Avalanche forecast

Stevens Pass

Mar 18 – 19, 2019Northwest Avalanche Center
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Avalanche danger

Wed
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

The snowpack is undergoing a major thaw. Large and dangerous natural wet avalanches are possible. This is a good time to step back, and allow the mountains to make the transition. 

Avalanche problems

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Avalanches may begin on northeast to east aspects early in the day and continue onto westerly and northwesterly aspects by afternoon.  With each day, you may see avalanches grow larger, entraining older snow layers, and may occur at higher elevations and on all but direct north aspects. Look for signs of instability such as wet, heavy surface snow, rollerballs, and fan-shaped avalanches. If you notice these clues, avoid going on or under steep slopes where loose wet slides may run naturally.

    It's a good idea to choose conservative terrain, or just stay out of the mountains until we get a solid freeze.Many slopes have produced wet avalanches, but until every steep slope has run, theres potential for more with this weather. Some slides may be quite large, gouging into old snow layers. Take note of how much the sun is affecting slopes by aspect, elevation, and slope angle to help you assess which slopes may be most problematic on a given day. If you do choose to go into the backcountry during these times of transition, get out of the mountains early, and off of steep slopes as soon as any superficial crust begins to break down. 

  • Wet Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Record warm temperatures are following an extended snowy and cold period. The snowpack has many layers that could begin to "wake up" as meltwater percolates down to them. The most concerning layer is about 3 feet down where old facets rest above a crust buried in early February. As each day warms and only superficial refreezes occur, larger and more dangerous avalanches may become more likely. The slide observed in Tunnel Creek and others from nearby zones show us that large, unpredictable slabs are possible. This will be the third day in a row with a poor overnight freeze, sometimes considered the magic number for widespread wet slab activity. 

    These avalanches are notoriously difficult to predict. When they do happen, they can be large and destructive. You can check how deep water is traveling through the snowpack by digging and looking for moist layers of snow. As day-time temperatures warm, stay off of and out from under large sun-exposed avalanche start zones, unsupported slopes, and terrain over 30 degrees where meltwater is reaching buried weak layers.

Discussion

Snow and Avalanche Discussion:

After a prolonged period of snow and below average temperatures, we are experiencing a heat wave with record high temperatures. The snowpack is making a transition from cold and dry, to a spring-like pack. Meltwater is beginning to make its way through the upper snowpack. Along with this, there is a high degree of uncertainty regarding avalanche size over aspects and elevations. The only dry snow left is on direct north facing slopes, elsewhere wet snow is commonplace. Cornices are beginning to sag and break off. The most active period for wet avalanches will likely be late in the afternoon.

From the 13th-18th, observers reported numerous natural wet loose avalanches on the southeast face of Lichtenberg Mtn and southwest aspects near the Skyline area and Windy Mtn. A couple of these avalanches were big enough to bury or kill a person. Significant wet avalanche activity has occurred at low elevations in the neighboring East Central zone with road closures due to avalanches in Tumwater Canyon. There have been several reports of wet slab avalanches on southerly slopes in the past three days. Most notably, a large slide into Tunnel creek likely occurring on the 17th in the afternoon. Also, a handful on Snoqualmie Pass, one at Crystal Mtn, Excelsior Ridge near Mt Baker, and one on lower Mt. St. Helens. 

The heat wave is on. Temperature graph from early March through current.

Snowpack Discussion

Coming soon. 

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