Lawinenlagebericht

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Northwest Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
4 · Gross
Waldgrenze
4 · Gross
Unterhalb
4 · Gross
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Kernpunkte

A powerful storm with warming temperatures is expected to create very dangerous avalanche conditions Sunday night through Monday. Avalanches could be deep, and run full path to valley bottoms, with destructive debris. Stay out of avalanche terrain, and well out from underneath steep slopes at all elevations while this natural cycle runs its course.

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Very Likely
    Größe
    2–3.5

    Heavy snow with rising temperatures, and strong wind are a perfect recipe for slab avalanches. Our current layered snowpack is fairly weak, and riddled with buried layers of surface hoar, facets, and poorly bonded suncrusts. Storm slabs were touchy on Sunday, and we expect these may begin to step down to the deeper crust layers as the storm delivers copious snowfall on Monday. More snow will accumulate at higher elevations, and combined with intense wind transport these slabs could become over well over 4ft deep and encompass entire start zones. Avalanches could step down beneath the Valentines Day Crust to a problematic crust from late January, and cause slides to carry much more power. 

    Storm slabs may build in the foothills at lower elevations if temperatures stay cold enough for snow. If temperatures warm up sooner than expected and heavy rain begins, we could see wet slabs or wet loose avalanches run from lower elevation forested hillsides and open sagebrush slopes. You don't need to jump on a snowmobile and ride up the highway to get into an avalanche, you only need a slope steeper than 30 degrees, unstable snow, and a trigger. 

    The recommendation is simple: Monday is not a day to mess around in or underneath any steep slope at any elevation, better to let this natural cycle run its course.

  • Deep Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2.5–3.5

    As the mountains receive heavy snowfall, deeper and destructive avalanches are expected to become possible on a crumbly layer of facets and crusts from late January. The layer is most likely to cause very large avalanches from W-N-E facing start zones near and above treeline. Near the crest, this layer has gained a lot of strength over the past two weeks, and is expected to be 3-4ft deep. Further east in the zone this layer is closer to the surface, but is likely much weaker and more likely to 'wake up'. This layer is a combination of weak sugar-like facets sandwiched between two crusts in many areas. Very wide avalanches were observed on it with a much smaller storm earlier this month. 

    Slides could encompass entire starting zones and run full path to valley bottoms. Stay off of, and well out from underneath steep slopes while this storm causes natural avalanches to run. 

    Example deep persistent avalanche crown from Mt Shuksan earlier this winter.

    Photo: Irene Henninger

Avalanche Discussion

Observers found touchy avalanche conditions on Sunday, with about a foot of new snow fracturing easily on steep slopes. One large soft slab up to 1.5ft deep was noted from a SE aspect at 6,700ft in Cutthroat drainage. Numerous small (D1-1.5) loose avalanches were also observed from upper elevations. NWAC Forecaster Josh Hirshberg found the late January crust about 2.5ft down on a N aspect in Cutthroat, with propagating results (PST 30/100 End). Observers have continued to get test results that show this problematic layer could create avalanches, given the right conditions. This storm (of the atmospheric river variety) with its warming temperatures, heavy snow, and strong wind should be a great test for it. It'll be a great time to stay out of complex mountainous terrain as large to very large natural avalanches are expected. Keep in mind lower elevation, unsuspecting slopes if over a foot of snow piles up in the valley. 

On Friday observers found surface conditions to consist of a mix of weaker snow grains; patchy surface hoar and near surface facets on northerly aspects while widespread surface hoar had grown on southerly aspects. These are the layers that are likely to become the next, widespread problem as snow piles up over it through Monday. 

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