Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Last night produced an additional 4 inches with storm totals of 14 to 23 inches of low-density snow.  A few more inches are expected today.  The new snow is adding stress to buried weak layers 2-4 feet below the surface on northerly aspects.  You can trigger a dangerous avalanche that breaks in old, weak layers on shaded slopes.  Use caution on shady slopes and leeward terrain features drifted by southwest to west winds.  The new snow is resting on slick crusts on southerly slopes and will be most dangerous where you find recent wind-loading.  Heed warning signs like cracking or collapsing and make thorough assessments before you enter wind-drifted terrain regardless of aspect.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    You can trigger slabs 2 to 4 feet thick that fail on a layer of faceted snow buried below the storm interface (the February "sand box" weak layer).  This problem is can be found on west through north through east slopes.  The new snow could awaken this weak layer and create dangerous avalanche conditions.  Slopes that avalanched a week ago remain suspect (see this observation for 'repeat offender' potential).  Persistent Slab avalanches can be triggered remotely from below or adjacent slopes.  Collapsing and cracking are clear signs that this weak layer has been overloaded and primed to avalanche.  Use caution on slopes facing the north half of the compass.  

    This photo from Cement Creek illustrates how persistent slabs can be triggered from a long ways away from the start zone.  The avalanche was remotely triggered by a snowmobile on 2/27.

  • Storm Slabs

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

    Avalanches could break at the storm interface or within the storm snow.  This problem will be more commonly found on sunny slopes where the storm snow fell on melt/freeze crusts.  Avalanches in the storm snow will be most likely on leeward terrain features where southwest to west winds drifted the new snow.  Look for cracking in the storm snow and find small test slopes to identify instabilities within the storm snow.  You can also perform hasty hand pits to check the bonding between the storm snow and old crusts. 10 inches or more of storm snow is a good threshold for raising your concern for triggering an avalanche in the recent snow.  Numerous small Storm Slab avalanches were observed below treeline yesterday.

    An example of an avalanche breaking at the bottom of storm snow below treeline on a steep convexity.  

Avalanche Discussion

A couple more inches of snow fell overnight.  This brings storm totals between 10 to 23 inches across the Northwest and Southeast Mountains.  Natural avalanches are possible today on slopes that receive wind-loading.  The simplest way to break down today's conditions is to divide avalanche concerns into two categories: avalanches that fail in the storm snow and those that fail in old snow.

Let's start with avalanches breaking into old snow.  On the north half of the compass, the February 'dry spell' weak layer is buried around 2 to 4 feet deep.  Uncertainty exists about the sensitivity of avalanches breaking in the old snow and exactly how much new snow it will take before it 'awakens' and produces dangerous avalanches.  There is not uncertainty about the potential for these avalanches to be dangerous and large in size.  Wind-loading near and above treeline should increase your suspicions of triggering an avalanche that breaks in old snow.  Avalanche paths that failed during the last storm cycle 10 days ago still have weak layers present beneath the storm snow and remain suspect.  You can take a look at this observation from an old bed surface on Friday to see that the weak layer remains.  Collapsing is a clear sign that the area you are traveling through has a sensitive Persistent Slab problem, but these warning signs may not always be present before a dangerous avalanche.  Choosing lower-angled slopes is the best way to manage the uncertainty of triggering a Persistent Slab avalanche.

The storm snow can produce several different avalanches.  Take a look at this observation from yesterday to see Loose Dry avalanches, Storm Slab avalanches, and a few suspected Wind Slab avalanches (visibility was too poor to confirm). The storm snow is resting on crusts on most sunny slopes and will be most dangerous where you find recent wind-loading.  Cracking is a good sign that you have found unstable storm snow.  You can use small test slopes to gather more information about the bonding between the new snow and the old crusts on sunny slopes.  Use caution around all wind-drifted terrain features on the south half of the compass.

In closing, on northerly terrain you should be most concerned with triggering an avalanche that breaks in old weak layers, and on sunny slopes, you should be most concerned with triggering an avalanche in the storm snow, particularly on wind-drifted terrain.

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