Posted:
Feb 11, 2021 @ 6:46 am

The avalanche danger in the west central Montana backcountry is CONSIDERABLE in wind loaded terrain and moderate elsewhere. 

Good Morning. This is Jeff Carty with the West Central Montana Avalanche Center advisory on Tuesday, February 11, 2021. This advisory is sponsored by Spark R&D. This advisory does not apply to operating ski areas and expires at midnight tonight. The USDA Forest Service is solely responsible for its content.

The snow continues to trickle in, 0.4” to 0.5″ of SWE in the last 48hrs. Temperatures have stayed cold, preserving light snow. Light snow showers are expected today and up to 6″ of new snow is forecast for the Bitterroots south of Hamilton. Winds have been westerly and moderate to strong at upper elevations. Northerly winds gusting to 40mph are forecast today. There is plenty of snow for transport.

Temperatures will drop today with the arrival of the arctic air mass. Lows tonight of – 27ºF and a high of -10ºF may negatively affect stability as the snowpack adjusts to the rapid drop in temperatures.

Windslab remains a concern. Fresh wind slab is currently building with wind and ample soft snow. Winds are shifting from west to east and northeast, as a result, multiple aspects may have loading. The central Bitterroot may have touchier and larger wind slabs as they are receiving the bulk of today’s snow. It is likely to trigger windslab in steep upper elevation start zones today. Where slides did not occur during the storm much of the wind slab sits on the rain crust/facet combo and has the potential to propagate long distances creating large destructive avalanches. 

Snowpack variability continues to be a theme throughout the forecast area with the new snow sitting on a variety of surfaces and bonding at different rates. Some areas in Lolo, and the central Bitterroot showed good bonding, while other areas in Lolo, the Rattlesnake, the central and southern Bitterroot, and the Seeley Lake area still have buried surface hoar, facets and the January 13 crust/facet layer that have not yet bonded and are capable of producing large avalanches. Investigate the snowpack often as you ascend to get an idea of the problems and instabilities. 

Facets and depth hoar are present where the snowpack was shallow prior to the last storm. These are still reactive and capable of the low likelihood/high consequence failures we have been concerned about for a while. On Sunday, an intentional skier triggered avalanche in the southern Bitterroot failed in depth hoar at the ground.  Collapsing occurred during travel yesterday at mid elevations on Mt. Ward, and in shallow areas in the southern Bitterroot, indicating poor structure capable of avalanching. Similar structure to this exists at mid elevations throughout the forecast area, at all elevations in the Rattlesnake, and in previous shallow spots such as windswept ridges.  Wind slab avalanches and cornice failures may step down to these deeper instabilities. Take care and stay heads up as you travel in these areas. It is best to avoid slopes over 30º where the weak faceted base exists.

Yesterday, widespread natural avalanches that failed over the weekend were visible in the southern Bitterroot. These primarily failed on east slopes indicating windslab. They likely slid on the Jan 13 rain crust/ facet layer.  Where slopes did not fail, weak layers are still present and may still fail with a trigger such as a skier or rider. 

Last week, there were 16 avalanche fatalities in the US, including one in the northern Swans on Saturday. This is the most avalanche deaths in a week since 1910. The snowpack throughout the western US is unstable and precarious, and backcountry use has increased dramatically this year. These deaths are a sobering reminder to make conservative choices. Given the variability of our current snowpack and the complexity this adds to decision making, it is a good time to enjoy the powder on low angle slopes, and allow more time before venturing into avalanche terrain.

The Bottom Line

It is likely to trigger windslab in upper elevation wind loaded terrain today. It is possible to trigger an avalanche on persistent weak layers underlying the new snow. It is possible to trigger avalanches on depth hoar in areas that were shallow prior to the storm. Avoid wind loaded terrain and stay alert for thin areas where basal faceting may exist, investigate that snow often to determine the bonding at the new snow/old snow interface. 

Carry a beacon, shovel, and probe. Remember to reassess conditions throughout the day and stay alert for signs of instability. Dig a pit. Look for red flags.

Upcoming Education Events

Please visit our education page for an up to date list of regional educational events and course offerings. Below are a few select events and opportunities to check out.

  • February 17th, 6-7:30 PM MST | FREE Online 1.5-hr Avalanche Awareness Session | Missoulaavalanche.org event | Delivered by A3 Pro instructors | Get more details and register HERE

Public Observations

Thank you to everyone who has taken the time to send in a public observation. Please keep sharing what you find and see while out in the backcountry. This online forum is a great resource to glean information about current conditions.

You can now text us your observations to (406) 219-5566 when you don’t have time to fill out the observations page. Texted observations won’t get posted on the website, but will be used in the development of the forecast.

Ski and ride safe.

 

READ FULL ADVISORY  

Problem 1 - Wind Slab and Cornices

  • TYPE

    wind-slabs

    Wind Slabs

    Release of a cohesive layer of snow (a slab) formed by the wind.  Wind typically erodes snow from the upwind sides of terrain features and deposits snow on the downwind side.  Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

  • SIZE

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-5

    Likely

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West
    NW - Northwest

Moderate to extreme winds are loading slopes with ample new snow. Winds are shifting from westerly to the northeast, as a result, all aspects may have some amount of windslab, including cross loading.

Wind slabs may be deep and destructive, especially in areas where they sit on persistent weak layers and where natural avalanches did not release during the storm. Human triggered avalanches are likely.

Cornices are growing and fragile. Cornice fall could trigger larger avalanches. Give them a wide berth.

Avoid wind loaded terrain and do not travel under cornices. Stay well away from the top, they can break far back onto flat ground.

Problem 2 - Persistent weak layers

  • TYPE

    persistent-slabs

    Persistent Slabs

    Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks.  Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Persistent, Deep-Slab.

  • SIZE

    5-6

    2-3 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West
    NW - Northwest

Throughout the forecast area, the new snow sits on a variety of faceted layers. We are seeing propagation on near surface facets, buried surface hoar, and the January 13 crust/facets combo. It is possible to trigger an avalanche on any of these surfaces and given the depth of the new snow these would be hard to survive.

Mid elevations and areas that were shallow prior to the storm have weak faceted snow as the base. It is possible to trigger these deeper layers. Shallower avalanches or cornice fall may step down to these deep persistent layers causing large destructive slides. A skier triggered avalanche in the southern Bitterroot failed at ground in these layers on Sunday, and they were collapsing during travel yesterday.

It is possible to trigger an avalanche remotely from the side, below and above.

Avoid slopes over 30º and provide wide margins below and adjacent to avalanche slopes.

VIDEO

FORECAST & OUTLOOK

Light snow continues today. North to northeast winds are currently developing, gusting as high as 40 mph by mid morning. The previously mentioned snow event Thursday into Friday will be shunted south and west by this cold air mass.The Bitterroot crest may still pick up 2 to 5 inches of snow, especially south of Hamilton. Dangerous wind chills of -20 to -30 are possible by Friday with blowing and drifting of snow. There is low confidence of another weather system over the weekend, bringing light snow to area mountains.

This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.