Good morning, this is Dave Zinn with a spring weather and snowpack update on Monday, April 28th. Daily avalanche forecasts have ended for the season. We will issue conditions updates on Monday and Friday mornings through April. This information does not apply to operating ski areas.
This is a volatile week in the battle between winter and summer. With temperatures in the upper 20s to low 30s F, a wet spring snowstorm is impacting the area to start off the work week, with the Bridger Range receiving the most significant precipitation. As of Monday, at 6 AM, the Bridger Range has received a whopping 1.3” of snow water equivalent (SWE) in the last 12 hours, with a more nominal 7” of snow at the Alpine Weather Station. The Red Chair Cam at Bridger Bowl shows 14” of new snow. The mountains near Cooke City, Big Sky, including the Taylor Fork, and south of Bozeman have received 0.5” of water with 0-5” of snow (meaning it rained in some areas). Light rain fell in West Yellowstone and Island Park.
The first storm of the week winds down Monday evening after an additional 1-4” of snow falls during the day. Tuesday will be warm and dry, but rain and snow return to the area Tuesday night through Wednesday.
The tail end of the week will be lovely spring weather with mountain temperatures pushing into the 50s and low 60s F.
All Regions
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).
Here’s what we know:
- The snowpack was mostly stable before Sunday/ Monday’s storm.
- The more new 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 storm slab and wind slab avalanches. Deeper depths of new snow generally equate to more significant avalanche danger.
- Rain on snow has a destabilizing effect as the bonds that hold the snowpack together break apart.
- Expect wet loose avalanches as the new snow warms. These may run long distances on underlying crusts.
What to do:
- Evaluate new snow depths and expect these to change with elevation.
- 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.
- 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.
Moving forward through the week, Tuesday's temperatures will climb well above freezing. Wet loose avalanches 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.
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.
We have had a long and wonderful season. So far, 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.
See our website for more general spring snowpack and travel advice.
If you encounter an error with our observation submission platform, please email your observations to mtavalanche@gmail.com
Give Big Gallatin Valley
Give Big Gallatin Valley is this week! (May 1-2, 6pm-6pm)
Please support the excellent non-profits of Gallatin County, including the Friends of the Avalanche Center. One Valley Community Foundation’s Give Big Gallatin Valley is a 24-hour celebration of giving to connect generous community members with causes they care about most in order to raise important funding for nonprofits serving Gallatin Valley.
Use this link to donate to The Friends of GNFAC during the Give Big event
Learn more about Give Big Gallatin Valley and help spread the word
Thank you to the Bozeman Ice Festival for matching $2000 in donations this year!
Your support goes toward free and low-cost avalanche education, beacon parks, weather stations, and avalanche center operations. This season, the education program reached over 3,600 students, including school-age youth, search and rescue groups, and motorized and human-powered users. 5,500+ people read our daily avalanche forecasts and more followed along on social media. We appreciate your help!
Sincerely, The Board of Directors, Friends of the Gallatin National Forest Avalanche Center, and GNFAC staff
Announcements
The Hyalite Canyon Road is closed to ALL MOTORIZED VEHICLES until May 16. This is a regular annual road closure to reduce road damage during the spring thaw. Bicycle and foot traffic are allowed. Contact the Bozeman FS Ranger District for more info.
Bridger Bowl Ski Area is closed for the season, and uphill travel is allowed. Please give employees and machinery plenty of room as they work to clean up from the season. Backcountry conditions exist. There are no ski patrol services, rescue or avalanche mitigation. Call 911 for any incidents requiring outside assistance.
Thank you for another successful season. Our success is directly related to support from our community and the Forest Service. Thanks to the readers of the forecast, everyone that sent in observations, took an avalanche class, or donated money, time or gear. We will issue conditions updates on Mondays and Fridays through April.
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!
Mark, Alex, Ian and Dave
For more spring travel advice see this article from our GNFAC forecaster blog.