Lawinenlagebericht

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Veraltet — dieser Lagebericht ist 2 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

You can trigger 15-17 inch thick slabs of new snow on many slopes today. Much larger avalanches involving very dense hard slabs can break 2-4 feet beneath the snow surface and send the majority of the season's snowpack downslope. Both types can be triggered from long distances away from the slope that avalanches. You can trigger slides both in sheltered terrain and where the wind has loaded more snow. If the sun comes out as much as is forecasted, loose snow avalanches will run naturally and be easy to trigger. 

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Yesterday's storm brought 17 inches of new snow and sustained strong winds. This new snow loaded on top of a variety of weak, slick surfaces. At upper and middle elevations, you'll find reactive slabs of new snow large enough to bury you and you may be able to trigger loose snow avalanches that run fast and far on weak, slick surfaces. Natural sluffs will run in the steep, wind-exposed terrain of the Sawtooths down to aprons. These slabs will be larger and more sensitive where the wind has been at work. Natural sluffs will run in the steep, wind-exposed terrain of the Sawtooths down to aprons. Given the sustained strong winds, expect to see slabs of new snow in unusual locations. While slabs will be smaller, expect low elevation avalanche paths to have wind-affected snow in them with avalanches extending down to highway levels in features such as road cuts. Steer clear of terrain traps such as gullies, depressions, and cliffs that can increase the consequences of new snow avalanches even in benign-looking terrain. 

    Importantly, natural and triggered slides involving the new snow can trigger the much larger, more destructive persistent slabs described below. Last night, Ben observed multiple natural storm slabs running on the slick new / old snow interface in the nearby Banner Summit Zone, with at least one stepping down to a deeper weak layer. He also was able to remotely trigger a storm slab from a distance. 

    If the sun comes out as much as is forecasted, it is possible for a round of loose snow avalanches to run on the new / snow interface. These will run far and fast due to the slick nature of the surfaces below the new snowfall. 

    (2/26/2024)

    This slab avalanche released during an intense period of precipitation midday on Monday. These slabs were failing at the base of the new snow, roughly 12-16 inches deep. 

    Photo: VandenBos

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1.5–3

    Several weak layers are buried within the upper 2 feet of the snowpack. With the additional 17 inches of snow that fell and strong winds yesterday, more stress is being added to these weak layers. Multiple human-triggered avalanches involving this snow were reported in the past 10 days (Profile drainage, Alturas Lake Creek, near Dollarhide Summit). This weak snow has produced avalanches on both shaded slopes and slopes facing the sun. On sunny slopes you'll find a sun crust with associated facets. On shaded slopes you'll find a few thin rain crusts with small facets sandwiched in between. 

    This weak layer is highly variable. On some slopes it is well bonded, while on others it is producing large avalanches. You may find good stability on one slope and then trigger an avalanche when you change to a nearby slope. Cracks shooting out from your skis or machine are a clear sign you've found an unstable slab. The human-triggered slides that have failed on this layer have been triggered from a distance (remotely). 

    ADDITIONAL DISCUSSION - DEEP PERSISTENT SLAB: The weak layer that produced widespread avalanche activity in January is now deeply buried here. If you dig down, you'll eventually find it buried underneath a very dense 3-5 foot thick slab of snow. Triggering one of these monsters is unlikely, but the result would be catastrophic. The most likely terrain for making one of these beasts is steep, rocky, wind-affected terrain where the snowpack has thinner spots that give you access to the weak layer. You can watch a video describing this problem here.

Avalanche Discussion

Yesterday's storm dropped 15-17 inches of new snow in this zone on top of a variety of weak surfaces from surface hoar to facets to crusts, along with sustained strong winds. This new / old snow interface has already proven to be reactive, as Ben observed multiple natural storm slabs running on the slick interface on nearby Copper Mountains last night. At least one of these stepped down to a deeper weak layer, and he remotely triggered another from a distance. This new weak layer, now 1-1.5ft deep in the snowpack, is just our newest interface to trigger at the top of the stack of slabs and weak layers that currently make up our snowpack. These storm and wind slabs will be most reactive where the wind has affected the snow, but expect these slides to still run in sheltered terrain. The sustained wind also loaded start zones that typically don't get loaded all the way down to roadway levels. These paths can run into terrain traps, like roadcuts, creeks, depressions and gullies, and increase the consequences of what may be smaller slides. If the sun comes out as forecasted today, solar-initiated loose avalanches can run on slick surfaces, gaining speed and mass as they run downhill. 

And this is just what lies near the surface. What does the rest of our slab-weak layer-slab-weak layer (etc.) stack — or snowpack — look like? Let's take a peek, from the top down:

Underneath our new storm snow and weak snow beneath it (2/25 weak layer) described above, we have roughly 2 feet of settled, dense snow from our storms in February. Below this slab, a variety of weak surfaces formed in different locations, from rain crusts, to sun crusts, to facets that developed at the end of January (2/1 weak layer). A smattering of natural and human-triggered avalanches involving this layer clearly indicate that it is capable of making avalanches, but stability varies significantly from slope to slope. On some slopes, you'll find a strong bond at this interface, while neighboring slopes may be harboring an unstable slab+weak layer combination. Avalanches that fail on this layer will break widely and will be plenty large to bury you. 

Closer to the bottom of the snowpack, we have a thicker layer of rotten, faceted snow that sat out during our December drought (1/6 weak layer) and has a dense, thick slab overlying it. While this weak snow is so deeply buried it is unlikely to trigger avalanches on, know that a slide running on this layer would almost certainly be unsurvivable with 3-5ft of snow moving downhill with you. 

Our slab-weak layer-slab-weak layer stack here in Idaho can create some excellent days skiing and riding out in the mountains — with cold temperatures and just a bit of sun, today will certainly be one of them. With that said, it is just as important to keep in mind what lies below the soft turns as it is to seek them out. The depth of our snowpack is increasing, as is the complexity of it. The intention of the Forecast is to provide you with the information to be able to make informed decisions that keep you safe in the mountains based on information you read here and observations you take in the field. Having a hypothesis of what the snowpack may look like from the forecast, heading into the field, and taking observations on your hypothesis matched up to actual conditions is how we make informed decisions in the hills, and grow as mountain travelers. Enjoy the soft snow, be safe, and continue to have a curious mindset when you head out today.

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