18-19

<p>The Lionhead area outside West Yellowstone received 6” of snow (.4” of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a>) with strong westerly winds. The snow fell onto a weak snowpack. Yesterday my partner and I rode and skied into this area and found a 2-2.5’ deep snowpack with two distinct weak layers breaking in our tests (<strong><a href="https://www.youtube.com/watch?v=rQ7fzq4X1-4&amp;index=2&amp;t=0s&amp;li…;). The most worrisome consists of loose, sugary facets at the bottom half of the snowpack. The second is a layer of small facets a few inches under the surface. Either of these are unstable enough to avalanche, especially on slopes with a wind-load. Triggering an avalanche is likely today. Collapsing and shooting cracks are warning signs to stick to low-angled terrain. Given the new snow and strong wind the avalanche danger is rated CONSIDERABLE on all slopes.</p>

<p>Snowfall totals range from 2” inches at Brackett Creek in the northern Bridger mountains (a trace at Bridger Bowl), to 3” in Hyalite and 5-6” around Big Sky and Cooke City. The <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a> is .3-.6”, enough weight to tip wind-loaded slopes toward instability. &nbsp;Last night the wind started to blow strong from the west to northwest and today there will be many thick and dense wind slabs ready to avalanche.</p>

<p>In the days leading up to this storm avalanches were being triggered on wind-loaded slopes throughout our forecast area. The two most recent and largest was a skier triggered slide on Bridger Peak on Monday (<strong><a href="https://www.mtavalanche.com/images/18/bridger-peak-avalanche-1-17-dec">…;, <strong><a href="https://www.youtube.com/watch?v=QgnQq4PQMi8&amp;list=PLXu5151nmAvRNl9ku…;, <strong><a href="https://www.mtavalanche.com/node/19330">details</a></strong&gt;) and a snowmobiler triggered slide on Buck Ridge on Sunday (<strong><a href="https://www.mtavalanche.com/images/18/snomo-triggered-avalanche-2nd-yel…;, <strong><a href="https://www.youtube.com/watch?v=mJRtrBZkgtY&amp;list=PLXu5151nmAvRNl9ku…;, <strong><a href="https://www.mtavalanche.com/node/19310">details</a></strong&gt;). Others include natural slides outside Cooke City (<strong><a href="https://www.mtavalanche.com/images/18/natural-avalanches-town-hill-near…;) and in the southern Madison Range (<strong><a href="https://www.mtavalanche.com/images/18/ernest-miller-avalanche-2">photo<…;). These avalanches were wind-loaded and broke on a weak layer of facets that formed during the cold, clear weather earlier in the month. <em>With today’s new snow and strong wind, this layer will be under strain and triggering avalanches is likely. Avoid wind-loaded slopes.</em> Today, the avalanche danger is rated CONSIDERABLE on all wind-loaded slopes and MODERATE on all others.</p>

<p>Take a look at the <u><a href="https://www.mtavalanche.com/weather/wx-avalanche-log">Weather and Avalanche Log</a></u> to get a handle on the recent activity, and also check out our <strong><a href="https://www.youtube.com/playlist?list=PLXu5151nmAvRNl9kuY3DxfWN1qiU7IeX…; to better understand the snowpack.</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>Weak and shallow snow define the Lionhead area. The snowpack is 2’ deep and the bottom foot consists of weak, sugary grains that will quickly become unstable from the weight of new snow. Eric shows us these facets draining out of the pit wall in last week’s <strong><a href="https://www.youtube.com/watch?v=kqiXL3X2coU&amp;t=0s&amp;index=8&amp;li…; </strong>Wind-loaded slopes are the most susceptible to being triggered, but if the storm lays down a heavy blanket of snow all slopes will be suspect. Snowfall and wind are on the menu today, and the avalanche danger will rise. This morning it is rated MODERATE, but could increase to CONSIDERABLE by day’s end.</p>

<p>Avalanches are still occurring. Yesterday a skier triggered a large slide on a wind-loaded slope on a NE aspect of Bridger Peak. The slide broke up to a foot deep and ran 100’ wide and 2,000’ vertical. It was triggered with a ski cut that broke on a weak layer of facets that formed during the cold, clear weather 12 days ago. No one was caught and the skier shared <u><strong><a href="https://www.mtavalanche.com/photos">pictures</a></strong></u&gt;, a <u><strong><a href="https://www.youtube.com/watch?v=QgnQq4PQMi8&amp;list=PLXu5151nmAvRNl9ku…;, and <u><strong><a href="https://www.mtavalanche.com/node/19330">details</a></strong></u&gt; of the avalanche. This is the same layer that collapsed with a loud “whumph” on Eric and Alex on the west side of the range on Saturday (<u><strong><a href="https://www.youtube.com/watch?v=QsSjIbhzEV4&amp;list=PLXu5151nmAvRNl9ku…;, <u><strong><a href="https://www.mtavalanche.com/images/18/collapsed-and-cracked-drift-bridg…;, <u><strong><a href="https://www.mtavalanche.com/node/19301">details</a></strong></u><u&gt;).</u></p>

<p>A weak layer of faceted snow is buried 6-18” under the surface in all our mountain ranges, but not necessarily on every slope. Avalanches are occurring on this layer which is outlined in our <u><a href="https://www.mtavalanche.com/weather/wx-avalanche-log">Weather and Avalanche Log</a></u>. Since December 10^th there have been 16 avalanches reported, all involving wind-drifted snow, and 8 of these were human triggered. Some were small slides and a couple were large, including a snowmobiler triggered slide on Buck Ridge that Alex and I investigated Sunday (<u><strong><a href="https://www.youtube.com/watch?v=mJRtrBZkgtY&amp;list=PLXu5151nmAvRNl9ku…;, <u><strong><a href="https://www.mtavalanche.com/images/18/instability-buck-ridge">photo</a>…;).</p>

<p>Outside Cooke City a few avalanches were reported since Saturday: two wind slabs naturally released (<u><strong><a href="https://www.mtavalanche.com/images/18/wind-slab-ynp-near-cooke-city">ph…;, <u><a href="https://www.mtavalanche.com/images/18/natural-avalanches-town-hill-near…;), and a snowbike triggered a small wind pillow next to a road (<u><strong><a href="https://www.mtavalanche.com/images/18/cooke-city-wind-slab-15-dec">phot…;). Guides are reporting the snowpack is getting weaker and an overall concern that widespread avalanches are on the horizon once the firehose of snow gets turned on.</p>

<p>Today, avalanches are possible on wind loaded slopes which have a <strong>MODERATE</strong> avalanche danger. On non-wind loaded slopes avalanche danger is <strong>LOW</strong>. Wind-loaded slopes should be assessed or avoided.</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>