Dec 12, 2015 @ 6:09 am

The avalanche danger is rated Considerable in the west central Montana backcountry above 6000ft. We have a very weak snowpack structure in the advisory area. Human triggered avalanches are likely. Dangerous avalanche conditions exist.

Good morning, this is Travis Craft with the West Central Montana Avalanche Center’s avalanche advisory for December 12, 2015.  We are now into our regular advisory schedule.  This danger rating does not apply to operating ski areas, expires at midnight tonight (Dec.12) and is the sole responsibility of the U. S. Forest Service.

Weather and Snowpack Discussion

This morning mountain temperatures are in the mid and upper 20’s  and winds are light with gusts of 10 mph out of the South-South-West. We picked up 1 inch of snow on throughout the advisory area.This morning scattered snow showers will move through the area dropping minimal amounts of snow. A potent weather system is forecasted to enter our area tonight and bring with it a large accumulation of snow and high winds which will rapidly increase the avalanche hazard in the backcountry.

The main problem we have in the advisory area is a persistent weak layer consisting of facets that formed during the early season freeze above 6000 feet. These facets have created a very weak snowpack structure.(profile1)(profile2)(video)  We have facets in the bottom of the snowpack that formed below a crust and to the ground with a well consolidated slab on top.(pit)   When touring and getting off our skis we are sinking all the way to the ground. We are also getting whumpfinging and collapsing, as well as, shooting cracks from our skis when touring throughout our advisory area.(pic)  In our ECT’s in the Rattlesnake and Lolo Pass we are seeing propagation on this layer of facets. In one of the tests on an East aspect at 7600 feet, this layer propagated and failed while isolating the column. All of these are significant signs of instability which tell us that we should not enter into avalanche terrain.  With all of these signs of instability, terrain should be carefully evaluated as to what is above you and if you are connected to steeper terrain where you could remotely trigger an avalanche. Runout zones should be avoided.

The second problem is that wind slabs have formed on leeward slopes sitting on this weak structure. The high winds that accompanied the last storm created  wind slabs on these leeward slopes, which are sensitive to triggers.  All wind loaded slopes should be treated as suspect due to the poor structure of the snowpack.

Weather Forecast and Avalanche Outlook

With the poor snow structure and and more snow in the forecast, any significant new loading will dramatically increase the instabilities in this weak snowpack.  Significant wind may accompany this storm so leeward slopes will also have an increased avalanche hazard. With these two factors I would expect the avalanche danger to increase rapidly. We will be in the field this weekend evaluating the snowpack and will issue the next advisory on Tuesday, December 14, 2015.  If you are out, please send us some public observations. We are now publishing three advisories a week: Tuesdays, Thursdays and Saturdays. Our beacon park at Lolo Pass is also up and running.


Problem 1 - Facets

  • 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.



    Likely/Very Likely

    The likelihood of an avalanche resulting from this problem.


We have a layer of facets creating a weak Snowpack.


Problem 2 - Wind Slabs

  • 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


    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.




    The likelihood of an avalanche resulting from this problem.


Leeward slopes have wind slabs sitting on a poor snowpack structure.


  • Danger Trend


    Increasing Danger

  • Area Forecast

    New Loading

The forecast calls for strong winds to move into the area this afternoon and then precipitation to start in evening. If eitherof these events happen I would expect the avalanche hazard to increase due to increased loading on a a weak snowpack.

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.