Good Morning. This is Ian Hoyer with a spring weather and snowpack update on Friday, April 16th. The Gallatin National Forest Avalanche Center has stopped issuing daily avalanche forecasts for the season. We will issue weather and snowpack updates on Monday and Friday mornings through April.
A trace to a few inches of new snow fell in the mountains this week. Skies have been generally cloudy, high temperatures have remained below freezing and winds have been moderate and easterly. Skies will clear today after a few lingering snow showers this morning. Saturday will be sunny with clouds increasing again on Sunday. High temperatures will be slightly above freezing. Winds will shift to the west on Saturday and remain moderate. There is a good chance for snow Sunday night into Monday. Precipitation may start as rain at lower elevations on Sunday afternoon.
All Regions
This weekend we’ll see pretty much the whole range of spring weather. Sun, clouds, shifting winds, temperatures above and below freezing, maybe rain, and then snow to cap it all off. While conditions are generally stable and avalanches are unlikely in the lower snowpack, this variable weather means that there are a number of different, relatively minor, hazards to watch out for.
Unusual easterly winds have drifted the little bit of recent snow into cohesive slabs on slopes that don’t normally have wind drifts. Skiers in the northern Bridger Range yesterday found some small wind drifts that cracked along their skintrack (photo). Assess these drifts before committing to steep slopes.
With temperatures rising above freezing and sunny skies, expect small, loose wet avalanches. These slides will only involve the new snow or the top couple inches of the snowpack as crusts break down. The major hazard posed by these slides is getting knocked off your feet and into rocks or off a cliff. Watch for the snow surface getting wet and rollerballs as warning signs that conditions for loose wet avalanches are developing.
Right now, precipitation is forecasted to begin Sunday evening. If snow starts a bit earlier than expected, watch out for small avalanches in fresh wind drifts on Sunday afternoon. If it rains more than a drizzle, avoid steep slopes as the snowpack may rapidly destabilize.
Continue to follow the same safe travel protocols as in the winter: ski or ride with a partner; always carry a beacon, shovel, and probe; and go one at a time on steep slopes.
We will issue spring snowpack and weather updates each Monday and Friday through April, or as needed, and we will share relevant avalanche and snowpack information on our website and social media. If you get out, please send us your observations no matter how brief. You can submit them via our website, email (mtavalanche@gmail.com), phone (406-587-6984), or Instagram (#gnfacobs).
Announcements, Avalanche Education and Events
Bridger Bowl is closed, and backcountry conditions exist (video). There is no avalanche mitigation or ski patrol rescue. Please stay clear of work areas, snowmobiles, chair lifts and other equipment.
See our education calendar for an up-to-date list of all local classes.
GENERAL SPRING SNOWPACK AND TRAVEL ADVICE
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.
NEW SNOW AND WIND LOADED SLOPES
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.
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.
WET SNOW AVALANCHES
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:
- Heavy rain,
- Above freezing temperatures for more than 24 hours,
- Natural wet avalanches,
- Rollerballs or pinwheels indicating a moist or wet snow surface,
- Or if you sink to your boot top in wet snow.
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.
CORNICES
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.
DISCLAIMER
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.
Have a safe and enjoyable spring and summer!
Doug, Alex, Ian and Dave
Snow will be around the mountains for a month or two and avalanches will continue. When warm temperatures return, wet snow hazards will increase. If there are big spring snowstorms, dangerous conditions may develop for new snow avalanches and wind slabs. With either extended warming or a big storm, there is still a lingering possibility of deep avalanches breaking on weak, sugary snow near the ground. See this article for some general spring travel advice.