Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

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

The likelihood of wind slab avalanches will increase throughout the day. Avoid wind loaded pockets below ridgeline and along the edges of gullies at upper elevation. It is possible to trigger an avalanche on buried surface hoar in the northern Bitterroot and Lolo Pass area on all aspects at middle and upper elevations. Avoid convexities and slopes over 35º. In the southern Bitterroot, deep persistent slab remains a low likelihood, high consequence concern. Avoid upper elevation north aspects over 35º.

Avalanche problems

  • Wind Slabs

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

    Moderate winds with strong gusts out of the west have built windslab on upper elevation easterly aspects that will grow throughout the day.  Just below ridgeline and along the leeward side of gullies is the most likely place to find wind slab currently.  As this evening's storm arrives, winds will increase, windslab will grow more rapidly, and become more widespread in distribution.

    Cornices, wind lips, and drifts all indicate wind direction and transport. Wind slab will be stiff and hollow sounding. Shooting cracks are a red flag that avalanche conditions exist, but you may not see them until you are on the slab and it is too late. Test small safe, representative slopes before moving into bigger terrain. The best way to mitigate this problem is to avoid wind-loaded slopes over 35º. 

    (1/20/2023) Photo: Jeff Carty

  • Persistent Slabs

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

    In the northern Bitterroot, surface hoar, now buried up to 17" deep under the new snow, was propagating in stability tests on all aspects Wednesday. In all pits we dug, it was laying flat, rounding, and not as touchy as well preserved surface hoar. We were not seeing obvious signs of instability like shooting cracks or whumping, so it gave the impression of stability, despite the lurking weak layer.  Given the persistent nature of buried surface hoar, triggering a slide is still possible. The best way to manage it is to avoid slopes and convex rolls over 35º. 

    Isolated pockets of surface hoar exist throughout the central Bitterroot, and they may catch you by surprise. Despite a week of observations indicating stable conditions throughout the central Bitterroot, skiers triggered a D1 slide, mid-slope on a 37º rollover, on Wednesday.

    In the southern Bitterroot,  buried surface hoar in sheltered north terrarin is 4"-8" deep but lacks a cohesive slab above it. With the arrival of new snow and loading by wind, it may become a problem. 

    If you choose to ski steeper terrain, carefully assess the snowpack for a lower-density stripe of buried surface hoar. Use small, safe, and representative convex rolls to test the reactivity of the layer to skier travel. Avoid traveling in or above terrain traps. You probably won't get red flags like shooting cracks or whumpfing, but if you do, step back to mellower slopes. 

    (1/20/2023) Photo: Jeff Carty

  • Deep Persistent Slabs

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

    In the southern Bitterroot, well developed facets can still be found at the base of the snowpack on upper elevation north aspects. This area has the shallowest snowpack, and as a result, the basal facets are not healing as we've seen in other parts of the forecast area. There have been no recent avalanches in this zone, but the structure is too poor to trust. It is unlikely to trigger this layer, but it could happen with just the right shallow trigger spot or a larger impact like a sledder trenching while high marking.  The best way to manage this problem is to stick to slopes under 35º and avoid shallow areas in upper elevation north aspects that may be trigger points, such as around wind-swept ridges and rocks. 

Avalanche Discussion

Since the last forecast, some snow has trickled into the northern portion of the forecast area. The Seeley zone received around 4”, the Rattlesnake around 3”, and the northern Bitterroot closer to 5”. The central and southern Bitterroot was passed over and received no new snow. 

Winds picked up in the same areas that received snow, and fresh windslab was building in Seeley the Rattlesnake and northern Bitterroot. The central Bitterroot had light snow transport at ridge top yesterday, while the southern Bitterroot remained calm. 

The Seeley and the Rattlesnake have mostly uniform conditions, with buried surface hoar and winds slab throughout each zone. The Bitterroot feels like three distinct zones currently, with widespread surface hoar buried up to 17” deep in the northern Bitterroot, generally stable conditions in the central part, and a shallow snowpack with lingering, deep persistent slab in the southern portion.

Today, our primary avalanche concern has shifted to windslab. It will continue to build with moderate winds and strong gusts. With the storm's arrival, this evening winds will increase to over 30mph throughout the forecast area. The avalanche hazard will rise to considerable in wind loaded terrain on Sunday. 

We’ve removed loose dry avalanches from the problem list. The surface snow has consolidated enough to limit sluffing to very steep and isolated terrain.  

Public observations are very important to understanding conditions. Please submit an observation and tell us how much new snow and wind you see over the weekend. 

Ski and ride safe

 

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