Avalanche forecast

West Slopes South

Northwest Avalanche Center
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Avalanche danger

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

A dynamic winter storm will sag into the West South zone on Tuesday bringing moderate snowfall, strong winds, and fluctuating temperatures. You could encounter overlapping avalanche problems and quickly changing conditions that will increase the avalanche hazard throughout the day. Throw in some uncertainty around buried weak snow layers and you’ve got the makings for a dangerous day in the mountains where you should be avoiding slopes over 35 degrees.

Avalanche problems

  • Storm Slabs

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

    It might take a while for the storm to reach the West South zone on Tuesday. But as the snow starts to pile up, red flag signs of instability such as shooting cracks, recent avalanches, or whumpfing collapses are the snowpack’s way of letting you know it is unhappy. If you’re not receiving this direct feedback, you can use upslope tests, quick hand pits, or small test slopes to help identify how well the new snow has bonded and if it is failing easily as blocks. A slight bump in temperatures in the afternoon hours could produce some slightly upside-down (heavy over weak) snow that you can look for in your tests.

    That doesn’t mean it’s going to be particularly warm, however. We’re still in the coldest period of the year. You could find that the new snow lacks a lot of cohesion and is acting more like a loose avalanche in sheltered areas. The flip side of this equation is that the winds should be howling most of the day, easily transporting pre-existing and newly fallen snow into thicker Wind Slabs in exposed areas. Regardless of what you want to name the new snow problem you encounter, it will be more likely on slopes over 35 degrees.

    (12/17/2022)

    Blowing snow in the Tattoosh Range near Mt Rainier, Saturday 12/17

    Photo: Andy Harrington

Avalanche Discussion

This is a bit of a tricky forecast for the West South zone on Tuesday. There is a lot of uncertainty surrounding the weather and how it will pan out. The timing and amount of precip received is the major question mark, but there is more certainty that the wind will be a major factor yet again. The storm may not fully impact the zone until the afternoon hours, meaning hazard should be increasing throughout the day. Since Sunday, multiple waves of stormy weather have impacted the West South, bringing low-density snow and strong to extreme winds.  This incoming wave should be the most significant yet and could start to tip the scales toward more widespread avalanche activity. As can often be the case in the West South, we may have a large discrepancy in snow totals across the zone from north to south, volcano to crest. Despite all our uncertainty and the spatial variability, in some respects the forecast is easy. We’re receiving a decent shot of snow, accompanied by strong winds, and we have known weak layers in the snowpack. So that lends to dangerous avalanche conditions likely developing by day’s end. 

The most recent 12/18 interface is the weak layer in the forefront of our minds for Tuesday, and it will receive its first true test. This is the interface you should be targeting when making your hazard analysis. Prior to this storm parade, weak snow in the form of surface hoar and near surface facets was found on most aspects. Some of this may have been wiped out by the winds, but in many areas light snowfall may have buried these weak grains intact. On slopes that are exposed to sunshine, a sun crust was observed prior to the storm and could make a great sliding surface for all of this new storm snow, whether that’s in the form of a loose or slab avalanche. Rimed precipitation particles/graupel were also identified near the 12/18 interface, adding another weak snow grain to the mix.  If avalanches start to fail on these persistent grain types, they could act in surprising ways by breaking above you, wrapping terrain features, or spanning far distances. Again, your best mitigation tool is to avoid slopes over 35 degrees until this storm plays out.

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