Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Thu
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

An Avalanche Watch has been issued for the Northwest Mountains forecast zone.  Substantial snowfall and wind loading on Wednesday will cause avalanche conditions to deteriorate throughout the day.  Drifted terrain is the most dangerous due to natural activity and very large avalanche size.  As snowfall stacks up in sheltered areas, human-triggered avalanches will increase in likelihood and size.  Avoid drifted slopes and steep wind-protected terrain on the north half of the compass.  Low elevation, southerly slopes are less likely to have fragile weak layers.  If storm totals meet or exceed forecasts, then very dangerous (HIGH danger) conditions could occur as early as Wednesday evening.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–3

    Last weekend's storm formed slabs 1 to 3 feet thick at all elevations over widespread weak layers of surface hoar and facets on most east, north, and west-facing slopes, and potentially faceted crusts on high-elevation southerly aspects.  Snowfall on Wednesday will increase the size of slabs.  These slabs are have been sensitive to human triggers, evidenced by numerous recent triggered and natural avalanches and likely become more sensitive as more snow accumulates today.  They can be remotely triggered from flat terrain, or propagate widely across terrain features.  Shooting cracks or collapses are a clear sign of danger.  This problem is generally D1 to D2.5 in size.

    There are old faceted weak layers near the ground that are more stubborn to human triggers. These dense old slabs may not give warning signs before they release; they are most commonly triggered from terrain features with shallow or variable snowpacks, or if avalanches in the upper snowpack gain enough mass to step down.  This problem is generally up to D3 in size.  

    The most reliable strategy for avoiding these issues is by riding on slope angles consistently less than about 30 degrees.  Mid to low-elevation southerly slopes are less problematic.   

    2/4: This avalanche on Mt. Axtell exemplifies our newest generation of persistent slabs (recent storm snow breaking on facets or surface hoar), which then stepped down and triggered older hard slabs deeper in the snowpack on old weak layers near the ground.  

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    A wet storm arrived early Wednesday morning and will deliver snow through late Thursday.  Issues in the storm snow could develop during the second half of Wednesday once storm totals reach or exceed about 10 inches.  Look for cracking in the surface snow by utilizing small test slopes.  The Storm Slab and Persistent Slab overlap in many places, but the Persistent Slab problem is far more problematic and dangerous.  Expect to manage Storm Slab problems only if you travel on lower elevations southerly slopes; on all other aspects and elevations, the Persistent Slab problem takes precedence.

    A skier triggered Storm Slab from this past Saturday.  Similar-sized slabs could become sensitive on low-elevation southerly slopes.

    Photo: Jake Hutchinson

Avalanche Discussion

An Avalanche Watch has been issued in preparation for very dangerous (HIGH danger) conditions to develop by Thursday in the Northwest Mountains.  Storm totals could reach or exceed two feet on Thursday.  Lesser snow totals of around a foot are expected for areas closer to Crested Butte and the Southeast Mountains.

For Wednesday, the most dangerous terrain will certainly be on leeward slopes as the potential avalanche size is very large and some could run naturally due to snowfall and wind-loading.  Take a look at these avalanches from last weekend to get a feel for the deadly size potential and keep in mind these avalanches occurred last weekend.  Avalanches that fail today will increase in size as slabs continue to grow(Axtel, Bellview, Purple Peak).  Traveling on or immediately below drifted terrain is a deadly proposition today.

There have been a handful of human-triggered avalanches in sheltered terrain during and after this past weekend's storm.  You can see examples from the Anthracites, Purple Palace, and Axtel.  1 to 2 feet below the surface there is a nasty layer of surface hoar and near-surface facets.  Slabs have been relatively soft in sheltered terrain limiting propagation and sensitivity to human triggers, however, snowfall on Wednesday may dramatically increase sensitivity and drive propagation farther resulting in easy-to-trigger, large avalanches.  There is uncertainty about how much more snow is required to see an uptick in sensitivity but a reasonable expectation is for it to happen today.  Remain very skeptical of steep sheltered slopes highlighted by the Persistent Slab rose.

Some areas in the Southeast Mountains have much less snow resting above the problematic layer of surface hoar and facets.  With less snowfall forecasted and thinner, softer pre-existing slabs, expect to find reduced avalanche hazard in sheltered terrain in the Southeast Mountains compared to the Northwest Mountains.  This is a product of smaller-sized avalanches and less sensitive conditions in the upper snowpack.  Deeper weak layers exist at the bottom of the snowpack and can still be triggered by backcountry travelers where the snowpack is less than about 4 feet.  

In both forecast zones, lower elevation southerly slopes present lower risk options.  Warm temperatures before last weekend's storm built thick crusts insulating deeper weak layers and destroyed the problematic surface hoar found on other aspects.  You can always take the safest path and surf the fresh snow on slopes less than 30 degrees.

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