Bulletin d’avalanche

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Sun
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

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.

Problèmes avalancheux

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    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. These conditions have produced numerous wet snow sluffs, many of which have been large enough to bury or injure a person (photo above/right).

    This dramatic warm-up is winding down, and temperatures today will be cooler than days past. However, with a poor re-freeze at lower elevations, and sunshine in the forecast, wet loose snow avalanches will remain possible today. 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.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    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, which have been more common in the Western Smokys than in the Sawtooths.

    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.

Régions couvertes