Bulletin d’avalanche

Stevens Pass

Apr 7 – 8, 2019Northwest Avalanche Center
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Danger d’avalanche

Tue
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

You can trigger avalanches in the recent snow that fell since Friday. Expect predictable, loose wet avalanches on steep slopes and some isolated wind slabs near ridge tops. Light precipitation with warm temperatures will continue for Monday.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    Wind slabs will be very localized to leeward slopes near the top of ridges. You can find fresh cornices and drifts up to 2 feet deep. Be especially aware of recently drifted snow if you plan on traveling to peaks above 5,500 feet or well into the above treeline band. Wind slab avalanches are becoming harder to trigger. Watch for slopes over 35 degrees where you see cracks in the snow or areas of snow that feel stiffer underfoot. Steer around obvious drifts and avoid being on or under cornices.

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    You may be able to easily trigger loose wet avalanches in the recent snow on steep slopes (40 degrees and steeper). If the sun comes out, you may see natural avalanches on sun-exposed slopes. Most loose wet avalanches will be small and predictable, involving the top 1 foot of the snowpack. It’s important to respect these avalanches as they can be heavy, forceful, and could be dangerous if they catch you off guard.

    If you don’t have experience with these avalanches, consider staying off of slopes steeper than 35 degrees. Use small test slope to check the bond of the recent snow. Make sure other backcountry travelers don’t trigger avalanches on top of you. Stop and regroup in safer areas, out from under slopes that are being traveled on.

Discussion

Snow and Avalanche Discussion:

About 1 foot of snow accumulated above 4,000ft on Stevens Pass since Friday. Westerly wind formed drifts up to 2 feet deep. On Sunday, observers reported a widespread cycle of natural and triggered loose wet avalanches in the recent snow on steep slopes above 4,000ft and on all aspects. One slab avalanche occurred on an east aspect, 5,000ft near the ridge at Gemini Pass. The recent snow is sitting on a thin crust over weak, wet snow.

Be sure to consider all the hazards that come with spring in the mountains. Factor in a good margin for error as hard to predict events like cornice fall, glide avalanches, icefall, rockfall, and a general “shed cycle.” When surfaces freeze and become firm, bring the right equipment to mitigate the hazard of sliding down icy slopes.

Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg

Snowpack Discussion

April 3rd, 2019

Spring snowmelt

The snowpack in much of the Cascades has changed dramatically in the past two weeks. The weather has shifted solidly to spring-like patterns. The spring warm-up started in mid-March with a prolonged period of relatively clear skies and warm temperatures. Moving into April, we’re seeing periods of unsettled spring weather bringing rain to many low and mid-elevation slopes and snow to upper elevations.

Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg

Since the peak height of snow in mid to late February, mountain weather stations in the 4,000-5,000ft range show an average of 27% decrease in height of snow. Looking at weather stations in nearly every zone, the percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. Traveling in the mountains the loss of snow coverage is most noticeable on southerly, sun-exposed slopes and below 4,000ft. On northerly aspects and slopes above 5,500ft, the snowpack has seen less dramatic changes and has even maintained some dry layers.

NWAC climatological snow depth data from April 1st. You can view it on our website here.

 

A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus

 

Spring avalanche considerations

As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:

  1. Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?

  2. What are the high and low temperatures of the past 24 hours as well as the forecasted temperatures during the time you’ll be in the mountains? Could these create weak, wet snow surfaces?

  3. How is the cloud cover contributing to the melting or freezing of surface snow? Did clear skies allow for a sufficient overnight freeze? Will the sun be strong enough to weaken surface layers?

Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg

Other considerations

In addition to daily avalanche hazard, the early snowmelt is creating some other travel considerations. Some roads and lower elevation slopes may not have enough continuous snow coverage for travel on snow machines. Holes melted around rocks, trees, and creeks could create a fall hazard. When nighttime temperatures and cloud cover allow for surface freezes, bring appropriate equipment to mitigate slip and fall hazard on steep slopes.

We are approaching the end of our daily avalanche forecasting season. The mountain weather forecast will continue into the spring, and the weather station data is available year round. Keep checking the forecast for conditions updates on the end of season information.

Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer

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