Lawinenlagebericht

Bitterroot

West Central Montana Avalanche Center
Auf der Karte ansehen

Lawinengefahr

Sat
Hochgebirge
1 · Gering
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
Open full map
Veraltet — dieser Lagebericht ist 3 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Large human triggered persistent slab avalanches are unlikely at upper elevations today. Small wet loose avalanches will become possible at middle and lower elevations. Signs of persistent slab instability, such as cracking and collapsing, will not be obvious so dig into the snowpack and look for weak snow 1.5-2.5 feet down that sits above a hard crust. Signs of wet snow include rollerballs, pinwheels, and recent wet avalanches. You can use slope angle to avoid the problems entirely by sticking to slopes under 30 degrees. 

Lawinenprobleme

  • Persistent Slabs

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

    Large persistent slab avalanches are unlikely on upper elevation slopes.  A slab of snow 1.5 to 2.5 feet deep rests on a weak layer of small facets above a crust. This was the weak layer in a large, widely propagating skier triggered avalanche in the Seeley Lake zone last week.

    It is doubtful that you will experience signs of instability like collapsing or cracking before you trigger an avalanche on this layer. If you do see these signs, stick to lower angle slopes. You can identify the problem by digging in the snowpack and looking for a layer of weak snow just below recent storm snow from last week and above a stout crust. However, the weak structure may be difficult to identify because the faceted snow crystals are small and rounding in some places.

    There are also still areas with weak faceted snow buried near the ground. We haven't seen an avalanche on these deeper layers since January, but the poor snowpack structure remains. You are unlikely to trigger one of these deeper layers, but a persistent slab avalanche in the upper snowpack could step down and release a much larger avalanche. You will most likely trigger a larger, more consequential avalanche in areas with a shallower snowpack and complex terrain.

    You can use slope angle to avoid the problem altogether by sticking to slopes under 30 degrees. If you do choose to enter into avalanche terrain, use terrain-based decision making. You can reduce your risk by avoiding large avalanche paths that will carry you a long distance or complex terrain with lots of hazards, such as cliffs, that can cause traumatic injury. 

    (2/16/2024)

    A large skier triggered avalanche from yesterday on an east aspect at 7000 ft on the west side of the Seeley Lake zone. 

    Photo: Arden Feldman

  • Loose Wet

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

    Small wet loose avalanches are possible at lower elevations and at middle elevations on warmer aspects today. Signs of instability include roller balls, pinwheels, recent avalanches, and wet surface snow. If you see any signs, avoid steep slopes with complex terrain above or below you.

    Wet avalanche debris can travel far and fan out more widely than expected. Even a small avalanche could knock you off your feet and carry you into hazards such as rocks and trees. It can also carry you into terrain traps such as cliffs and gullies. A sluff of wet snow can step down and trigger a larger persistent slab avalanche in areas with poor snowpack structure, creating a more consequential avalanche.

    (2/21/2024)

    A small wet loose avalanche that initiated from a rock band and ran full path.

Avalanche Discussion

Today will be predominantly warm, clear, and dry before a significant storm system begins entering our forecast region tonight. No significant precipitation is anticipated overnight, but cloud cover and wind speeds will increase with gusts up to 35 mph.

There is more confidence today that the distribution of the persistent slab problem is limited to upper elevations. Recent fieldwork has shown a generally stable snowpack on Lolo Pass and in the Rattlesnake. Continue to be suspicious of upper elevation slopes. Because you are unlikely to see signs of instability before triggering an avalanche, the best way to identify this problem is to dig into the snowpack and look for poor structure. The facets are small (less than 1mm) and will feel slightly softer and weaker than the slab sitting above them (one hand hardness step down). The crust is hard and difficult to penetrate in many places. The most suspect areas are where the overlaying slab is deeper and stiffer.

You are also most likely to find this poor structure in the Seeley Lake zone. The close call last week was on the west side of this zone. Furthermore, this zone has a more similar snowpack to the Flathead Avalanche Center's forecast region, where multiple large, human triggered avalanches recently occurred. The avalanches failed in the same structure: weak, faceted snow above a crust. Follow the links for more information about these incidents: Skier caught, carried, and injured, another skier caught and carried, sympathetic avalanche occurrence.

Much of the snowpack is well settled and will be refrozen after some warm weather yesterday, but keep an eye out for sluffs of wet snow with continued warm temperatures and even sunnier conditions today. While you are unlikely to be buried, a small avalanche could carry you into consequential terrain or step down into a deeper avalanche. Wet snow can be more difficult to escape than dry snow and can travel farther than expected.

We are looking at generally stable conditions across all three zones, but remember, low danger does not mean no danger. There is still some uncertainty around the persistent slab problem, and detection is difficult. Terrain-based decisions are the most reliable way to avoid trouble. Sticking to low angle slopes is the simplest way to manage the problems. But you can choose to avoid complex and consequential terrain. Couloirs, chutes, long avalanche paths, cliffs, and exposed rocks or trees increase your risk. Use smart travel practices like going one at a time, stopping in safe places, and regrouping at critical decision points.

Please submit an observation of what you're seeing out in the backcountry. Snowpit reports will be very helpful for us to further pinpoint the distribution of the persistent slab problem. Reports of stable conditions, weather, and surface conditions are also valuable information.

Ski and sled safe!

Abgedeckte Regionen