Jan 21, 2021 @ 6:09 am

The avalanche danger is MODERATE in the west central Montana backcountry today. Human triggered avalanches are possible today.

Good Morning. This is Travis Craft with the West Central Montana Avalanche Center advisory on Tuesday, January 21st, 2021. This advisory is sponsored by LB Snow. This advisory does not apply to operating ski areas and expires at midnight tonight. The USDA Forest Service is solely responsible for its content.

Weather and Snowpack

Mountain temperatures range from 18 degrees to 25 degrees F this morning. Winds are 4 mph gusting to 8 mph out of the West. The advisory area received no new snow overnight.

Persistent weak layers in the snowpack and wind drifted snow are the main avalanche problems. The likelihood of triggering an avalanche is decreasing slowly. It is still possible to trigger a wind slab on steeper slopes greater than 35 degrees at higher elevations near ridges. Cornices are getting large and getting weaker. The persistent weak layers are less reactive in pit tests. We could get facets below the rain crust from 01/13/2021 to propagate yesterday in the Lolo Pass area. In drainages in the central Bititterroot and Sweeny, this layer did not propagate. The buried surface hoar layers deeper in the snowpack (2 feet) are becoming less and less likely triggered. Overall we are gaining strength in the snowpack.

The shallowest snowpack in the forecast area is the Rattlesnake. There are still many shallow spots on slopes where triggering these weak layers is possible. The places to avoid on slopes are near cliff bands, rocks, and roll overs. These areas will have shallower snowpacks.

Terrain choice is the only option to reduce our exposure to avalanches. ATES is a way to describe terrain using the definitions simple, challenging, and complex. Here is a link to help understand the terminology and ranking. Looking at today’s avalanche problems I would choose simple terrain. Then after evaluating the snowpack with multiple pit tests and avoiding steep wind-loaded start zones, slowly ease into challenging terrain. This builds in safety margins.

Bottom Line

Today, choose sheltered lower angle slopes. Avoid slopes with weak sugary snow. Do multiple pits investigating the stripes in your pit wall. Avoid cornices and wind loaded terrain. Choose simple terrain that does not expose you to terrain traps. Avoid likely trigger points on slopes.

Travel one at a time in avalanche terrain, carry a beacon, shovel, and probe. Remember to reassess conditions throughout the day and stay alert for signs of instability. Dig a pit. 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.

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. This online forum is a great resource to glean information about current conditions.

Remember, you can submit your observations through the observation page anonymously. When submitted anonymously, the forecasters review the observation and utilize it when generating the forecast. The information does not appear on the public observation page.

Ski and ride safe.


Problem 1 - Wind Drifted Snow

  • TYPE


    Wind Slabs

    Release of a cohesive layer of snow (a slab) formed by the wind.  Wind typically erodes snow from the upwind sides of terrain features and deposits snow on the downwind side.  Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

  • SIZE


    2 (Large)

    The potential size of avalanche resulting from this problem.




    The likelihood of an avalanche resulting from this problem.

Upper elevations, just below ridgeline are where these are most likely found.

Watch for pillows and drift like formations. Shooting cracks are a warning sign.

Avoid wind loaded start zones and cross-loaded slopes greater than 35º.

Problem 2 - Persistent Weak Layers

  • TYPE


    Persistent Slabs

    Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks.  Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Persistent, Deep-Slab.

  • SIZE


    2-3 (Large)

    The potential size of avalanche resulting from this problem.




    The likelihood of an avalanche resulting from this problem.

Faceted layers can be found throughout the forecast area. Shallower snowpacks have worse structure than deeper areas and are more suspect. In some places, persistent slabs are up to 40 inches thick and becoming low likelihood, high consequence problems. The worst faceting is in the Rattlesnake.

The problem facets tend to be concentrated around crusts. Look for these and soft, sugary snow above and below as you assess the snowpack.

The rain crust from 01/13/2021 has facets below it that can propagate. Located at the surface or 1 foot deep in the snowpack depending on location. It can be found up to 8500 ft in some areas.

In other areas problem layers are in the upper two feet of the snowpack and involve buried surface hoar and near surface facets.

The extended column test (ECT) and the propagation saw test (PST) are best for assessing the stability of these layers.

Observations 01/20/2021


Later today light snow is expected to enter the area into Friday. 1 to 3 inches is forecasted for the next couple of days. Look for this and winds able to transport snow to keep the avalanche danger steady at Moderate. See the forecast.

This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.