Bollettino valanghe

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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In sintesi

The intense warm-up during the past few days has resulted in a number of large wet avalanches—both slabs and sluffs. Cooler temps today will make wet avalanches less likely, but continue to monitor snow surfaces and avoid being on or beneath steep slopes with wet, mushy snow. Weak layers in the top 3 feet of the snowpack are stabilizing—triggering a dry slab avalanche on upper elevation, northerly-facing slopes is becoming less likely but not impossible.

Problemi valanghivi

  • Wet Slabs

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

    Over the past several days, very warm daytime temperatures, intense sunshine, and weak overnight freezes have pumped a lot of water down into the snowpack on all but the highest and most north-facing slopes. As this meltwater makes its way deeper into the snowpack, it can destabilize weak layers (facets and/or crusts) and produce wet slab avalanches. We've observed a number of natural wet slab avalanches, primarily in the Boulder and Pioneer Mountains (photo above/right) where a weaker snowpack exists, but also in the Smoky Mountains.

    This dramatic warm-up is winding down, and temperatures today will be cooler than days past. Still, there's a lot of water lingering in the snowpack and this will take time to drain through the snowpack. So while wet slabs will be less likely today, they're not completely off the table. Wet slabs can be unpredictable and produce avalanches that you want no part of. The best way to manage the problem is by monitoring the snow surface conditions—avoid steep slopes with wet, soggy snow or where you're beginning to punch through previously supportable crusts. Given the significant spring transition we're experiencing, it pays to be on the conservative side—don't overstay your welcome if the snow starts to become wet.

    ADDITIONAL DISCUSSION:The same conditions described above make wet loose avalanches and sluffs possible again today. With cooler temps today, they will be less likely than in days past. Manage this problem the same as the wet slab problem—by avoiding slopes once they thaw and become wet.

  • Persistent Slabs

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

    The persistent slab problem has provided an exciting wrap up to our forecast season. Alternating storms and sunny spells in March produced several layers of facets and crusts. From March 28th to April 4th, there were six human-triggered persistent slab avalanches, several of them triggered remotely (from above, below, or to the side of the slope). The storm early this past week produced a new round of large natural avalanches breaking on these persistent weak layers (also, see photo above/right).

    While the recent heatwave worsened this problem initially—leading to some warming-induced slab avalanches—there are signs that it's beginning to help. With enough time, the warm temperatures help these previously weak layers to bond together and gain strength. This process will be happening most slowly on the highest and shadiest slopes. While triggering a persistent slab avalanche has become unlikely, given its rap sheet over the past few weeks, I wouldn't completely write off this problem on upper elevation, northerly-facing slopes.

Avalanche Discussion

As the Sawtooth Avalanche Center winds down our daily Avalanche Forecasts (the last one is tomorrow), we've all had to buckle our seatbelts and hold on for a wild ride. It's been an exciting past two weeks:

  • COVID-19. The pandemic hit Blaine County and the Wood River Valley hard, leading to a stay-at-home order and the early closure of the Sun Valley Ski Area. There has been a significant increase in backcountry traffic, especially on Galena Summit. This prompted us to write a blog post about backcountry recreation during COVID-19.
  • Human-Triggered Avalanches. From March 28th to April 4th, there were six human-triggered avalanches failing on persistent weak layers that formed in March. All of these were large enough to bury or injure a person. This was the highest concentration of triggered avalanches we've had all season.
  • An M6.5 Earthquake. On March 31st at 5:52PM, a large earthquake struck in our Banner Summit zone during a period of HIGH danger. Incredibly widespread avalanching occurred as a result, primarily in the northern half of the forecast area but extending as far south as Ketchum.
  • An Intense Heatwave. Beginning earlier this week, we quickly transitioned from a cold, snowy pattern to the warmest temperatures of the year. Temps in the mountains soared into the 40s and 50s F, and valley bottoms hit the mid-60s F. Not unsurprisingly, this warm-up led to widespread wet snow avalanching, including some wet slabs.

So what does the future hold? Most of the forecast area is in a spring snowpack pattern where the avalanche hazard is driven primarily by daytime thawing due to warm temperatures and solar radiation. Persistent weak layers, while showing signs of improvement, remain a concern on the highest and shadiest slopes. Late-spring storms and squalls could lead to storm slab instabilities, especially when accompanied by wind.

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