Avalanche forecast

Seeley Lake

West Central Montana Avalanche Center
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Avalanche danger

Wed
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Fresh wind slabs at upper elevation are the primary concern today. The southern Bitterroot will have the largest and touchiest wind slab, and slides are likely just below ridgelines. The northern portion of the forecast area will have less wind and snow, but human-triggered wind slab avalanches are possible. Avoid wind-loaded slopes over 35º. Persistent weak layers continue to be a problem with avalanches possible on facets in the Rattlesnake, Swan, and central Bitterroot. Large avalanches on faceted snow at the snowpack base are still possible at upper elevations in the southern Bitterroot.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Fresh snow and productive winds continue to build, and wind slabs can be expected throughout the forecast area today. Temperatures rose higher than forecast yesterday, creating a smooth crust that will allow new wind slabs to slide on easily at mid-elevations. Wind slabs may build over softer snow at higher elevations and shaded aspects. 

    Winds are generally light to moderate today with strong gusts. Upper elevations will see stronger winds, and wind slabs will continue to build throughout the day. The southern Bitterroot will be most affected.

    Look for cornices above the slope, hollow-sounding pillows of drifted snow, and shooting cracks on small loaded test slopes. Avoid wind-loaded slopes greater than 35º.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2.5

    We've identified a layer of shallowly buried near-surface facets in the central Bitterroot and possibly yesterday in the Swan Range. However, the layer was very isolated, difficult to distinguish, and unreactive in our column tests. Last Monday, the thin layer of sugary crystals was the weak layer in a small slide at Gash Point. These layers are not consistent throughout the forecast area but are worth looking for wherever you tour. Check for these layers 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. 

    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. Cold temperatures last week likely promoted further faceting.  

  • Deep Persistent Slabs

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

    A layer of old, sugary, weak snow is 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. Last week, high pressure, cooler temperatures, and snowfall tapering may have continued to weaken these layers at the base of the snowpack, especially where the snowpack is shallow and where facets already exist. 

    The southern Bitterroot is receiving the brunt of the storm, and the deep slab problem is being loaded. Increased caution is warranted where this problem exists-above 7400' on shaded aspects. 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.

    Today, fresh snow and wind may keep this weak snow teetering on the edge of stability. Any avalanche that fails on these layers will be large and destructive. These types of slides may behave in ways that surprise you. They can wrap around terrain features like ridges and may be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. Avoiding these areas is wise until they get buried deeper and have time to round out. 

Avalanche Discussion

A significant storm is barreling down on the forecast area this week. Expect light to moderate snowfall with moderate southwesterly winds and strong gusts. Snow totals are in the 2 to 8-inch range, with the southern Bitterroot receiving the brunt of the storm.

Since our last forecast, we've had high pressure with temperatures in the single digits to mid-teens and increasing southwesterly winds. Loading will increase throughout this storm. Loaded N-NE-E aspects are most suspect; however, local topography variations can affect a slope's wind speed, direction, and loading characteristics.

The temperature moderated ahead of the storm, and temperatures rose higher than forecast yesterday, 01/03/22, with freezing levels as high as 6500' in the central Bitterroot. Lingering wind slabs are likely stubborn but remain possible where colder temperatures lasted. Fresh wind slabs are the biggest concern today. This storm will have variable snow levels and snow densities throughout the week. Expect denser cohesive snow on top of lighter, weaker snow.

Cold temperatures for the past weak promoted faceting throughout the forecast area, and we saw a general trend toward destabilization of the snowpack. Faceted layers became more sugary and less rounded. Near-surface and basal facets are weak, low-density layers that could fail under the weight of new snow. In places where slabs overlaying these layers are thinner, it will be easier to trigger an avalanche involving that weak snow. In areas where the slab is thicker, an avalanche that you initiate will be larger but harder to trigger. How much it will take to overcome these weak layers is unknown. If it does fail, it will create large deadly avalanches.

The hazard will rise with continued snow and wind. As the storm intensifies, it's best to step it back to slopes under 30º, especially in areas of suspected facets.  

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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