The avalanche danger in the west central Montana backcountry is moderate this morning, rising to considerable this afternoon. Mild overnight temperatures and above average daytime temperatures will allow rapid warming of the snowpack today. Large wet slab avalanches are possible.
Good Morning. This is Jeff Carty with the West Central Montana Avalanche Center advisory on Thursday, March 18, 2021. This advisory is sponsored by the Montana Backcountry Alliance. This advisory does not apply to operating ski areas and expires at midnight tonight. The USDA Forest Service is solely responsible for its content.
Mountain temperatures peaked at 45º at Stuart Peak, and 55º at Lolo pass yesterday. Last night they dipped to 30-33º, and are expected to rise to above 50º today. Winds will be southerly and moderate.
There hasn’t been much change in conditions in the last 48hrs. Any aspect that catches sun has been heavily affected. As a result, loose wet avalanches have been abundant. A few more wet slab avalanches have been spotted and dead north aspects above 6500’ remained mostly dry yesterday. Today’s high temperatures coupled with a very weak freeze overnight increase the likelihood of wet slab avalanches. Most aspects and elevations will be affected by today’s warmth.
The most abundant hazard right now is wet loose avalanches during the afternoon. These can be seen on all aspects but north and will continue with daytime warming and sun, peaking in late afternoon. Last night’s freeze was very weak with temperatures reaching or dipping just below freezing, most surfaces above 6000′ should have refrozen, it may be just the surface. Check for wet snow below the surface, this indicates that the snowpack is not refreezing overnight. When this happens, bonds in the snowpack have broken down and liquid water can lubricate weak layers leading to wet slab avalanches. If you can push your pole down through a crust and into moist snow, or you are sinking in past mid boot, it’s time to move to head home or to shadier aspects if there is a safe route to and from them. Avalanche crowns spotted in the Swans yesterday indicate that wet slabs are possible, one of these was 500 yards wide. Wet loose avalanches had begun by 10am in the central Bitterroot indicating a weakly frozen snowpack. Warmer temperatures today will lead to more unstable conditions. Avoid traveling on or under avalanche slopes in these conditions. Lower elevations where temperatures remained elevated overnight are at higher risk. The chance of wet slab avalanches will be most likely with strong sun warming later the day. However, due to the unpredictability of wet slabs, avoid all avalanche slopes all day today.
The past week’s warmth has helped bond persistent weak layers. Yesterday was the first day we’ve had without propagation in the areas we dug. In isolated areas, persistent weak layers may still linger on north aspects. Shallow spots at ridgetop, around rocks, or where wind-scoured are most suspect. If in doubt avoid convexities and slopes 35º or greater.
Cornices and glide cracks will be with us for the rest of the season. These are two unpredictable avalanche problems. Cornices were sagging yesterday. With continued warmth and sun, the likelihood of cornice fall increases. Give them a wide berth and stay out from underneath. Large cornice fall could trigger lingering persistent slabs or wet slabs. Glide cracks can slowly open as the snowpack creeps downhill, accelerating as with warming. They can also fail unexpectedly, creating full-depth avalanches. Stay off slopes where they are present.
Bottom Line
Wet loose avalanches are likely with sun on south and west aspects and large wet slab avalanches are increasing in likelihood. Avoid traveling on or under avalanche slopes today. Persistent slab may be present on isolated north aspects. Avoid traveling under large cornices and give them a wide berth on ridges. If roller balls show up, move to shadier aspects or head home. Avoid glide cracks.
Carry a beacon, shovel, and probe. Reassess conditions throughout the day and stay alert for signs of instability. Dig pits. Look for red flags.
Upcoming Education Events
Please visit our education page for an up-to-date list of regional educational events and course offerings. Below are a few select events and opportunities to check out.
- March 20th | Avalanche Rescue Course | Get more details and register HERE
Public Observations
Thank you to everyone who has taken the time to send in a public observation. Please keep sharing what you find and see while out in the backcountry. You can now text us your observations to (406) 219-5566 when you don’t have time to fill out the observations page.
Ski and ride safe.
