Avalanche forecast

Snoqualmie Pass

Northwest Avalanche Center
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Avalanche danger

Thu
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Recent very large and unpredictable natural avalanches should give you pause and make you think twice about entering large start zones, avalanche paths, or their runouts. Warm and sunny weather on Wednesday may help to destabilize the snow and trigger a number of different avalanche problems.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Very large natural avalanches have been occurring across the Cascades and at Snoqualmie Pass as recently as Monday. Many of these avalanches are suspected to have run on the Mid-January crust/facet combo, which has recently been measured close to 10 feet deep (300cm). We have had several rounds of deep slab avalanches over the past few weeks and many have been erratic and hard to predict. We could go several days without seeing another deep slab release, or we could see multiple happen tomorrow. Know that if you decide to enter large start zones, avalanche paths, or runout zones, that you are putting yourself at risk. Many of these slides have run very far distances and broken trees as they descend, reaching valley bottoms and areas normally considered as “safe”. Being caught in one of these slides would almost certainly be fatal and those consequences should shape your decision-making and terrain selection.

    (3/1/2021) A very wide, very deep avalanche released from the east bowl of Chair Peak at some point between Sunday night and Monday morning (NE-E-SE 5800; HS-N-D3.5-R4-O). Photo: Bryce Hill

  • Cornices

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

    The recent low-density snow and windy conditions around the pass allowed cornices to form on multiple aspects. The largest and freshest cornices may be found on east aspects, but take a look to the ridgelines on any aspect and see if these beasts lurk above you. Sunny weather and rising freezing levels may be enough on Wednesday to destabilize cornices and let them fall on the slopes below. On Saturday, a cornice fall was responsible for triggering a large avalanche on Bryant Peak in the Alpental Valley. These large triggers could be the cause of slab avalanches to fail or may have plenty of snow themselves to pose a hazard. Cornices notoriously break further back than expected, so stay far away from them if you’re on a ridgeline and stay clear of slopes that are below them.

    (3/1/2021) The cornices around Snoqualmie Pass have grown very large recently. This cornice was observed on Tinkham Peak on Monday and the remnants of a large avalanche crown are on the slopes below. Beware of large cornices triggering large avalanches. [~5300' E-S aspects]

  • Loose Wet

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

    Much of the snowpack around Snoqualmie Pass has recently seen some sunshine and warm weather, but you could still see some Wet Loose avalanches on Wednesday. These avalanches will be more likely on slopes that see direct sunshine and will initiate at a point such as near rocks or trees. These slides can entrain significant snow, fan-out, and even trigger slabs as they descend downhill. Rollerballs, the snow surface getting warm and sticky, or snow shedding from trees and rocks will be your first indicators of Wet Loose avalanches. If you notice these signs or recent avalanches with fan-shaped debris piles, head for shadier slopes keeping aware of the terrain above you.

    (2/28/2021) Rollerballs and Wet Loose activity were observed on Sunday (2/28) in the Alpental Valley.

Avalanche Discussion

On Monday morning, we received word of a very large natural avalanche (D3+) that occurred on Chair Peak in the Alpental Valley with a crown depth estimated to be over 10 feet in the deepest part. This is just one of many slides of this kind that have been happening all around the Cascades for the past few weeks and have been shaping our decision-making and terrain selection. Many of these slides have reached valley bottoms or areas people often consider as “safe”. In the Chair Peak slide, debris was found to have overrun Source Lake and the berm, which is a popular picnic spot for skiers, snowshoers, and hikers. Last week, a slide from Denny Peak ran all the way to Franklin Falls, which is a popular hiking and snowshoeing destination. These examples help to illustrate that although a deep persistent slab avalanche may not initiate at lower elevations, they are capable of running into them, meaning that generally safe avalanche conditions do not exist, even below treeline.

On Wednesday, sunshine and warming temperatures have us concerned for a number of reasons. When the snow starts to warm up, it generally starts to fall off of things. This means that cornices may fall and trigger deep slabs, or blocks of snow and ice may fall off rocks and cliffs to trigger avalanches on the slopes below. Even snow falling off trees and rocks may start as a simple snowball and grow into a large avalanche as it descends. Although some east winds may help to keep things a bit cooler at pass level to start the day, we don’t expect conditions to stay that way throughout the forecast period. Let the deep persistent slab problem continue to guide your decision-making on what terrain is within your acceptable level of risk and stay safe out there.

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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