Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
Open full map
Non aggiornato — questo bollettino è scaduto 7 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

A prolonged dry spell and previous strong winds have reduced the threat of avalanches on many slopes.  Two types of terrain present slab avalanche hazards: slopes where you encounter surface hard slabs (like this skier-triggered slab on Tuesday), and on slopes that retain slabs over basal weak layers (typically on the north half of the compass near and above treeline).  You can reduce your risk (and find better riding conditions) by avoiding slopes with signs of recent drifting, choosing wind-protected terrain, or wind-eroded terrain at upper elevations. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1.5–2.5

    Although instabilities are becoming increasingly harder to find, the most hazardous concern is the lingering threat of triggering a large slab that fails 2 to 4 feet deep on depth hoar, facets, and faceted crusts without obvious warning signs.  In the Northwest Mountains, these slabs are most likely to be triggered in terrain where slabs are thinner or variable in depth from past wind events. The most reliable way to manage this problem is by choosing slope angles less than 30 degrees.  Southerly aspects, low elevations, and heavily wind-eroded terrain typically offer safer snowpacks, where slabs are less common.  

    A secondary issue comes in the form of thin, hard lens-shaped slabs that formed during last week's wind events.  This problem exists at all elevations, though most commonly an issue on the north half of the compass, where it forms over weak facets.  Although they are typically stubborn to trigger now, skirt around these harder drifts in consequential terrain to avoid the threat of getting pushed into trees, rocks, or gullies.    

    A persistent slab avalanche that likely failed during the recent wind event (1/16 to 1/17)

    Photo: Crested Butte Avalanche Center

Avalanche Discussion

Last week’s wind events scoured the snowpack and redistributed limited transportable snow into pockets of thin, hard wind slabs on multiple aspects and at all elevations. Extreme northwest winds on Friday and Saturday produced the most consistent drifting and slab formation. The best way to identify this problem is by reading the snow surface and watching for hard lenses in concavities and other catchments. While most drifts are now difficult to trigger, a skier-triggered wind slab on Tuesday shows they remain a concern in consequential terrain, especially where slabs formed over facets on colder aspects.

Persistent slab avalanches failing in November and December weak layers are becoming unlikely. Nearly two weeks of dry weather and recent wind erosion have made this problem increasingly isolated and stubborn. In the Southeast Mountains, a thinner snowpack and widespread erosion have reduced the likelihood of slabs, and we have higher confidence in lower danger.  This video demonstrates the typical situation. The persistent slab threat is also diminishing in the Northwest Mountains, but isolated instabilities there could still be large, which is why we have held MODERATE danger as it better fits travel advice.  Slopes of concern are shadier aspects with poor structure that received wind loading rather than erosion during the weekend wind event.  Caution is warranted if you find suspect slopes that appear to have been loaded versus eroded at upper elevations.

A storm system arriving Friday morning will increase the avalanche danger throughout the day and Saturday.  The new snow will fall on a mix of wind board, melt/freeze crusts, and weak faceted grains.  The Northwest Mountains are favored for this storm, so expect a greater increase in hazard where more snow accumulates.  

 

Upcoming Outreach Events:  

Fireside Chat:  Jan 23, Dave Scheefer "Historical avalanches in Gunnison County."  Mallardi Theatre 5-7 pm.

Trailhead Day: Jan 24, Slate River, 10 am-2 pm.

Regioni coperte