Avalanche forecast

Seeley Lake

West Central Montana Avalanche Center
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Avalanche danger

Sun
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Fresh wind drifts may be sensitive to the weight of you or your machine today. The wind has been chiefly light to moderate out of the southwest loading northerly aspects across the forecast area. Shifting winds may have loaded various slopes in the Missions, Swan, and Seeley Lake zones. Avoid wind-loaded slopes over 35º. Persistent weak layers continue to be a problem with avalanches possible on facets in the Rattlesnake and central Bitterroot. Deep old, sugary, weak snow is buried 4 to 5 feet down at upper elevations in the southern Bitterroot. Triggering a slide on this layer is unlikely. However, where the slab is thinner, like steep rocky alpine terrain or areas of previous wind scoring, triggering is still possible. An avalanche on this layer would be large and destructive. We recommend avoiding slopes over 30º on upper elevation in the Southern Bitterroot until this layer is buried deeper and has time to round out.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    Plenty of snow remains available for wind transport, and the potential for triggering slabs of drifted snow 1 to 2 feet thick is possible on leeward aspects. We've had light to moderate winds out of the southwest across the forecast area. Areas of the Missions, Swan, and Seeley Lake zone likely saw recent periods of moderate northerly winds, which may have loaded southerly aspects as well. Today, we will have light to moderate southwesterly winds with high ridgetop gusts.  

    Wind slabs may still be reactive on leeward slopes. Cornices above the slope, hollow-sounding pillows of drifted snow, and shooting cracks on small loaded test slopes are signs of wind slabs lingering nearby. Avoid wind-loaded slopes greater than 35º.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    A layer of old, sugary, weak snow is now buried 4 to 5 feet down in the southern Bitterroot. It has been identified on northerly aspects above 7400 feet; however, it could also exist outside of this. This weak layer of old surgery snow hasn’t improved and still shows reactivity in tests. High pressure, cooler temperatures, and snowfall tapering this weekend may continue to weaken these layers at the base of the snowpack, especially where the snowpack is shallow and where facets already exist. This continues to be a low likelihood of triggering, but an avalanche on this layer would be significant and destructive. 

    We now classify the weak snow near the ground in southern Bitterroot as a deep persistent problem. This is the same layer described in the persistent slab problem in previous forecasts. The main difference here, and why we are now classified as a deep persistent slab, is the overall weak layer depth. 

    You may be able to trigger an avalanche on this layer where the slab is thinner, like steep rocky alpine terrain, or areas where previous wind scoring has eroded the overlying slab. Large cornices falling onto slopes, as we saw on Dec. 22nd, and smaller slab avalanches running downslope can also trigger deep persistent slab avalanches.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1.5–2

    We've identified a layer of shallowly buried near-surface facets in the central Bitterroot. On Monday, the thin layer of sugary crystals was the weak layer in a small slide at Gash Point.   Recent observations have confirmed that this layer still exists about 2 feet deep (50 cm) and may remain reactive. This layer is not consistent throughout the Bitterroot but is worth looking for wherever you tour. Check for this layer 20 to 30 inches below the surface, and watch for shooting cracks as you tour. Isolated areas on shaded aspects that are steep and unsupported, like rollovers or cliff bands, are the likely areas where it's possible to trigger. A slide on this layer would be deep enough to bury or injure you.  

    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. The current cold temperatures are promoting further faceting.  Avoiding these areas is wise until they get buried deeper and have time to round out. 

Avalanche Discussion

Artic air-induced uplift has allowed the forecast area to receive light to moderate snowfall, adding about 4 to 6 inches in the past 48 hours. The wind has been chiefly light to moderate out of the southwest for the Bitterroot, Lolo, and  Rattlesnake. Areas of the Missions, Swan, and Seeley Lake zone likely saw periods of moderate northerly winds depending on the extent of the cooler air; however, the dominant flow has remained southwesterly there as well. Temperatures dropped as low as -3ºF but have generally stayed in the single digits to mid-teens throughout. 

For the most part, the snowpack has received the load of new snow well. The snowpack is right-side up (denser underneath and lighter on top), and we have yet to see widespread instabilities or natural avalanches in the past week. 

However, areas of weak sugary faceted snow do exist in the forecast area, and the questions remain; how much has the snow that sits on the ground changed? And will it continue to hold up to further loading next week?

It’s best not to be lulled into complacency by the apparent lack of obvious signs of snowpack instability. Check for layers of sugary crystals if you plan to travel on slopes steeper than 35 degrees, watch for shooting cracks as you tour and look for opportunities such as small safe, representative slopes for tests. 

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/

Thank you, and Happy New Year!

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