Avalanche forecast

Snoqualmie Pass

Northwest Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

The avalanche hazard should increase throughout the day as a transition from dry to wet avalanche problems takes place. When you notice that the sun’s influence is starting to warm the snow surface, you should adjust your mindset and head toward shady low-angled slopes.

Update 6:52pm: Preliminary information has been received of an avalanche involvement. See the forecast discussion.

Avalanche problems

  • Loose Wet

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

    As temperatures creep up and the sun grows stronger throughout the day, expect wet avalanches to become more prominent on steep sunny slopes. Your first indicators that the upper snowpack is destabilizing will be rollerballs and pinwheels, snow shedding from trees and rocks, and the surface becoming wet and sticky. If you notice these signs, expect Wet Loose avalanches to follow and head toward shadier slopes. These slides often start at a point such as near trees or rocks, can entrain significant snow, and will fan out as they descend, possibly growing large and traveling long distances toward lower elevations. 

    (4/9/2021) Rollerballs and small Wet Loose avalanches were observed on 4/9 when the sun made an unexpected appearance after fresh snow had fallen the day before.

  • Wind Slabs

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

    Plenty of wind-affected snow still exists as you ascend in elevation, and cold dry snow will be more likely to escape the sun’s influence on northerly slopes later in the day. Take a look for freshly built cornices, textured snow surfaces, or new drifts behind trees or other objects to help you identify wind-loaded slopes to avoid. Wind Slabs could be hidden under a couple of inches of fresh snow, so feel for subtle changes as you travel to help you determine if you stumbled upon one without knowing. Wind Slabs will be more likely near rocky features such as the confined gullies or at the base of cliffs, on leeward slopes near ridgelines, or on convex rollovers. You can use uptrack tests or test slopes to determine how the recent storm snow is reacting to your weight and if slab potential exists. If you find poorly bonded snow or suspect terrain to be wind-loaded, you should seek out lower-angled slopes. 

    (4/9/2021) This Wind Slab was triggered by a skier on 4/9 on Snoqualmie Mountain. Wind Slabs are more likely near rocky features just like the one here. [N 5000' Photo: Andrew Pringle]

Avalanche Discussion

Update: We just received word of an avalanche involvement where a skier in the Alpental Valley triggered an avalanche and was caught, carried, and injured on a N/NE slope at approximately 5300'. This is preliminary information but is the second avalanche involvement in as many days in the Alpental Valley.

Snoqualmie Pass has received around 2 feet of cold dry snow this week, which is sitting atop a stout crust (4/7 interface). You may find a smaller (~2cm) crust separating this week’s two storms (4/9 interface) 8-10” below the surface on sunny slopes. Throughout the recent storm snow, weak graupel has been observed in some locations, giving you plenty of points of failure to investigate. This includes sitting atop the 4/7 crust, which was the cause of multiple recent human-triggered avalanches including an avalanche involvement where a skier was caught and carried on Friday.

As we transition away from cold and snowy toward sunnier weather, Sunday should be day 1 of what projects to be a multi-day shedding event/wet avalanche cycle. Transition days can be difficult, and your morning concerns could be more focused on dry avalanche problems such as Wind Slabs or Dry Loose. As the sun gains influence and warms surfaces, the transition to wet avalanche problems may happen quickly, manifesting in different types of avalanches. NWAC forecasters witnessed two dry loose avalanches occur within 15 minutes of the sun coming out at Snoqualmie Pass on Saturday, triggered by snow shedding from rocks. This process will continue Sunday, when snow will continue to shed from trees and rocks, falling onto the slopes below and possibly triggering Wind Slabs or starting loose avalanches - either wet or dry. Glide slopes that have not previously failed may reach their tipping point or may have been reloaded during this week’s stormy weather and fail for a second time. We have reports that a cornice fell and triggered a small slab in the Alpental Valley during a period of sunny weather on Friday. Both fresh and mature cornices exist throughout the zone, and more could fall under the sun’s influence, Sunday. We expect most avalanches to occur above the 4/7 interface, but given a large enough trigger, moist grains below the 4/7 could fail and allow an isolated avalanche to grow very large. 

Wet avalanches are generally not well understood and difficult to predict, so it could take a few days for all of these scenarios to play out. But Sunday should be the start of this transition and you should keep in mind that multiple near-misses and avalanche involvements have occurred around the Cascades and specifically at Snoqualmie Pass over the past week as we have navigated flip-flopping springtime weather. This recent history combined with the uncertainty around these shedding events should lead you toward a more conservative mindset and terrain selection.

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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