Dec 29, 2020 @ 6:37 am

The avalanche danger in the west central Montana backcountry is MODERATE

Good morning; this is Jeff Carty with the West Central Montana Avalanche Center’s avalanche advisory for December 29, 2020. This advisory does not apply to operating ski areas, expires at midnight tonight, and is the sole responsibility of the U.S. Forest Service.

Weather and Snowpack

Mountain temperatures are 16º F this morning and are forecast to reach the high 20’s today. Clear skies and cold nights have dominated since the last forecast and extensive surface hoar has grown throughout the forecast area. Winds were calm to mild yesterday and will be light with moderate gusts today. Wind slabs are not expected to build in the next 12 hours, but increasing winds tonight will start to transfer snow to leeward slopes.

The snowpack is varied throughout the forecast area. Deeper areas have better bonding and are not showing consistent failures in stability tests. Weak layers still exist in isolated areas and avalanches are possible in these zones.

In shallow areas, generally under 5 feet in depth, the snowpack structure is still poor. Facets and crust combinations are reactive in pit tests. While the strength is increasing, propagation in tests shows us that slab avalanches are possible. These are becoming a low likelihood high consequence problem as persistent weak layers get buried deeper.  

Lolo Pass had two concerning faceted layers yesterday, 8” and 31” deep that were propagating in extended column tests. These exist on multiple aspects, and staying on slopes under 33º is wise if you are riding in the Lolo Pass area.

A natural avalanche was reported in the Swans yesterday. This area had extensive faceting before Christmas that is likely still lurking and contributed to this slide. Extended column tests on Pyramid Peak were failing on buried facets on the 24th. Investigate the Swan snowpack carefully before committing to steeper slopes.

The southern Bitterroots, outside of Lost Trail has also had a shallower, faceted snowpack.

Extensive surface hoar has grown throughout the forecast area and will be a problem layer with our next snow load. 

Bottom Line

Careful snowpack assessment is required as persistent weak layers exist in specific areas. Digging a pit, looking for sugary snow layers, and performing stability tests is crucial for making informed decisions. Where persistent grains are present stick to slopes under 33º that are not connected to steeper slopes. Practice safe travel protocols. Travel one at a time in avalanche terrain, carry a beacon, shovel, and probe. Remember to re-assess conditions throughout the day and stay alert for signs of instability. Look for red flags.

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 is a great resource to glean information about current conditions. Here is the link to Public Observations.

Ski and ride safe.




Problem 1 - Persistent slabs

  • 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 (Large)

    The potential size of avalanche resulting from this problem.




    The likelihood of an avalanche resulting from this problem.


    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West
    NW - Northwest

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. In other areas problem layers are in the upper foot of snowpack and will become more of a concern with the next snow load.

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

Where facets are found stick to low angle slopes and avoid traveling under steep terrain as remotely triggered avalanches may be possible



A few light snow showers are possible Monday night into Tuesday as a weak system drops southward. Westerly ridge top winds will increase Tuesday night into Wednesday as a Pacific storm system enters the Pacific Northwest. Westerly flow will focus mountain snowfall along the Idaho and Montana border Wednesday night into Thursday. Confidence is growing for a period of heavy mountain snowfall next weekend into early next week, particularly for the Bitterroot Mountains.


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.