Lawinenlagebericht

Bitterroot

West Central Montana Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

Winds have drifted snow that could be easily triggered at upper elevations today. Assess wind features carefully and avoid loaded slopes over 35º. Weak snow buried 1 to 2 feet deep sits over a slick rain crust formed in early March at elevations between 6,000 and 8,000 feet. Avalanches could step down to or trigger on these weak layers and may break wider and deeper than expected. Expect rising temperatures, melting snow, and an increased possibility of wet avalanches on slopes facing the sun if it comes out. 

Lawinenprobleme

  • Wind Slabs

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

    Productive westerly winds over the past 24 hours have drifted snow onto easterly aspects. Yesterday we observed natural wind slabs near Gash Knob. It’s likely that fresh wind slabs will linger and remain reactive today. 

    Look for clues like wind textured snow, drifts, and wind lips to indicate wind direction. Hollow-sounding snow and shooting cracks are signs that wind slabs are nearby and reactive. 

    Avoid wind-loaded slopes with angles over 35 degrees.

    Large cornices linger over east-facing ridgelines. They deserve space and respect. Avoid traveling underneath them.

    03/16/2022  A small (D1) natural wind slab avalanche, on a leeward east-facing slope near Gash Knob, 7,600 feet, in the central Bitterroot.

    Photo: Ryan Sorenson

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1.5–2.5

    It may be possible to trigger avalanches on near-surface facets sitting over the slick rain crust formed by the atmospheric river event in early March. They are buried 1 to 2 feet deep. Elevations between 6,000 and 8,000 feet where the crust provides a bed surface and temperatures remained cold enough for facets to linger are most suspect. We experienced whoomphing in the central Bitterroot yesterday, and recent column tests show that it is reactive.

    As these layers get more snow on top and warm temperatures and wind stiffen the overlaying snow into a slab, it will become more reactive. There is always a fair amount of uncertainty with persistent weak layers. Where they exist and the degree of their reactivity varies drastically throughout the forecast area. You could find it on multiple aspects. The weight of you or your machine may trigger persistent slabs, and avalanches can break deeper and wider than anticipated. The potential exists for wind slab or cornice fall to step down to these layers. These layers tend to last, and given the recent test results and observations, it will take some time to round out and bond. Until then, it’s best to avoid areas you may trigger it-convexities over 35°, and carefully consider the consequences of triggering an avalanche before stepping into avalanche terrain -slopes over 30°.

  • Loose Wet

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

    It’s March and the sun is high in the sky. If it comes out expect rising temperatures and melting snow on slopes that receive its rays-southerly aspects. 

    There is a possibility that water trickling through the snowpack could lubricate the facet layer sitting above the rain crust formed earlier this month, causing it to release as a slab avalanche on the crust. 

    Stay off slopes that you are sinking past mid-boot in wet sloppy snow. Plan your exit from the mountains to avoid passing under or across wet saturated slopes and stay alert for overhead hazards.

Avalanche Discussion

Showery snow squalls brought 4 to 12 inches of fresh snow and productive moderate to strong westerly wind since our last forecast. Snow totals are variable across the forecast area. In the past 48 hours, SNOTEL’s show 0.6 SWE for the Seeley zone, 1.0 SWE for the Rattlesnake, 1.3 SWE at Lolo, 1.0 SWE for central Bitterroot, and 0.7 SWE for the southern Bitterroot. Temperatures fluctuated from the low to mid-30s °F on Tuesday with snow lines in the 5,500 to 6,000 feet range. Yesterday's temperatures were in the mid to upper 20’s °F range with snow lines falling to the 3,500 to 4,000-foot range. 

It was snowing an inch an hour yesterday morning at the Gash and reports indicate similar squalls in Rattlesnake and Swan. During and after the squall strong westerly winds were transporting a lot of snow onto easterly aspects. We observed two natural wind slabs on east slopes between 7,800 and 8,000 feet. Recent reports indicated easily triggered wind slabs near ridgelines and storm slabs in the trees in Rattlesnake and reactive cornices and wind slabs at ridgelines in the Swan. 

The snowpack below 6,000 feet is well consolidated and shallow in many areas. From approximately 6,000 to 7,500 feet the fresh snow sandwiches low-density snow and near-surface facets sitting above the rain crust formed from the atmospheric river in early March. We experienced a whoomph on low-angle terrain an east aspect near 6,800 feet at the Gash and similar collapses were reported across the forecast area. Recent column and propagation saw tests on this layer show that it may be possible to trigger a slide on these facets. Above 7,500 feet the crust gets thinner and deeper, and freshly deposited wind drifted snow becomes the primary concern.

Please let us know what you are finding out there. You can submit observations here.

Ski and ride safe.

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