Avalanche forecast

Avalanche forecast

Payette Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

The avalanche hazard is Moderate today. Multiple weak layers exist in the upper two to three feet of the snowpack. Persistent slab avalanches will be difficult to trigger but the size would be destructive. The weight of a snowmachine is more likely to impact these deeper weak layers than the weight of a person.

Wet loose avalanches will increase throughout the day. As temperatures warm today, watch for the new snow becoming saturated and sliding easily on slick surfaces like buried crusts. Pay attention to hazards overhead like rocks and cliffs and stay out from underneath steep slopes and gullies.

Avalanche problems

  • Persistent Slabs

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

    In some locations, we have a decent slab overlying a persistent weak layer. This weak layer is composed of 1-1.5 mm facets sitting on or near the interface from the 03/01 storm that produced rain up 7800'. The likelihood of triggering a persistent slab is trending down. However, we have not seen anything that would eliminate this problem completely. On steep slopes that face north, this problem may still be lingering going into this week's warmup.

    Triggering a persistent slab avalanche will be difficult unless you find a shallow spot in the snowpack (see video). However, the consequences are fairly high as an avalanche would involve the top 2 + feet of dense snow.  There is a lot of variability with these persistent weak layers. The sensitivity may vary depending on aspect and elevation. Below 7800' exhibit caution on steeper slopes that face the northern half of the compass. Pay attention to collapsing of weak layers like the sounds of whumpfing. If you experience a collapse, stay off of steep slopes in that terrain.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    Today, temperatures are forecasted to be in the low 40's around 6500'. This Spring warmup is anticipated to continue through the week. With the higher, March sun angle, you can expect to begin seeing wet loose avalanches today. These releases will likely begin with the new snow received from Sunday. On solar aspects, there are several crusts in the upper snowpack that would make a firm, slick surface for wet snow to slide on. Watch for new snow releasing off rocks, cliffs, and other features that absorb solar radiation. If you start to feel the snow getting soft and wet, it's time to head to shaded slopes with cooler, drier snow.

    Oftentimes, wet loose avalanches begin as small point releases and can easily grow in size as they travel and grab more wet snow in their path. (See photo on the right). Pay attention to the terrain as temperatures warm and avoid terrain traps like gullies and avalanche paths.

    (3/19/2022)

    Wet loose avalanche in one of the slide paths off Sawtooth shoulder. Observed 03/19, likely occurred 03/18. West aspect, looks to have started as a point release at about 6600' and entrained more wet snow in the path.

    Photo: PAC

Avalanche Discussion

Today's Advisory is sponsored by Franklin Building Supply just South of McCall on Highway 55. Thank You For Supporting the Payette Avalanche Center!

Since early January, we have experienced little in terms of significant snow. Over the past few months, the avalanche hazard would rise and fall in conjunction with incoming storms. You may remember reading about shallow wind slab avalanches or dry loose avalanches. Within the past week, we have been dealing with a different type of moderate hazard. A persistent slab problem presents different challenges than a wind slab or dry loose avalanche. Persistent slabs are more difficult to trigger and are much more destructive. These buried weak layers lurk under feet of snow and can make it seem as they are not present most of the time. Although triggering an avalanche on these buried weak layers is decreasing in likelihood, persistent slab should be part of your discussion today with your partners. I would be alert to activity on buried weak layers on slopes below 7800' that face east through west on the northern half of the compass. You are more likely to experience collapsing or whumpfing than you are shooting cracks. Collapsing of a buried weak layer is a red flag and you need to adjust your terrain selection accordingly if you experience any red flags.  

For better or worse, Spring is here and the temperatures will reflect it. Temperatures are expected to climb through the week to 5-15 degrees above normal. With full sun, we will undoubtedly experience wet avalanches. Wet loose avalanches are typically fairly predictable and manageable. As the day gets warmer and the snow becomes more saturated, the hazard of wet loose avalanches rises. Something to keep in mind as the week progresses is the other type of wet snow avalanche, and that is a wet slab. Wet slabs are different beasts than wet loose and are much more difficult to predict.  Wet slabs can be much larger and more destructive than loose avalanches.  Why am I mentioning wet slab avalanches in the discussion already?

Unlike dry slab avalanches that fail due to overloading stress of a weak layer (i.e. lots of snow, a skier, a snowmobiler). Wet slab avalanches fail due to decreased strength of a weak layer (think water running through a snowpack and further weakening a buried weak layer). Our snowpack has several key ingredients that leave me suspicious of the possibility of wet slab avalanches later this week.

  1. Persistent weak layers buried within the upper 2-3 feet of the snowpack
  2. A variety of different crusts in the upper snowpack
  3. Abnormally warm temperatures that will lead to saturation of the snowpack

 

As the weather changes this week and becomes abnormal. We need to be thinking of changing avalanche hazards and the possibly of abnormal avalanche conditions.