24-25

<p>The weather and spring storms are the primary drivers of this week’s avalanche danger. To start off the week, there are significant differences in the weather throughout the forecast area, with significant snowfall in some areas (Bridger Range), moderate accumulation in others, (south of Bozeman, Big Sky, and Cooke City) and a few ranges receiving more rain than snow (Lionhead and Centennial Ranges).</p>

<p><strong>Here’s what we know:</strong></p>

<ul>
<li>The snowpack was mostly stable before Sunday/ Monday’s storm.</li>
<li>The more <em>new </em>snow there is, the more likely you are to encounter avalanches and the larger their potential size. Slides will fail within the new and wind-drifted snow as <strong>storm slab </strong>and<strong> wind slab avalanches. </strong>Deeper depths of new snow generally equate to more significant avalanche danger.</li>
<li>Rain on snow has a destabilizing effect as the bonds that hold the snowpack together break apart.</li>
<li>Expect <strong>wet loose avalanches</strong> as the new snow warms. These may run long distances on underlying crusts.</li>
</ul>

<p><strong>What to do:</strong></p>

<ul>
<li>Evaluate new snow depths and expect these to change with elevation.</li>
<li>If your travel plans take you to slopes steeper than 30 degrees, assess how the new and wind-drifted snow is bonding to the old snow surface by watching for clear signs of instability, such as avalanches and shooting cracks. A quick, extended column test 1-2 feet deep can provide additional information.</li>
<li>If rain fell on the snowpack and as temperatures warm, breaking through an unsupportable and slushy snowpack, seeing pinwheels of wet snow and small wet slides are indicators of wet snow instability.</li>
</ul>

<p><strong>Moving forward through the week, </strong>Tuesday's temperatures will climb well above freezing. <strong>Wet loose avalanches </strong>will run long distances on firm crusts underlying the new snow. These slides will be most concerning in areas where the most snow fell. A shift to cooler, north-facing aspects may be enough to ward off the wet snow danger.</p>

<p>Depending on midweek snowfall amounts, the avalanche danger may increase again Tuesday night into Wednesday before settling into the spring rhythm of stable conditions in the morning while snow surfaces are cold and frozen, and less stable by late morning as temperatures climb into the 50 and 60 degree F range under sunny skies toward the end of the week.</p>

<p>We have had a long and wonderful season. <em>So far</em>, there have been no avalanche fatalities within our forecasting area. Keep up the good work, and don’t let your guard down. We want to keep it that way. Assess how the new snow is bonding, avoid slopes with active wind-loading, and evaluate the snowpack for changing conditions as temperatures warm and cool again.</p>

<p><span>See our website for more general spring snowpack and travel advice. </span></p>

<p><span><span><span><span><span><span>Avalanche hazards today, and over the next couple days, will generally be small and isolated.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>If you get up onto steep, shady slopes that still harbor dry snow, watch out for triggering small </span></span></span></span></span></span><span><span><span><strong><span><span>Wind Slab avalanches. </span></span></strong></span></span></span><span><span><span><span><span><span>Without much recent snowfall, any wind drifts will likely be less than a foot deep and most of them will be well bonded at this point. You’ll want to look out for drifts that aren’t well bonded - cracks shooting out from your skis or sled are the bullseye data to watch for. If snowfall kicks in earlier than expected on Sunday, fresh wind slabs may become a more substantial issue later in the day.&nbsp;</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>When the sun is out and not obscured by clouds (mostly today, but also maybe in places on Saturday),</span></span></span></span></span></span><span><span><span><strong><span><span> Wet Loose avalanches</span></span></strong></span></span></span><span><span><span><span><span><span> will be a concern. In most places the snow surface has been worked over by multiple days of warm temperatures and won’t readily destabilize. Mostly watch for small slides in the softer, newer snow. A sticky, wet snow surface and roller balls running down from your tracks indicate that the conditions for these slides are developing.&nbsp;</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>While both wet and dry slides will be generally small, remember that any type of avalanche can be hazardous in terrain where they could push you into rocks, trees, over a cliff, or carry you a long way down a steep slope. Before riding steep slopes, assess the terrain for consequences of being knocked over by a small slide.&nbsp;</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Daily forecasts are done for the season, but avalanches will continue. Remain diligent with your snowpack and terrain assessments, and be ready to adapt your plans to changing conditions. See our website for more general spring snowpack and travel advice.</span></span></span></span></span></span></p>

<h3><span><span><span><strong><span><span>GENERAL SPRING SNOWPACK AND TRAVEL ADVICE</span></span></strong></span></span></span></h3>

<p><span><span><span><span><span><span>Spring weather can be highly variable and create a mix of avalanche problems. Snow conditions and stability can change drastically from day to day or hour to hour. Anticipate rapid change and plan accordingly. Abundant snowfall over the winter with more spring snow to come makes avalanches possible into summer.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>NEW SNOW AND WIND LOADED SLOPES</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Spring storms are notorious for depositing heavy amounts of snow in the mountains. Even with a deep and generally stable snowpack throughout the advisory area, heavy and rapid loads of new snow will decrease stability. The main problems to look out for are avalanches breaking within the new snow, wind slabs, and loose snow avalanches. The likelihood of triggering an avalanche spikes during and immediately after snowstorms. New snow instabilities tend to stabilize quickly, but it’s a good idea to give fresh snow a day to adjust before hitting big terrain. New snow instabilities can be challenging to assess, and spring storms bond to old snow differently across aspects and elevations. Conservative terrain selection is essential during and immediately following storms. Avoid wind-loaded slopes and slopes steeper than 35 degrees for 24-48 hours after new snow and wind.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>New snow can quickly change from dry to wet on a spring day, and stability can decrease rapidly with above freezing temperatures or brief sunshine. New snow may bond well early in the morning and then easily slide later. Wet loose slides are likely during the first above freezing temperatures or sunshine immediately after a storm. Anticipate changes in snow stability as you change aspect or elevation and over the course of the day. An early start is always an advantage. Be ready to change plans or move to safer terrain at the first signs of decreasing stability.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>WET SNOW AVALANCHES</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Spring and wet snow avalanches go hand-in-hand. Above freezing temperatures, rain, and/or intense sunshine cause the snow to become wet and weak and make wet avalanches easy to trigger or release naturally. Conditions tend to become most unstable when temperatures stay above freezing for multiple days and nights in a row. Avoid steep terrain, and be aware of the potential for natural wet avalanches in steep terrain above you, if you see:</span></span></span></span></span></span></p>

<ul>
<li><span><span><span><span><span><span>Heavy rain,</span></span></span></span></span></span></li>
<li><span><span><span><span><span><span>Above freezing temperatures for more than 24 hours,</span></span></span></span></span></span></li>
<li><span><span><span><span><span><span>Natural wet avalanches,</span></span></span></span></span></span></li>
<li><span><span><span><span><span><span>Rollerballs or pinwheels indicating a moist or wet snow surface,</span></span></span></span></span></span></li>
<li><span><span><span><span><span><span>Or if you sink to your boot top in wet snow.</span></span></span></span></span></span></li>
</ul>

<p><span><span><span><span><span><span>In general, if the snow surface freezes solid overnight, the snowpack will be stable in the morning and stability will decrease through the day as snow warms up. The snow surface hardness, rate of warming, duration of sunshine, aspect and elevation determine how fast stability will decrease through the day. Be aware that sunny aspects may have a wet snow avalanche danger while shadier slopes still have a dry snow avalanche danger. Getting off of steep slopes should be considered when, or before, the above signs of instability are present. Wet snow avalanches, whether loose snow or slabs, can be powerful, destructive and very dangerous. Conservative terrain choices, starting early in the day, and careful observations can keep you safe. See Alex’s recent video, and this article for more spring travel advice.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>CORNICES</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Cornices along ridgelines are massive and can break under the weight of a person (photo). Prolonged above freezing temperatures and rain make them weaker and possible to break naturally. They can break off suddenly and farther back than one might expect. Cornice falls can also entrain large amounts of loose snow or trigger slab avalanches. Stay far back from the edge of ridgelines and minimize exposure to slopes directly below cornices. Regardless of whether a cornice triggers a slide or not, a falling cornice is dangerous to anyone in its path.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>DISCLAIMER</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>It does not matter if new snow falls or not, avalanches will continue to occur until the existing snowpack is mostly gone. Always assess the slope you plan to ride with diligence and safety in mind. Do not let your guard down. Travel with a partner, carry rescue gear and only expose one person at a time in avalanche terrain.</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Have a safe and enjoyable spring and summer!</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>Mark, Alex, Ian and Dave</span></span></span></span></span></span></p>

<p><span><span><span><span><span><span>For more spring travel advice see this&nbsp;</span></span></span></span></span></span><a href="https://www.mtavalanche.com/blog/transitions-spring-snow-avalanche-prob… our GNFAC forecaster blog</span></span></span></span></span></span></p>