Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Thu
Alta quota
4 · Forte
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Two to three feet of new snow + gusty winds = very dangerous avalanche conditions.  Avoid all wind-loaded avalanche terrain. Be very careful on and near all steep slopes - there's a real possibility of triggering a slide large enough to bury people. If the clouds part, smaller avalanches could 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.  

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–3

    With 2-3 feet of snow in the past 2 days, we're facing a variety of "new snow" problems:

    • Wind slabs: Expect to find soft and stiff, 2-4 foot thick wind slabs in exposed terrain. They will be most widespread near ridgelines at upper elevations, but yesterday'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: Chris and Ben noted an "upside-down" feel to the storm snow yesterday in the northern Sawtooths and Copper Mountain area. With so much new snow, you're likely to find some mid-storm layers that are capable of producing avalanche activity on slopes completely sheltered from the wind. 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 between the old surface and the storm snow. 
    • Loose snow avalanches: You're likely to trigger dry loose slides (sluffs) within the new snow on slopes approaching about 40 degrees in steepness. If the sun shines through the clouds, expect some natural loose snow activity. The April sun can quickly cause things to quickly 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 likely grew quite large in the past 2 days. Stay away from the edges, and give them a wide berth. 

    Approach all steep slopes with caution today. 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 more forgiving terrain. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    Weak layers of snow buried in the upper 4 feet of the snowpack produced two human-triggered avalanches last weekend (photo, photo). Both of these avalanches failed before the heavy load from the recent storm. The faceted layers responsible for 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. I have to admit I didn't expect to be writing about this topic at 6:00 AM this morning. I also have to admit it's refreshing to think about something other than COVID-19 for a moment. 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 crews. This was the deadliest known natural disaster on Mount Everest.  
  • What about aftershocks? Several aftershocks occurred in the minutes and hours following the major earthquake yesterday. Some of the recorded aftershocks were stronger than documented earthquakes that produced avalanche activity. The USGS expects many MW 3-4 aftershocks in the coming days, and an event larger than the MW 6.5 cannot be ruled out. 
  • 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 in the hills.  

Are there some slopes that, without yesterday's earthquake, would be teetering on the edge of avalanching but did release because of the earthquake? Probably. Can we assume that the overall snow stability, over a large scale, is now better because of the earthquake? Unfortunately, not with any certainty.  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 see 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 giant 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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