Lawinenlagebericht

Bitterroot

West Central Montana Avalanche Center
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Lawinengefahr

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

1 to 2 feet of recent snow and strong winds have created dangerous avalanche conditions. Skiers and riders are likely to trigger large avalanches on all aspects. Upper elevation wind loaded terrain is the most sensitive. Sticking to slopes under 30 degrees without steep terrain above them is the best way to manage the hazard. Extreme cold will exacerbate the consequences of any avalanche today.

Lawinenprobleme

  • Storm Slabs

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

    Over the last couple of days, the forecast area received 1 to 2 feet of fresh snow and strong winds. In many areas, the new snow fell on a weak old snow surface. Large human triggered storm slab avalanches are likely on all aspects today. Upper elevation terrain will be especially sensitive. 

    This problem is combined with wind slab today. Expect sensitive wind slabs on all ridges and exposed terrain. Wind slabs often appear smooth and rounded. Winds shifted from SW to NW to NE during the storm cycle, so all aspects could be wind loaded. 

    Recent avalanches, cracking, and collapsing are all red flags telling you to avoid avalanche terrain. With up to 2 feet of low density snow in areas, loose dry sluffing is a concern in steeper terrain as well. Smaller avalanches could step down and release full depth persistent slab avalanches. Avoid slopes steeper than 30 degrees to manage the hazard today. 

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    1 - 2.5 inches of snow water equivalent has fallen in the last several days, adding significant stress to an already poor snowpack structure. The two primary layers of concern are facets deep in the snowpack, and a layer of surface hoar buried 3 feet down. 

    Very large and destructive human triggered persistent slab avalanches are possible. These avalanches could break up to 5 feet deep and be triggered from above, below, or to the side. Middle and upper elevation northerly terrain is the most suspect. You are more likely to trigger these around shallow areas of the slab, convexities, or buried rocks. Choosing slopes under 30 degrees is the best management plan. Leave a wide margin between yourself and avalanche terrain. The snowpack needs time to adjust to the new load. 

Avalanche Discussion

1 to 2 feet of recent snow combined with strong winds and bitter cold temperatures have created dangerous avalanche conditions today. Over the last couple of days, the entire forecast area has received at least a foot of snow with at least an inch of SWE (snow water equivalent).  The central Bitterroot received the most snow, with totals at Twin Lakes SNOTEL of 26” of snow and 2.5” of SWE. Strong winds shifted around the compass from southwest to northwest to northeast as the storm progressed, and finally mellowed out overnight. 

Observers found unstable snowpit test results and propagating failures on test slopes yesterday. The new snow fell on a weak old snow surface of near surface facets. In some areas, the new snow is not very cohesive and struggles to produce propagation. However, in wind affected areas, the new snow forms a stiffer slab that is more sensitive. 

Due to the cold and lack of visibility, it was hard to get extensive observations yesterday. However, based on snow totals and the known poor structure alone, it’s easy to conclude that the snowpack needs time to adjust to the new load. High spatial variability of persistent weak layers exists throughout the forecast area. We have multiple buried layers of surface hoar, depth hoar, facet/crust combinations, and buried near surface facets. Northerly aspects have the worst structure, but all aspects have layers of concern. 

The slab above these weak layers is now at least a foot thicker and stressed by the additional weight. Layers like these are notoriously unpredictable, but they are currently primed to fail. Although natural avalanches are unlikely, the weight of a single person may be enough of a trigger. An avalanche on these layers will be very large (4-5 feet deep) and destructive and has the potential to run full track. These layers can be remotely triggered from above, below, or to the side. Leave a wide margin between yourself and any steep terrain around you. Smaller avalanches in the storm snow could step down and trigger larger persistent slab avalanches. Signs of instability like recent avalanches, cracking, and collapsing are significant red flags. Careful route finding and attention to conditions are necessary to travel in the mountains today. Choosing slopes under 30º is the best plan to manage the hazard. 

The avalanche danger scale operates on a spectrum, and today, we are at the upper end of Considerable. The lack of extensive observations brings some uncertainty to the forecast. High danger may exist in isolated areas that received more snow and wind. Use caution and be prepared to take a step back if you find evidence of significant avalanche activity. 

The winds have died off this morning, but temperatures are near record lows. Even with sunny skies today, temperatures will still struggle to rise above zero Fahrenheit. Think about the consequences of your travel decisions today. Even a small injury or equipment issue could prove deadly. Tonight is not the night to be stuck out overnight. 

If you find good visibility today and see evidence of recent avalanche activity, please let us know by submitting an observation. 

Ski and ride safe. 

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