Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Mother nature deposited 4 - 9 inches in the Northwest Mountains accompanied by gusty southwest winds.  For areas with 7 inches or more of new snow anticipate finding sensitive slabs, particularly on leeward terrain with fresh drifting.  In sheltered, northerly-facing terrain you could trigger shallow soft slabs in the storm snow or loose snow avalanches.  You will find a CONSIDERABLE avalanche danger rating in areas with wind-loading and in parts of the forecast zone with the deepest storm totals in the Ruby Range.  Expect the size of avalanches to increase as compared to the past week.  Use caution around leeward terrain features and anticipate moving snow on sheltered, northerly-facing terrain that can accumulate enough mass to bury or injure you.        

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    You can trigger avalanches in the storm snow on leeward terrain features where the wind drifted the new snow into fresh slabs.  In sheltered terrain, you can trigger shallow, soft slabs in terrain that faces the north half of the compass where the new snow is sitting on a very weak and faceted surface.  Expect triggered avalanches on the north side of the compass can entrain a significant of weak, old snow creating avalanches big enough to bury or injure you.  Simply use caution around drifted terrain and use small test slopes in sheltered terrain to get gauge if the new snow will fail as a soft slab.

    An example of a shallow, soft slab from early this season in sheltered terrain.  Slabs that fail in the storm snow like pictured here could accumulate a dangerous amount of old snow as it runs.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Strong northwesterly winds last week raked the mountains and mixed into typically more sheltered locations.  The incremental wind drifting built hard slabs ranging from just a few inches to several feet thick that is now buried and obscured by recent storm snow.  Although you can find these slabs at any elevation, we've seen the most significant wind loading and slab formation occurring nearing treeline, especially in terrain with large fetches favored by northwest winds.   These slabs are slower to heal because they formed above exceptionally weak facet layers.  They can also be remotely triggered from below or adjacent to steep slopes where the drifted snow spans onto lower slope angles.   The problem is localized to leeward terrain features behind tree fences, on the lee sides of ridged terrain features, and in concave catchments such as gullies.   Collapses and shooting cracks are clear warning signs of the problem.  Give wind-loaded terrain a wider berth than normal as surface clues have been buried by the recent storm snow: there have been several near misses this week with some slides breaking into lower-than-usual slope angles.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Dry loose avalanches (sluffs) are a concern in wind-sheltered terrain on the north half of the compass.  Where the snow surface is weak and cohesionless, you can easily trigger sluffs that entrain the top 8" to 14" of loose snow on slopes steeper than 35 degrees.  The recent storm has increased the snow available for loose avalanches to entreain.  Ski cuts, moving across the fall-line while descending, or choosing lower-angled slopes are effective techniques to manage this problem. These point-release avalanches will accumulate more mass in long-running, steep terrain. Use caution above gullies where an avalanche can pile more deeply and above trees and cliffs where the consequences of getting caught in a sluff will be magnified. This problem is most common in sheltered terrain below treeline, but you can also trigger sluffs on some near treeline slopes and a few above treeline slopes.    

Avalanche Discussion

Wednesday's storm delivered 2 to 9 inches of snow.  The Northwest Mountains accumulated 4 to 9 inches with the highest totals in the Ruby Range.  The Southeast Mountains picked up 2 to 5 inches of storm snow.  This difference in snow totals is the key point in the diverging danger rating between the two zones.  There is a variety of avalanche problems to manage today depending on aspect, elevation, and location in the CBAC forecast zone.  What follows will help you make sense of the variety of surface instabilities you may encounter today.

Wind Drifted Terrain:  Leeward terrain features will have formed fresh slabs at upper elevations from southwest winds that will be sensitive to the weight of a person.  Fresh wind-drifted slabs will be the biggest and most concerning for areas that accumulated the most storm snow.  Finding more than about 7 inches of drifted snow is a good clue that you have found increased avalanche hazard.  Cracking or collapsing is a clear sign of instability that you should heed.  Keep in mind that there is an older hard slab problem stacked beneath the recent storm snow on some leeward terrain features, triggering an avalanche in the storm snow could release a hard slab below like in this ob from Schuylkill Ridge yesterday.  Use caution around drifted terrain today as the storm snow may be very sensitive and it is now very hard to tell if an old slab exists below the recent storm.

Sheltered Terrain:  You can be confident that steep, sheltered terrain facing the north half of the compass will produce moving snow.  The question is will it be a shallow soft slab or simply a loose avalanche.  Either way, the new snow will add volume to avalanches as they run and accumulate mass.  Expect avalanches to be large enough to hurt you or potentially bury you in snow-favored areas.  Sheltered terrain facing the south half of the compass could produce small avalanches but are much less likely and will be smaller in size due to crust below the storm snow.

Although this storm was not all that impressive, it will bring a fairly distinct increase in avalanche danger for areas that accumulated the most storm snow.  Take a cautious approach to the day to make sense of the conditions you are traveling through and expect things to be different from what you have been managing the past few weeks.

 

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