Avalanche forecast

Stevens Pass

Mar 2 – 3, 2019Northwest Avalanche Center
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Avalanche danger

Mon
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Sun and east wind will be the main weather factors on this quiet Sunday. Watch for triggering lingering avalanches in wind drifted snow and older weak layers 2-3 feet below the surface. Although there haven’t been many recent avalanches, it’s still important to minimize your exposure to avalanche terrain.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Wind slabs could still be dangerous if triggered in the wrong spot. Cracks shooting through the snow are an obvious sign that you can trigger a wind slab avalanche. The easiest way to avoid these avalanches is to stay off of leeward slopes near and above treeline. Steer around areas of stiff, supportive, drifted snow.

    Wind slab avalanches are now generally difficult to trigger. With more east wind on Sunday, there’s potential for fresh slabs to build but there may not be enough snow available on windward slopes to be transported. There are a couple of interfaces in the upper snowpack (about a foot below the surface) that could make it easier to trigger avalanches. Use extra caution if you find buried surface hoar or facets under drifted slabs. From February 24-27th a series of east wind events drifted snow near and above treeline along the Highway 2 corridor, Nason Creek, and other major drainages that funnel east wind. On the 27th, a skier triggered and got caught in wind slab avalanche on the northwest side of Mt McCausland. Observers reported a few other avalanches around this time from Stevens and Snoqualmie Passes and the East Central zones.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2

    The name “persistent” describes these avalanches well. They stick around. You’re not going to trigger them on most slopes. In isolated areas, you may still be able to trigger a dangerously large avalanche on facets buried in early February. The most likely place the trigger a deep persistent slab avalanche is in extreme terrain that’s very steep, rocky, with a variable height of snow.

    There has been just enough information to keep this low likelihood but high consequence problem in the advisory. You can watch a video of test results I found on Friday on a WNW aspect at 5,200 feet near Mill Creek. I thought I’d be looking at another unreactive profile that would help confirm that deep persistent slabs are no longer an issue. Instead, I saw impressively reactive test results and more indication that you could trigger an avalanche on the Feb 8th facets than I’ve seen in well over a week. Additionally, observers have reported a couple of recent avalanches, where the Feb 8th facets can’t entirely be ruled out as the culprit weak layer.

Snowpack Discussion

March 1, 2019

Here we are at the beginning of March. The days are noticeably longer, the sun feels warmer, but the winter is far from over. This may be a good time to take stock of February’s events, where we are now, and what we might expect in the future.

A February to Remember

Many longtime Cascade travelers continue to remark about the long span of high-quality travel conditions in February. Indeed, we’ve experienced three weeks of very cold and snowy conditions. Backcountry travelers frequently reported, “best conditions in years”. February also saw a few rounds of natural and human triggered avalanches. These avalanches were different from our more frequent Cascade storm slabs; here today and gone tomorrow. Most avalanches failed on buried weak persistent layers. As such, most zones spent much of February with Persistent or Deep Persistent Slabs in their forecasts.

Natural avalanche in Icicle Creek Feb 13, 2019. Photo: Matt Primomo

With the notable absence of high elevation rain events, snowpacks around the northwest grew remarkably. On the west slope, most weather stations added 2-4 ft of snow depth. East-side locations added 1-2 ft. Mt Hood locations added 4-5 ft. Even with these impressive snowpack growths, many Cascade Snotel sites are still recording below average snow depths for the winter.  

During most of February, snow conditions were soft and powdery making for fun recreating in many areas. Photo Matt Primomo.

Where We Are Now

Lack of avalanche activity on the layer of buried facets from Feb 8 allows us to gain confidence in many areas. In some areas, snowpack tests are showing less than sudden results. These two pieces combined have allowed us to drop persistent slabs from some forecasts and shift others to “unlikely.” The exception is the East Cascades where a shallow snowpack has preserved buried facets, and persistent slabs will remain possible through the weekend. So, we’re out to the woods, right? Well, maybe. The same cold and stormy weather that brought us excellent snow quality, also allowed us to bury new weak layers. So far we haven’t confirmed any avalanches on these shallow weak layers, but they have our attention and we’re tracking them.

Recently our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow. Several wind events from various directions placed wind slabs on a variety of aspect. With the colder than normal temperatures, wind slabs may not heal as quickly as normal.

A small natural wind slab avalanche in the Crystal Mt backcountry. Photo; Dallas Glass

What Might We Expect

As we move into March, it’s anyone’s guess what specific weather patterns lay in store of us. However, there are two items that stick out in our minds.

  • The strength of the March sun: You can already feel it just walking around town. As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see slab avalanches become easier to trigger, natural loose avalanches originating from sunny slopes, or surface snow become thick and heavy. Remember, the sun frequently brings change.

  • High elevation travel: Frequently March begins to usher in a period where we push higher and deeper into the mountains. Remember, we don’t have as much information about these far-flung locations. If you use the longer days of March to travel to bigger objectives, keep your eyes open. When observations don’t line-up with the avalanche advisory, you may be experiencing a different snowpack. It’s times like these we need to stop and reevaluate.

Mt Baker on a nice day in early February. Photo: Lee Lazzara

February was amazing! but March is here… there’s still plenty of winter left. Stay safe out there.

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