. You can help us map the extent of this weak layer by digging a snowpit and <a href="https://www.mtavalanche.com/node/add/snow_obs"><span>letting us know</span></a> if you find a strip of sugary snow 1-3” thick buried a foot deep and telling us if it fails and propagates during your assessment. If you find this instability, avoid avalanche terrain.</li>
<li>Weak layers buried 3-6 feet deep, depending the total depth of the snow, make large avalanches breaking near the ground an unlikely but dangerous possibility (<a href="https://www.youtube.com/watch?v=B1BjS3G4ZWA&amp;list=PLXu5151nmAvTi1DBS… week’s Cooke City video</span></strong></a>). During avalanche mitigation work yesterday, the Big Sky Ski Patrol triggered a small but deep avalanche showing that it remains with us even though the last human-triggered deep slab avalanche in the backcountry was a couple of weeks ago (<a href="https://www.mtavalanche.com/node/27148"><strong><span>details</span></s…;).</li>
</ol>

<p>Doug gave us some good advice that applies to all of these concerns in his <a href="https://www.youtube.com/watch?v=RNMts6wt0JQ"><strong><span>video</span>…; from Hebgen Lake last week, “Dig and test. If there is any doubt, stay out of avalanche terrain.”</p>

<p>Human-triggered avalanches are possible, and the danger is rated MODERATE.</p>

<p><u>Roof Avalanches:</u> Yesterday, many slick roofs in the valley shed the winter’s snow. Avoid playing under sloped metal roofs still holding snow (<a href="https://www.mtavalanche.com/images/22/roof-avalanche-bozeman"><strong><…;).</p>

<p><span>Please share avalanche, snowpack or weather observations via our<a href="https://www.mtavalanche.com/node/add/snow_obs"><span&gt; </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). </span><span><span>&nbsp;</span></span></p>

<p><span>Today, consider three avalanche-related concerns in Island Park. First are slides breaking 3-8” deep in wind-drifted snow. Second is a weak layer buried one foot under the surface that demonstrated instability nearby at Lionhead Ridge (<a href="https://www.youtube.com/watch?v=F8HKebJ_OJA&amp;list=PLXu5151nmAvTi1DBS…;, <a href="https://www.mtavalanche.com/node/27323"><strong><span>observation</span…;). We will assume it exists in the Centennial Mountains until proven otherwise. And third, large avalanches failing on weak layers buried near the ground are an unlikely but dangerous possibility. Dig down, test for instability, and if there is any doubt, stay out of avalanche terrain because dangerous, human-triggered slides are possible.</span></p>

<p><span><span><span><span><span><span>For Christmas we might get the best gift of new snow in the mountains, but it arrives with dangerous avalanche conditions on wind-loaded slopes. Strong westerly wind overnight has drifted recent snow into thick slabs that can break under the weight of a person and avalanche on steep slopes. Today’s new snow will also be drifted by moderate winds and continue to add weight and depth to wind slabs. Additionally, weak layers buried 1-4 feet deep may cause avalanches to break deeper and wider, potentially very large. Avoid slopes where the wind has recently deposited thick drifts of snow. If you travel across non-wind loaded steep slopes, first dig to look for and assess stability of buried weak layers.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Doug and I rode at Lionhead Ridge yesterday, and we found a weak layer of sugary facets buried 1 foot deep which produced unstable results in our snowpack test (</span></span></span></span></span></span><a href="https://www.youtube.com/watch?v=F8HKebJ_OJA&amp;list=PLXu5151nmAvTi1DBS…;, </span></span></span></span></span></span><a href="https://www.mtavalanche.com/node/27323"><span><span><span><strong><span…;). Riders at Buck Ridge near Big Sky found a similar layer around 1 foot down, as well as a crust below yesterday’s 3-4” of new snow (</span></span></span></span></span></span><a href="https://www.mtavalanche.com/node/27324"><span><span><span><strong><span…;).</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Weak layers near the bottom of the snowpack also remain a concern, especially where the snowpack is relatively shallower, around 3 feet deep, like in the Bridger Range, Big Sky, Hyalite, Bacon Rind and Hebgen (</span></span></span></span></span></span><a href="https://www.youtube.com/watch?v=RNMts6wt0JQ"><span><span><span><strong>… Lake video</span></span></u></span></strong></span></span></span></a><span><span><span><span><span><span>). We have not seen recent avalanches breaking on these weak layers, but the potentially major consequences require conservative decision making. Do not forget about these deeper weak layers, especially as today’s fresh drifts and new snow continue to add weight. </span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Today the avalanche danger is CONSIDERABLE on wind-loaded slopes and MODERATE on all other slopes.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><u><span><span>Roof Avalanches:</span></span></u></span></span></span></span></span><span><span><span><span><span><span> With above freezing temperatures and rain possible at lower elevations, be cautious of roof avalanches for the next few days. The last two months of snow and cold temperatures have allowed thick slabs to build which may slide off of metal roofs. Look up to see if this hazard exists above where children play or where you park, walk or stand around buildings.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Please share avalanche, snowpack or weather observations via our</span></span></span></span></span></span><a href="https://www.mtavalanche.com/node/add/snow_obs"><span><span><span><span>…; </span></span></span></span></span></span><span><span><span><strong><span><u><span><span>website</span></span></u></span></strong></span></span></span></a><span><span><span><span><span><span>, email (</span></span></span></span></span></span><span><span><span><strong><span><span>mtavalanche@gmail.com</span></span></strong></span></span></span><span><span><span><span><span><span>), phone (406-587-6984), or Instagram (#gnfacobs). </span></span></span></span></span></span></p>