Lawinenlagebericht

Stevens Pass

Mar 17 – 18, 2019Northwest Avalanche Center
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3 · Erheblich
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3 · Erheblich
Unterhalb
3 · Erheblich
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Kernpunkte

Continued warming and sunny skies will create dangerous avalanche conditions in the middle and second half of the day. Large and dangerous natural wet avalanches are possible. Plan your travel to move off of steep, sunny slopes before the surface snow gets weak and wet.

Lawinenprobleme

  • Loose Wet

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1

    Avalanches may begin on east and southeasterly aspects early in the day and continue onto westerly aspects by afternoon. Early in the warming trend, wet avalanches occurred first on the steepest, most directly solar aspects (SE-SW). With each warm day, you may see avalanches continue at higher elevations and on east, west, and even northerly 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.

    The loose wet avalanche cycle has been prolonged and slow to run its course. Many slopes have already produced wet avalanches. There's potential for some large avalanches, possibly gouging into old snow layers. With increased uncertainty around avalanche conditions, it's a good idea to choose conservative terrain. Use extra caution in the second half of the day. 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.

  • Wet Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    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.

    Increased daytime warming is making each day the warmest we've seen since January. This is the second day in a row with a poor overnight freeze. As water melts into the snowpack, dangerous wet slab avalanches are becoming possible. These avalanches are notoriously difficult to predict. When they do happen, they can be large and destructive. 

    A number of interfaces exist in the upper snowpack including sun crusts on southerly aspects and facets and surface hoar on northerlies. Most concerning is a layer of old facets resting on a crust that was buried in early February. This interface is about 3 feet below the snow surface. You can check how deep water is traveling through the snowpack by digging and looking for moist layers of snow.

Discussion

Snow and Avalanche Discussion:

The avalanche hazard will be minimal at daybreak but will increase to Considerable danger by afternoon. Warm temperatures and sun are the main drivers of our avalanche concerns. Keep typical spring concerns on your mind. Important considerations include wet snow, cornice falls, and timing your travel with progressively warming snow surfaces. 

From the 13th-17th, 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 two reports of wet slab avalanches on southerly slopes in the past two days. One at Crystal Mtn and the other on Excelsior Rideg near Mt Baker.

Snowpack Discussion

March 14, 2019

It’s starting to feel a lot like spring. The sun is stronger. Temperatures are warmer. The days are longer. Each year March brings a period of transition for the snowpack. Despite the feel of spring, this transition could take quite a while. Right now, its still winter in the mountains and most of the snowpack is cold and dry.

A northeast through southeast facing bowl in Tunnel Creek, Stevens Pass. The red line shows the transition from northeast to east aspects. Crusts were on the surface of east aspects (right of the line). Surface snow on northeast was soft and dry. March 11th, 2019. Photo: Josh Hirshberg

Aspect dependent snowpacks

A common trend this time of year is the distinct difference between snowpacks on shaded slopes and those on sunny slopes. We might refer to this as aspect dependent snowpacks. The strong March sun can create crusts and send melt-water through the snowpack on sunny slopes.

That process is beginning, right now. As of the 14th, sun-exposed aspects have a crust at the surface. Otherwise, these slopes hold mostly dry layers in the upper 3 feet of snow. The sun now affects more aspects than in the earlier half of the winter. You may find sun affected and melt-freeze crust on any slope that received direct or indirect sunshine. This could include some northwest or even northeast aspects.

Shaded slopes are still holding onto cold winter like snowpacks. That doesn’t mean you won’t find any melt-forms in these areas, but the snowpack looks, feels, and acts more like mid-winter. Avalanche problems can follow a similar trend by aspect. You may see wet avalanche problems on sunny aspects and lingering dry slab problems on shaded slopes.

Two photos taken on March 10th in the Snow Lake area. The left is N facing terrain on Chair Peak with cold, dry surface snow. The right photo shows loose wet avalanches on E facing slopes of Mount Roosevelt. Photo: Ian Nicholson

Balancing problems

So, how do we balance these sometimes opposing snowpacks? In a word, observations. Transitional periods like this require diligence and planning. It seems obvious, but observations you make on a south-facing slope aren’t applicable to N facing terrain. Consider this as you plan your day. Secondly, you’ll want to make observations to verify the avalanche forecast. As we’ve discussed, snowpack structures can diverge during these transitional periods. When your observations line-up with the information you find in the daily avalanche forecast, the forecast is likely valid. If you see observations that don’t match, it’s time to stop, reevaluate, and possibly choose more conservative terrain options.

Making snowpack observations to verify the forecast. Photo: Pete Durr

Less elevated danger and changing trends

You may not find low avalanche danger on every avalanche forecast, but spring is a time of year when less significant hazards can allow backcountry travelers to explore deeper into the mountains. The longer days and often lower avalanche hazard can support stepping into bigger terrain. Remember that low danger doesn’t mean "no danger." Additionally, avalanche conditions can change quickly during spring due to rapid warming over the course of a day or snowfall from potent storms.

We forecast for very large regions. You travel on individual slopes. You may encounter anomalies in the landscape where conditions don’t match the forecast. As with any time you travel in snowy mountains, you can stay safe by using standard travel protocols to minimize your exposure to avalanche terrain and the hazards of the day. Keep your eyes open and be ready to adapt to changing conditions.

What's next?

Winter is far from over. Keep checking the forecast, tracking conditions, making observations, and telling us what you find.

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