Posted:
Jan 16, 2021 @ 6:45 am

The avalanche danger in the west central Montana backcountry is moderate.

Good Morning. This is Jeff Carty with the West Central Montana Avalanche Center advisory on Saturday, January 16th, 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.

A surprise storm that exceeded the forecast amount brought up to 0.7” of SWE to the area, more than 7″ of fresh snow. The northern portion of the forecast area received more than the south. Mountain temperatures range from 24º to 31º F this morning and are forecast to reach 32º above 6000′ today. Winds are light with moderate gusts out of the west.

In the wake of the storm, stability has improved. We’ve seen reports of mid-storm avalanche activity from the Missions, Swans, Rattlesnake and Lolo Pass. More slides likely took place. Cold temps have dried out the snowpack and some bonding has taken place. Unfortunately, it does not seem like we received enough rain to lock up all the problem layers. In some cases, the addition of moisture to the snowpack followed by cold temperatures has promoted further faceting. 

At Lolo, near-surface facets have formed under the rain crust and this is failing on isolation in pits. While not an issue currently it will be something to watch when it gets a load of snow on top of it. Deeper in the snowpack, buried surface hoar is still propagating in extended column tests. While it has gained strength, this layer could fail on steep slopes.

The southern Missions were showing better bonding with no propagation in tests, but the structure is too poor to trust on slopes over 33º yet. Windslab, which is currently forming with fresh snow and stronger ridgetop winds in that area is the main concern. D1-D2 avalanches are possible. 

Reports from the central Bitterroot indicate that the refreeze has resulted in partially stable conditions. Given the variability of the Bitterroot, investigate the snowpack for problem layers and stability where you are skiing.

The Rattlesnake is sketchy! Don’t trust the snowpack. Dense wind crust has formed at ridge top, and rain crust is abundant, but we have nowhere near enough snow to create any sort of bridging effect. The moisture from the rain promoted faceting at all elevations. Buried surface hoar was failing with moderate force down 2 feet. Facets at the base of the snowpack are advancing to depth hoar and failing dramatically in tests. Natural slides in Jenny Bowl had stepped down to deeper layers on low angle sections of the bowl. This is scary moderate hazard territory. If you triggered a slide on the basal facets it could be 3+ feet of hard slab that would be very destructive. Stay off steep terrain in the Rattlesnake. The snowpack will start to heal once we get more snow and build depth, but it will take a while.

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. Avoid anything with soft facets or propagation.

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

It is possible to trigger an avalanche today. Problem layers range from wind slab to deep persistent layers and require continued assessment to manage. Avoid slopes over 30º where facets are present and practice extra caution in the Rattlesnake.

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 worst faceting is in the Rattlesnake.

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 and near surface facets.

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

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-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

Moderate winds and are loading leeward slopes with fresh snow. 

Windslab will be growing with the wind today. These could become touchy.

Upper elevations, just below ridgeline are where these are most likely found.

Watch for pillows and drift like formations. Shooting cracks are a warning sign.

Avoid wind loaded start zones and cross-loaded slopes greater than 35º.

 

VIDEO

GALLERY

Wind damage, Rattlesnake, 01:15:2021
Wind damage, Rattlesnake, 01:15:2021
Rattlesnake test results, 01_15_2021
Rattlesnake test results, 01_15_2021
MtFuji2021--01-15
MtFuji2021--01-15
Icy trees, Rattlesnake, 01:15:2021
Icy trees, Rattlesnake, 01:15:2021
2021-01-15-crust
2021-01-15-crust
Seeley Lake-15-Jan.jpg
Seeley Lake-15-Jan.jpg
Extended column test poster (1)
Extended column test poster (1)
Fractures in depth hoar (2)
Fractures in depth hoar (2)

FORECAST & OUTLOOK

  • Danger Trend

    same

    Same Danger

  • Area Forecast

    Overcast

A low amplitude ridge of high pressure will be over the region today. A low stratus deck will linger over the area for much of the morning potentially causing frost to form. Another disturbance will be making its way across the region Sunday into Monday. This feature will bring 3 to 6 inches along the southern end of the Mission Mtns. While the Bitterroots, Sapphires and over the Anaconda Range should receive up to 4 inches. Another concern will be the increase westerly winds in the higher terrain, especially above 6000 ft as gusts up to 40 mph will be possible.

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.