Posted:
Jan 12, 2021 @ 6:45 am

Moderate avalanche danger will rise to conciderable in the west central Montana backcountry today with the arrival of the storm.

Good Morning. This is Jeff Carty with the West Central Montana Avalanche Center advisory on Tuesday, January 12th, 2021. This advisory is sponsored by LB Snow. This advisory does not apply to operating ski areas, expires at midnight tonight, and is the sole responsibility of the U.S. Forest Service.

Weather and Snowpack

Mountain temperatures range from 24 º to 35º F this morning. Winds are moderate. The advisory area has received no new snow.

We are on track for an atmospheric river to pass over us today and tomorrow. Up to 9″ of snow, the potential for rain above 5000’ and winds to 43mph are forecast today. Up to 3” of SWE are expected by tomorrow night. Avalanche hazard will increase with the storm. Of particular concern is the effect of warm temps, heavy snow, and rain on mid-elevations where the snowpack has the worst structure. Wind slab will be growing throughout the day as well. Widespread surface hoar will likely create problems once the storm deposits enough snow to create a slab. 

Variable is the best way to describe the snowpack throughout the forecast area. Minutely variable. Wide-ranging conditions can be found in very close proximity making stability assessments challenging. Yesterday at Gash Point the 6’ deep snowpack at 7300’ on the north aspect was well consolidated with no concerning layers. 500’ lower at 6800’ on the same aspect the snowpack was 3’ deep, comprised entirely of facets. It has truly terrible structure that cannot be trusted. Shallow mid-elevation and wind-scoured areas have similar faceting throughout the forecast area.

In the southern Missions yesterday, a small human triggered avalanche ran on buried surface hoar, but extended column tests (ECT) adjacent to the slide were not propagating. A similar buried surface hoar problem exists throughout the forecast area.

The north aspect of Mt. Fuji over the weekend was showing increased strength and consolidation, while the south face has developed depth hoar and failing in tests.

In the southern Bitterroot yesterday, small wind slabs were touchy, and faceted layers remain a concern.

What this means is you need to be really heads up. Deeper faceted layers will not give warning signs if they fail. Slight changes of aspect and elevation can result in drastically different snowpack and stability. One pit and stability test will not suffice to give you a good handle on problem layers. 

The best way to deal with the myriad of current problems is to investigate the snowpack often and only commit to lines if you can be certain of the distribution of problem layers. Avoid anything with soft facets or propagation. As the storm roles in today dial it back to low angle slopes.

The Rattlesnake may have the shallowest snowpack in the area and as a result the worst structure. Some spots are touchier than others but the whole zone requires caution. Staying under 30º is wise.

Assessing instability in facets can be challenging, and it is easy to fall into decision making traps, especially if it is a problem you are unfamiliar with. Please see this extended column test refresher for some tips. 

The Bottom Line

Stability will deteriorate with snow, wind, rising temperatures, and potentially rain at mid-elevations today. Avalanches will become likely. Various facet, surface hoar, and windslab layers exist throughout the snowpack. In some places, depth hoar and advanced facets are low likelihood/ high consequence problems. Careful, and frequent snowpack assessment is required with the variable snowpack. Sticking to lower angle slopes is the best plan. Expect the avalanche hazard to rise Wednesday.

Travel one at a time in avalanche terrain, 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.

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.

Remember, you can submit your observations through the observation page anonymously. When submitted anonymously, the forecasters review the observation and utilize it when generating the forecast. The information does not appear on the public observation page. 

Ski and ride safe.

 

 

 

 

READ FULL ADVISORY  

Problem 1 - Persistent slabs

  • 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

Faceted layers can be found throughout the forecast area. Shallower snowpacks have worse structure than deeper areas and are more suspect. In some places, persistent slabs are up to 40 inches thick and becoming low likelihood, high consequence problems.

The problem facets tend to be concentrated around crusts. Look for these and soft sugary snow above and below as you assess the snowpack.

In other areas problem layers are in the upper two feet of the snowpack and involve buried surface hoar.

The extended column test (ECT) and the propagation saw test (PST) are best for assessing the stability of these layers.

The touchiness of existing facets will increase with rising temperatures and snow load today.

Where facets are found stick to low angle slopes and avoid traveling under steep terrain as remotely triggered avalanches are possible

Problem 2 - Wind slab

  • 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
    NW - Northwest

Moderate to strong winds and are loading leeward slopes.

Windslab was touchy in some areas yesterday and will be growing with new snow and wind today. These will become a dangerous problem.

Avoid wind loaded start zones and cross-loaded slopes greater than 33º

 

Problem 3 - Wet snow problems

  • TYPE

    wet-slabs

    Wet Slabs

    Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet slabs can be very destructive.

  • SIZE

    3-4

    1-2 (Small-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

Rising temperatures and potential rain make wet avalanche problems possible today at elevations up to 7000′

Just how warm and wet it will get is hard to predict, but any rain on snow is bad for avalanche conditions. Instabilities could range from roller balls to wet slabs.

Above freezing temperatures will loosen bonds within the snowpack and free water can lubricate layers. Wet snow problems are difficult to predict and standard snowpack tests are not effective in their assessment.

Mid elevations are where we have the worst snowpack structure, and the potential for larger persistent slab avalanches, compounding the issue.

As temperatures rise and if rain falls, be very cautious of any slopes over or close to 30º.  Stay well away from the runout zone of avalanche paths.

Rising overnight temperatures could affect elevations to 8000′. Wednesday morning will have the highest hazard of wet slides.

VIDEO

FORECAST & OUTLOOK

While steady precipitation is expected, two periods of heavy rain and snow are expected to impact the area over the next 24 hours. The snow level is still expected to rise throughout the day, approaching 7000 and perhaps even 8000 feet for a short time early Wednesday. Gusty southwesterly winds are also expected, especially ahead of and immediately following the cold front passage Wednesday morning. Liquid amounts of 1.00-2.00 inches are possible in the Bitterroot Mountains, and 0.50-1.00 inch possible in the Swan Range by Thursday morning. Localized amounts of near 3 inches may be possible along the Idaho/Montana border.

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.