Lawinenlagebericht

Seeley Lake

West Central Montana Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

The hazard remains moderate today, but the complexity of the problems has increased in the past 72 hours. Wind slab remains an issue and may be harder to detect due to new snow masking previously developed wind slab. Near surface facets have developed in the central Bitterroot and were a sliding layer yesterday. Facets remain a concern on specific slopes in the Rattlesnake and southern Bitterroot. It is possible to trigger a slab on multiple layers, dependent on where you are touring. Careful assessment and terrain management are required today.

Lawinenprobleme

  • Wind Slabs

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

    Winds have been mild, and minimal new windslab has been built in the past 48hours. However, where present it is showing some reactivity. Older reactive wind slab is still present in some areas and may be covered by a layer of low-density powder, making it more difficult to identify.

    Winds are generally mild today, except strong gusts may build new windslab as the day progresses.

    Look for cornices, wind lips, and drifts to indicate where wind slabs are present. Avoid wind-loaded slopes over 35º.

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2.5

    There are three separate persistent problems in different portions of the forecast area.

    In the central Bitterroot, shallowly buried near surface facets failed and propagated 40 feet wide in steep terrain (>40º) yesterday. The slide was of relatively low consequence, but the destructive size will quickly grow with additional loading from wind or snow. . Avoid slopes over 35º where these facets are found. 

    In the southern Bitterroot facets in the old snow are still reactive. They have been identified on northerly aspects above 7400 feet; however, they could exist outside of this as well. This is a low likelihood, high consequence layer. It’s best not to tease it; the right trigger point could cause avalanches up to 4 feet deep. Avoiding slopes over 30º where these facets are present is the best practice at this time. 

    In the Rattlesnake isolated basal facets exist. These are present at formerly windswept sites where snow remained after the early December thaw and in wind-scoured areas around rocks and outcroppings. Spots like the top of Too Steep to Tele and the south face Burgandy are great places to find these facets. Avoiding these areas is wise until they get buried deeper and have time to round out. 

     

Avalanche Discussion

Since the last forecast up to 6 inches of new snow has fallen, with about 2 inches in the past 24 hours. Temperatures have dropped as low as 0 ºF and will stay low through January 1st. 

Winds have been ligh out of the southwest and are expected to be westerly and light with strong gusts today. 

There has been minimal new load on the snowpack from either snowfall or wind loading, and therefore little change in the hazard. Wind slab has bonded well in many portions of the forecast area, although reactive slabs are still lurking. Shooting cracks were observed in the southern Bitterroot in wind-affected snow, and steep wind-loaded terrain was a concern in the Missions. In the central Bitterroot, reactive wind slab was isolated to ridgetop. Test small representative slopes before stepping into larger terrain. 

Colder temperatures may have promoted near-surface faceting in shallowly buried layers. Yesterday in the central Bitterroot, a 6 inch deep soft slab propagated 40 feet wide on a thin layer of facets in steep (>40º) terrain. These slabs are isolated to steep terrain. They pose a slight risk to skiers unless combined with a terrain trap such as a gully or cliff band, which will increase the depth and consequences. As new snow or wind slab land on this faceted layer, slabs will become more consequential as well.

Yesterday in the Missions, sluffs ran long distances on steep northerly aspects.

The southern Bitterroot continues to have a four-foot-deep faceted layer on shaded aspects above 7400 feet. Slides on this layer may take a large trigger such as cornice fall, or just the right shallow trigger point to fail, making them low likelihood. However, the consequences of a slide on this layer would be serious. Avoid slopes over 30º where this problem exists.

In the Rattlesnake, isolated pockets of basal facets exist in shaded spots like ridge tops and around shallowly buried rocks. There remains a slight possibility of triggering a slab in these areas.

As you travel in the backcountry today, look for signs such as cornices, wind lips, and drifts to indicate where wind slab may be found. It may be hiding under a few inches of low-density powder. Carefully evaluate the snow before stepping into steeper wind-loaded terrain. In general, avoiding wind-loaded slopes over 35º is a good approach. 

Many thanks to the folks who have submitted observations. Please keep sending them in. You can submit observations here: https://missoulaavalanche.org/submit-an-observation/

Ski and ride safe.

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