Avalanche forecast

Snoqualmie Pass

Northwest Avalanche Center
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Avalanche danger

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

Expect very dangerous avalanche conditions on Thursday as a dynamic storm with heavy snowfall and gusty winds continues into the morning hours. Rising temperatures on Thursday should result in a switch to rain at lower elevations. Thursday morning is the expected height of instability with uncertainty about how the rest of the day will pan out. Travel in avalanche terrain is not recommended during this time period.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    With more precipitation on the way, Storm Slab depth will continue to increase on top of the ~1.5 feet of snow already accumulated on Wednesday. Slabs will be deeper and more reactive as you ascend in elevation, particularly above the rain/snow line later in the day. Slabs will also be larger on leeward slopes, at the base of cliffs, or on the sides of gullies where the storm snow was redistributed by the wind. Thursday will not be a day to be traveling in avalanche terrain, including where avalanches can run or stop.  But if you choose to go into the mountains, stick to low angle terrain and gather some information for another day. Watch for shooting cracks, collapses, recent avalanches, and active wind loading. Use test slopes and hand pits to look at the bond between the new and old snow, along with any interstorm layers that may be present. You can also look at the 12/21 interface to see if it is still a point of failure. 

    (12/30/2020) This Storm Slab released naturally on Wednesday in the Kendall Lakes area and propagated the width of the slide path, connecting terrain features.

  • Loose Wet

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

    As temperatures rise on Thursday, a switch to rain is expected at lower elevations, creating the potential for Loose Wet avalanches below ~4500 feet. There’s a little bit of uncertainty around the timing of this event and if the East flow can keep the lower elevations cool enough to remain all snow. Our best guess is that sometime in the mid-morning hours for the possible switch. Loose Wet avalanches are capable of entraining lots of snow and running far distances, even on low angled slopes. The height of this instability should occur when rain begins falling on cold dry snow that has fallen during the storm. At this onset of rain on snow, you should avoid traveling in and below all avalanche terrain.

Avalanche Discussion

Avalanches were releasing naturally and being triggered by humans at Snoqualmie Pass on Wednesday, making conditions perhaps a bit spicier than we had originally anticipated. At least one observed avalanche in the Kendall Lakes area broke about 6” deep, connecting terrain features, and involving the full width of the slide path. Although the grain type this slide failed on is unknown, it leads us to believe that there are some layers of concern in the snowpack that will continue to be loaded, or possibly reloaded on Thursday. Additionally, Alpental Pro Patrol reported sensitive Storm Slabs up to 15” deep releasing easily with ski cuts during control work, Wednesday. With continued snow Wednesday night into Thursday morning, we suspect peak instability should be early in the day before the precipitation rates start to taper. A natural avalanche cycle is expected to occur at some point, but the timing of them can be tricky to predict. You need to keep your guard up throughout the forecast period and realize that slopes that slid on Wednesday could reload and slide again on Thursday. Other slopes that haven’t yet slid could be approaching their tipping point with the added precipitation. To add some complexity, a warm-up is expected to occur on Thursday which could lead to multiple outcomes. We could see rain at lower elevations, driving wet avalanche problems before helping the snowpack to stabilize. We could also see heavy snow start to overlay weaker snow sitting on top of a crust at multiple elevations, making a favorable setup for natural and human triggered avalanches. We’ll have to see how the weather plays out on Thursday, but it will certainly be a heads up day.

Leading up to this storm, the pass saw multiple human triggered avalanches occur, with some slides breaking in surprising ways. The 12/21 crust remains a point of reference in the snowpack for you to dig down to. Look at all the snow and interfaces above the 12/21 and evaluate it carefully for points of failure. Weak snow grains had been observed on the surface prior to Tuesday night’s snowfall and may now be buried, creating a weak layer. Let us know what you’re finding out there by submitting an observation.

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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