Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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

Triggering slides large enough to bury people is possible. You may be able to trigger slides remotely: from flatter terrain above, below, or to the side of a steep slope. Large avalanches are most likely in wind-loaded terrain but are possible on sheltered slopes too. Smaller avalanches may release naturally (on their own) in very steep terrain directly facing the sun. The small slides could trigger larger slab avalanches as they descend slopes.  

COVID-19: The medical system cannot spare the resources required to care for injured backcountry skiers and snowmobilers. Search and Rescue will be delayed or unavailable. Please recreate responsibly close to home, and follow social distance requirements to protect yourself and others.  

Avalanche problems

  • Storm Slabs

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

    Many slides released naturally Monday and Tuesday. With 2-3 feet of snow earlier this week, we're facing a variety of "new snow" problems:

    • Wind slabs: Expect to find soft and stiff, 1-4 foot thick wind slabs in exposed terrain. They will be most widespread near ridgelines at upper elevations, but the recent storm's erratic winds may have built slabs in unusual places lower on slopes as well. Watch for cracks shooting from your boards or sled, a red flag that says "caution, unstable snow." If the snow gets stiffer or deeper while you're traveling, you found a wind slab. 
    • Storm slabs: We observed many natural avalanches that appeared to fail within the storm snow or at the interface between the storm snow and the old surface. Push, pull, and play with the snow as you're traveling to see if it breaks cleanly within the new snow or at the interface. 
    • Loose snow avalanches: You're likely to trigger dry loose slides (sluffs) within the new snow on slopes approaching about 40 degrees in steepness. On slopes directly facing the sun, expect some natural loose snow activity. The April sun can quickly cause things to come unglued; watch for signs things are changing, and think on a scale of minutes rather than hours. 
    • Cornice: Small pieces of cornice could naturally break, triggering wind slabs or loose snow avalanches on steep slopes beneath them. Cornices grew quite large this week. Stay away from the edges, and give them a wide berth. 

    Approach all steep slopes cautiously. Think about the consequences of an avalanche on any slope you're considering skiing or riding. Where will an avalanche carry you? If you would be carried through trees, dragged into a creek or gully, or pushed through rocks, the prudent move is to head for gentler or more forgiving terrain. 

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    Yesterday, skiers remotely triggered an avalanche on an ENE-facing slope near 9300' on Avalanche Peak near Galena Summit (photo). Snowmobilers may have remotely triggered a wide avalanche in similar terrain in the Little Beaver Creek drainage near Smiley Creek (photo); if they did not trigger it, then it released naturally. Weak layers of snow buried in the upper 4 feet of the snowpack produced two human-triggered avalanches last weekend (photo, photo) before the heavy load from the recent storm. The faceted layers responsible for all of these slides exist across the majority of the advisory area.  You are most likely to find these faceted layers on slopes that face E-NE-N-NW. On these aspects, assume the weak layers exist and are capable of producing large avalanches. 

    Triggering one of these slides is more likely in wind-loaded terrain but is possible in sheltered terrain as well. Persistent slab avalanches fail in ways that catch even experienced backcountry travelers by surprise. They may break much wider than you expect, wrapping around terrain features that typically confine wind and storm slabs. These types of slides can be triggered remotely—from flatter terrain above, below, and to the sides of avalanche starting zones. Avoid consequential avalanche paths where these weak layers exist. Build a wide safety margin into your travel plans today. 

Avalanche Discussion

EARTHQUAKES & AVALANCHES

On Tuesday, March 31st, a 6.5 magnitude earthquake shook south-central Idaho just before 6 PM. The quake's epicenter was in the SAC Banner Summit zone, about 10 miles north of Banner Summit proper. We received reports of people hearing significant avalanches running in the Sawtooth Mountains near Stanley, making noise for up to a minute after the earthquake ended. Wednesday, we were able to view many of the hundreds of avalanches that released on Tuesday (links). Our initial observations indicate many of the slides were triggered by the earthquake, especially in the Sawtooth Mtns. Here's a brief FAQ-style overview of the topic:

  • Do earthquakes cause avalanches?  Yes. They occur frequently enough that there is a formal Snow, Weather, and Avalanche Guidelines (Greene et al, 2016) trigger code for earthquake-caused avalanches: NE. Significant avalanche activity has been attributed to large earthquakes in at least 22 cases in the Himalayas, Japan, Russia, the Andes, New Zealand, Alaska, northwestern Canada, in the mainland United States, and in other locations (Podolskiy et al, 2010).  
  • How do earthquakes cause avalanches? The working model for avalanche release involves small cracks initiating within a weak layer and then growing to a critical size where the weak layer collapses vertically. After the weak layer collapses, the slab releases and moves downslope. The motion caused by earthquakes likely causes many cracks to initiate in the weak layers, ultimately producing avalanches. Loose snow avalanches can be "shaken" off of very steep faces, similar to explosives-released loose snow slides. 
  • Have people been killed by earthquake-triggered avalanches? Yes. Fairly recently, an April 2015 magnitude 7.8 earthquake in Nepal's Himalaya Mountains triggered several large avalanches. One swept through the Mount Everest Base Camp, killing at least 22 climbers and support crew members. This is the deadliest known natural disaster on Mount Everest.  
  • What about aftershocks? Several aftershocks occurred in the minutes and hours following the major earthquake Tuesday. Some of the recorded aftershocks were stronger than previously documented earthquakes that produced avalanche activity. The USGS expects many MW 3-4 aftershocks in the coming days, and an event larger than the initial MW 6.5 cannot be ruled out. It's worth considering this "objective hazard" when deciding to ski or ride in significant avalanche terrain this week.  
  • Are steep slopes that were impacted safe now if they didn't already slide? A few documented cases of avalanche activity occurring minutes to hours following earthquakes exist (Singh et al, 2002). Scientists continue to debate whether a 2017 avalanche in Italy that destroyed a hotel and killed several people was a delayed-earthquake event (Geggel, 2018). Limited research and evidence surrounding delayed avalanche activity after earthquakes or large explosive detonations suggests that once several hours have passed from the seismic event or explosion, the snowpack is probably as stable as it was before the earthquake or explosion. Remember that aftershocks are likely to continue jostling the snowpack for days, so it will be difficult to tell when the last "insult" occurred while you're out playing.  

Are there some slopes that, without Tuesday's earthquake, would be teetering on the edge of avalanching but did release because of the earthquake? Definitely. Can we assume that the overall snow stability, over a large scale, is now better because of the earthquake? Unfortunately, not with enough certainty to risk our safety.  We're sailing in uncharted waters. 

Experiencing a strong earthquake at the tail end of a storm that dropped 2-3 feet of snow (HIGH avalanche danger) is a very rare event. We (SAC Forecasters) are as interested as everyone else to observe and study how the snowpack reacted to the earthquake. When evaluating stability gets complex (large storm + weak layers in the upper snowpack + lingering depth hoar at the bottom of the snowpack + earthquakes), we need to base our terrain decisions on simple data and red flags: we just received feet of snow, it was windy, persistent weak layers exist in the snowpack, and the storm continued after the earthquake. If you see evidence of large avalanches, the slopes that released are obviously safer than those that didn't. Other than that, assume the earthquake did not make slopes more stable than they would have been if the earthquake didn't occur. 

 

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