Lawinenlagebericht

Silver Valley & Bitterroot Mountains

Idaho Panhandle Avalanche Center
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Lawinengefahr

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

Kernpunkte

Steady light snowfall and elevated winds continues to thicken wind-drifted slabs on slopes facing northwest through north to southeast. Traveling on slopes around 35 degrees with about 8 inches of freshly drifted snow warrants careful snowpack evaluations and may increase your chance to find trouble. Be wary of steeper slopes if you encounter signs of unstable snow like surface cracking or audible collapsing. These same slabs are slowly adding more stress to deeper weak snow layers on similar aspects and avalanches in these deeper layers may occur with little warning signs prior.

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2

    Faceted grains lurk around an early-December rain crust. This weak interface ranges 18 to 24 inches deep across our forecast zones and the overlying slab continues to settle and stiffen faster than these weak grains are gaining strength. Most observed natural avalanches from our last storm cycle appear to have failed around this crust. Since signs of instability have wanned in recent days, you may get no warning signs before a slope avalanches.

    Avoid shallow areas near exposed rocks or below cliffs where it may be easier to impact weaker snow below a thinner slab. It may also take a larger trigger like a snowmobile or multiple riders descending the same slope to find the "not-so-sweet" spot and provoke one of these more dangerous slides. Persistent Slab avalanches can behave in surprising ways failing from adjacent to or below a slope. Give steeper, bigger terrain a wider berth and choose slopes with clean, open runouts without gullies or depressions below you.

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

    Since December 23, snow totals range 6 to 8 inches. These wind-drifted slabs sit above a layer of softer snow that’s directly above a thin rain crust from December 18. Steady saltation winds have done an efficient job forming thicker slabs on leeward-facing slopes and in gullies. Triggering an avalanche from a deeper drifted area could break wider and run farther than you expect. Continually assess snow depths as you move around and choose wind-sheltered slopes for safer travel options.

Avalanche Discussion

December 21 marked the beginning of a stormy period that delivered predominantly incremental snowfall to our forecast zones. The Selkirk and Cabinet Mountains came out on top once the dust settled and snow totals range 10 to 22 inches. The Bitterroot Mountains received 6 to 10 inches but higher elevations areas that do well on southwest flow likely received double the snowfall – the Lost Lake SNOTEL (6,110 feet) just south of our forecast boundary reports 18 inches of new snow over the same period. Although the snow, surface refresh is welcomed, the new snow fell during relatively mild air temperatures and is denser than the snow below and well within the range of human triggering.

Newly formed slabs will likely be more reactive and larger in the Northern Mountains that received higher snow totals. Moderate winds and stronger gusts likely broadened slab formation on leeward-facing terrain, and snow safety teams report drifting in unusual places. Pay attention to how soft or stiff the snow surface feels and go around steeper slopes or convex rolls. Slopes with more than about 8 inches of recent snowfall are the more likely spots to find trouble or be caught off guard. Shooting cracks under foot or extending out in front of your machine are telltale signs of unstable snow as well whoomphing sounds.

These slabs are sitting above multiple rain crusts in the top one to two feet of the snowpack and they remain vulnerable to failing in lieu of a smaller avalanche in the storm snow. These crusts vary in thickness and in how advanced faceting is above and below them. However, the common theme we’ve found throughout the region is snowpack test continue to produce failures with varying applied force. The most worrisome failures continue to rear their head in the lower crust from December 7.

As these weaker interfaces continue to get buried deeper, it makes it more difficult to impact them. Finding the shallow spot on a slope, a larger trigger like a snowmobile, or a rapid new snow load seems to be the more likely scenario to increase the chances to trigger a deeper, more dangerous avalanche. Manage your terrain selection to avoid our storm snow problem and you will likely avoid our larger Persistent Slab avalanche problem that always involves a higher degree of uncertainty.

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Avalanche classes are filling up fast! IPAC has a few openings left in a couple of our classes. With the snowpack becoming more complex, it's the perfect time to give a loved one the holiday gift that shows how much you care! Learn more here.

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