Avalanche forecast

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Northwest Avalanche Center
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Avalanche danger

Thu
Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

A natural avalanche cycle is likely to occur Tuesday night into Wednesday morning with another round of heavy snow and very strong winds. Stay off of, and out from underneath steep slopes through Wednesday while this meaty storm runs its course. You could trigger slopes from a distance and create very large and destructive avalanches.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Heavy snow is sitting over less dense snow from Tuesday's storm. Even if this last round finishes 'right side up' expect to find a reactive weak layer within the storm snow. Signs of instability are shooting cracks and recent slab avalanches. These may occur even on small, steep features and could be 2ft deep below treeline. At higher elevations slabs could be well over 4ft deep during the day tomorrow. At lower elevations (below 4,500ft or so) rain on snow could saturate the snowpack and cause wet loose avalanches to occur as well. Tuesday is a good day to plan to avoid avalanche terrain altogether, staying well away from any steep slope.

    Deeper weak layers exist in areas with a shallow snowpack such as the Winthrop foothills, and the lowlands around Twisp. In these areas, you could trigger a persistent slab avalanche on old snow near the ground. These would be difficult to manage, could be triggered from a distance, and may occur in unusual locations of the lowlands/eastern slope.

    Example large storm slab

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Very strong winds will create intense snow transport at upper elevations and exposed locations on Wednesday. Wind slabs may release naturally into the morning hours and could be very large. Signs of wind slab instabilities are shooting cracks, whumphing collapses, and recent slab avalanches from leeward and crossloaded slopes. Cornices may be quite reactive and growing during the morning. If you do get up onto a ridgeline, give these a wide berth as they could fail from a distance and trigger very large avalanches below. 

    Example of intense wind loading off a ridgeline.

Avalanche Discussion

Temperatures have warmed to 32 degrees at Washington Pass, and round 3 of the Atmospheric River Train is imminent. With a high freezing level, heavy precipitation, and very strong winds Tuesday night into Wednesday, we can expect a significant avalanche cycle. So far (5pm Tuesday), we've gained 1.5" of Snow Water Equivalent at Harts Pass, and added 12" to our snowpack depth at Washington Pass.

Before the storm, the previous snowpack was already deep and strong. For the most part, avalanche concerns are limited to the recent storm snow and wind events (still quite deep). However, in the eastern part of the zone the snowpack is more shallow. Here, persistent weak layers may be found near the ground, and these may become problematic with this big load. Last week, an observer near Aspen Lake reported an avalanche on old, weak layers beneath the recent storm snow, whumping collapses, and propagating test results. In areas like the Winthrop foothills, the lower Twisp River, the Lost River drainages, and areas east of Mazama the snowpack may be shallow and weak. If you're venturing into these eastern mountains, expect a weak and dangerous snowpack on Wednesday.

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources. Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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