Lawinenlagebericht

Bitterroot

West Central Montana Avalanche Center
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Lawinengefahr

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

Large wind slab avalanches are possible on upper elevation north and east aspects today. Larger and more destructive avalanches could also break deeper on persistent weak layers buried in the middle of the snowpack at upper elevations. Signs of instability in wind loaded terrain are cracking, collapsing, and recent slab avalanches. Use safe travel practices to reduce your risk. If you encounter any red flags or want to avoid avalanche problems completely, stick to slopes less than 30 degrees.

Lawinenprobleme

  • Wind Slabs

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

    Large wind slabs are possible on upper elevation north and east aspects today. Anticipate small, fresher wind slabs that formed yesterday, especially in the Bitterroot. Triggering a 1-2 ft deep wind slab formed within the last few days is also possible. A wind slab avalanche could step down and trigger a larger persistent slab avalanche, as described below.

    Wind slabs can be found on or just below the downwind side of exposed ridgelines, behind rocky outcrops, on rolls or convexities, or in cross loaded terrain features such as gullies. They can look smooth, rounded, and pillowy.  Snow will feel more dense and consolidated underneath your skis or sled than surface snow that has not been wind affected. It may even be hard and sound hollow, like a drum. Be on the lookout for rippled, uneven snow surfaces, indicating that wind has transported snow, and wind slabs may be lurking nearby.

    Red flags include cracking, collapsing, and recent slab avalanches. Cracks shooting out from your skis or machine clearly indicate that you've found an unstable slab, however, if the snow is very hard, you may not see cracking before its too late. Stick to wind sheltered slopes less than 35 degrees if you find signs of instability.

    (3/1/2024) An unnamed peak on the divide above the Kootenai Lakes flagging from the SW winds.

  • Persistent Slabs

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

    Although unlikely, you could still trigger a large persistent slab avalanche on all upper elevation aspects today. A variety of persistent weak layers are buried in the snowpack, including rounding facets near the ground, small facets above and below a crust in the middle of the snowpack, and buried surface hoar. 

    Recent snowfall and wind loading throughout the week have been adding stress and testing the strength of the buried weak layers. Additional load from new snow and wind yesterday and today could be what finally tips the scale. Before triggering an avalanche, you will unlikely see obvious signs of instability, such as cracking and collapsing. Smaller avalanches, such as fresh wind slabs described above, can step down and trigger a larger avalanche on persistent weak layers.

    Dig down into the snow, and if you find softer snow around hard crusts, or loose sugary snow below a firmer slab, recognize the potential for a large and dangerous avalanche. Use your individual or group risk tolerance to dictate how steep of a slope you are willing to expose yourself to. You can reduce your risk by defaulting to shorter, planar slopes with runouts free of surface obstacles and terrain traps. Use slope angle to avoid the problem altogether by sticking to slopes less than 30 degrees.

    (3/1/2024)

    An unstable propagation saw test (PST) result in a snow pit on a northeast aspect at 7900 ft in the Northern Bitterroot. 

    Photo: Arden Feldman

Avalanche Discussion

This week started out with a bang when we got a large storm that dropped 1.5-2 feet of new snow. We then saw continued small storms that incrementally deposited a few inches here and there. An additional 1-2 inches are forecasted for today. As we close out the week, SWE (snow water equivalent) totals since Sunday (the 25th) morning are:

Seeley Lake (North Fork Jocko SNOTEL): 3.1" SWE

Rattlesnake (Stuart SNOTEL): 2.2"

Northern Bitterroot (Lolo SNOTEL): 3.2"

Central Bitterroot (Twin Lakes SNOTEL): 4.6"

Southern Bitterroot (Saddle Mountain SNOTEL): 2.2"

In the last 24 hours, the Bitterroot saw the most precipitation. Lost Trail ski area reported 5 inches of new snow. Unfortunately, the Point 6 weather station (our best upper elevation wind data for the forecast region) has been rimed up and not functional since Friday morning. Alternate wind data showed light-moderate wind from the south overnight last night throughout the forecast region. Observations from the Bitterroot yesterday confirmed gusty winds predominantly from the southwest. Today will have light wind from the west with 1-2 inches of low density snow, favoring the northern end of the forecast region.

With continued new snow and wind, wind slab avalanches remain a concern. Fresh wind slabs will be small and confined to upper elevations, predominantly in the central and southern Bitterroot. You may encounter older, deeper wind slabs lingering in atypical locations such as middle elevations or cross loaded terrain features from very strong wind that blew on Thursday and Friday night. Learn to recognize the signs of wind transport, such as wind scouring, flagging, rippled or uneven snow surfaces, and cornice development. These signs can tell you where wind loaded terrain lurks nearby.

The Bitterroot has seen the most significant load this week, and observations showed more sensitivity in buried weak layers within this zone. Two observers marked the crowns of natural avalanches that may have released recently within these layers. Although the little snow forecasted for today and recent snow from yesterday is unlikely to add any more substantial load, it could be just enough to finally cause failure if triggered. If you were to trigger a persistent slab avalanche, it would be large and consequential.

Be on the lookout for sluffs of dry loose snow today. Although small, a sluff could knock you off your feet or machine and carry you into hazards such as rocks, cliffs, and trees.

We'd love to know what you see out there today! Please submit an observation.

Ski and sled safe!

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