20-21

<p>It is snowing in the southern mountains. Already ten inches has fallen at the <a href="https://www.mtavalanche.com/weather/stations/carrot-basin"><u>Carrot Basin SNOTEL</u></a> in Taylor Fork measuring 5% density cold-smoke powder (.5” of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent, SWE</u></a>). Outside West Yellowstone there is 8” of snow (.4” SWE), but that will deepen today and tonight. Wind will blow today’s new snow into drifts.&nbsp;</p>

<p>The avalanche danger is rising. The snowpack is weak and the weight of new snow and windblown snow will make slopes unstable. A thick, widespread layer of facets at the ground will become easier to trigger. Another weak layer of feathery surface hoar 1-2 feet deep is also unstable, but it’s distribution is spotty (<a href="https://www.youtube.com/watch?v=Sw_8eiygoSc"><strong><u>Bacon Rind video</u></strong></a><strong>, </strong><a href="https://www.youtube.com/watch?v=hkLoYCcemg0"><strong><u>Taylor Fork video</u></strong></a>). Today’s snowpack analysis is simple: it is snowing, the instability is rising and it’s likely you’ll trigger an avalanche. Visibility will be poor and with a rising danger it is best to avoid steep slopes and flatter areas under steep slopes, called <a href="https://avalanche.org/avalanche-encyclopedia/#runout-zone"><u>runout zones</u></a>. Today the avalanche danger is rated CONSIDERABLE on all slopes.&nbsp;</p>

<p>The mountains from Big Sky to Hyalite to the Bridger Range have only gotten a trace of new snow. The avalanche potential in these mountains involve two weak layers: a widespread layer of sugary snow at the ground (<a href="https://www.mtavalanche.com/images/21/frazier-basin-avalanches"><strong… of slides in Frazier Basin</u></strong></a>) and a layer of thin, feathery surface hoar found in isolated areas. Cracks or “whumpfs” of the snowpack are signs that a rider or skier could trigger a slide. On our field day to Buck Ridge a week ago (<a href="https://youtu.be/fUK0nUTBSvU"><strong><u>video</u></strong></a&gt;) we had the snow break in our tests and we saw many slides that were triggered. Not much has changed since. Triggering avalanches is becoming more difficult, but still possible, and the danger is rated MODERATE on all slopes.</p>

<p>The mountains around Cooke City got a couple inches of snow with south wind gusting to 40 mph. Skiers noted cracking of wind blown snow near Republic Pass yesterday, a sign of instability. Many slopes in the area are stable, but a few are problematic. I investigated three avalanches on Monday that were triggered by snowmobilers over the weekend (<a href="https://www.youtube.com/watch?v=Ui1wdUHE56E&amp;list=PLXu5151nmAvQDzKmH…;, <a href="https://mtavalanche.com/images/21/snomobiler-triggered-slide-near-huckl… photo</u></strong></a>, <a href="https://mtavalanche.com/images/21/2-avalanches-sheep-mtn"><strong><u>Sh… Mountain photo</u></strong></a>). These broke on a weak layer buried 12-18” deep (<a href="https://www.mtavalanche.com/images/21/surfae-hoar-crystalcloseup"><stro… hoar</u></strong></a> or small-grained facets). Skiers got a spiderweb of cracks on this same layer on Monday (<a href="https://mtavalanche.com/images/21/shooting-cracks-meridian-peak"><stron…;). Furthermore, a thin, weak layer of sugary facets buried 3 feet deep is still worth remembering. Cracking, collapsing or recent avalanche activity are signs to stay out of avalanche terrain no matter the weak layer. For today the avalanche danger is rated MODERATE on all slopes.</p>

<p>The mountain snowpack from Lionhead through the Bridger Range is shallow with two distinct weak layers. The first is a thick layer of weak, sugary snow near the ground. This layer is nearly universal and has caused many avalanches this season, most recently a snowmobile-triggered avalanche in the Taylor Fork (<a href="https://mtavalanche.com/avalanche-activity"><strong>2020-21 avalanche activity list</strong></a><strong>, </strong><a href="https://mtavalanche.com/node/23920"><strong>Taylor Fork details</strong></a>). It will take weeks or months to fully trust this layer of depth hoar, but it is adjusting to the <em>current </em>load and avalanches are less likely. The second is a thin layer of feathery surface hoar buried 6-18" deep. This layer is not everywhere and we have seen variations in its reactivity. During recent field days, Alex and I found it very reactive at Bacon Rind and in some areas of the Taylor Fork (<a href="https://www.youtube.com/watch?v=Sw_8eiygoSc"><strong>Bacon Rind video</strong></a><strong>, </strong><a href="https://www.youtube.com/watch?v=hkLoYCcemg0"><strong>Taylor Fork video</strong></a>). We will watch this layer to see if it begins causing more problems in the future.</p>

<p>Stability in these ranges is currently similar, but today's snowfall will heavily favor the mountains around West Yellowstone. Stability will decrease as the snow adds up. Watch for new avalanche activity and shooting cracks and listen for "whumphing" as clear signs to retreat from steep slopes. Get your shovel out and perform an extended column test if you don't observe these obvious signs of instability.</p>

<p>Today, travel in and around steep terrain requires a conservative mindset, careful route-finding and thorough snowpack assessment. Human-trigger avalanches are possible and the danger is rated MODERATE.</p>

<p>The challenge in Cooke City is that many slopes are stable while some have one or two buried weak layers. The first is a recently problematic layer of surface hoar buried 12-18" deep and the second is a more dormant layer of sugary facets buried 2-3' deep. Last weekend, snowmobilers triggered three avalanches near Sheep Mountain and skiers saw shooting cracks and hear loud whumphs. Learn more by watching Doug's new <a href="https://www.youtube.com/watch?v=Ui1wdUHE56E&amp;list=PLXu5151nmAvQDzKmH…; and looking at the photos from Cooke City (<a href="https://mtavalanche.com/images/21/snomobiler-triggered-slide-near-huckl… photo</strong></a>, <a href="https://mtavalanche.com/images/21/cornice-triggered-avalanche-miller-ri… Ridge photo</strong></a>, <a href="https://mtavalanche.com/images/21/2-avalanches-sheep-mtn"><strong>Sheep Mountain photo</strong></a>, <strong><a href="https://mtavalanche.com/images/21/shooting-cracks-meridian-peak">shooti… cracks</a> </strong><strong><a href="https://mtavalanche.com/images/21/shooting-cracks-meridian-peak">photo<…;). Both weak layers are adjusting to their current load and, while avalanches remain possible, they are becoming less likely.</p>

<p>Variability between slopes requires riders and skiers to be attentive to changes in conditions, to plan routes carefully and to regularly search for signs of instability as a reason to back out of steep terrain. The danger is rated MODERATE.</p>

<p>If you get out, please send us your observations no matter how brief. You can submit them via our <a href="https://www.mtavalanche.com/node/add/snow_observation"><strong>website<…;, email (<a href="mailto:mtavalanche@gmail.com"><strong>mtavalanche@gmail.com</strong></a…;), phone (406-587-6984), or Instagram (#gnfacobs).</p>