Posted:
Jan 19, 2021 @ 6:15 am

The avalanche danger is MODERATE in the west central Montana backcountry today. Human triggered avalanches are possible today.

Good Morning. This is Travis Craft with the West Central Montana Avalanche Center advisory on Tuesday, January 19th, 2021. This advisory is sponsored by LB Snow. This advisory does not apply to operating ski areas and expires at midnight tonight. The USDA Forest Service is solely responsible for its content.

Weather and Snowpack

Mountain temperatures range from 11 degrees to 22 degrees F this morning. Winds are primarily calm today. The advisory area received no new snow overnight.

The snowpack is drastically different from drainage to drainage. This is spatial variability. Yesterday, a skier triggered a slide on Saddle Mountain by hitting the sweet spot on a cornice. The crown was 75′ wide and 1-2 feet deep on a 40-degree starting zone. The slide ran 500 ft of the 1500 ft path stopping in lower angle terrain. In the Rattlesnake, weak layers are propagating in pit tests. In the Lost Horse drainage, weak layers are not propagating but are still present. Gash Point, there are wind slabs at 8000 ft that are easily triggered and in lower elevations, widespread collapsing on buried weak layers. The rain crust seems to peter out around 7500 ft throughout the advisory area.

The definition of moderate danger is HEIGHTENED AVALANCHE CONDITIONS ON SPECIFIC TERRAIN FEATURES. EVALUATE SNOW AND TERRAIN CAREFULLY; IDENTIFY FEATURES OF CONCERN.
What does this mean for our snowpack? Look for signs of wind loading. Dig multiple pits as you change elevation and aspect. Choose sheltered lower angle terrain to get a feel for the snow. Use hand pits and small test slopes to get a picture of the snow.

Bottom Line

Today, choose sheltered lower angle slopes. Avoid slopes with weak sugary snow. Do multiple pits investigating the stripes in your pit wall. Avoid cornices and wind loaded terrain. Choose simple terrain that does not expose you to terrain traps. Avoid likely trigger points on slopes.

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 - Wind Drifted Snow

  • 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

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

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.

Problem 2 - Persitent 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.

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.

VIDEO

Observations 01/18/2021

GALLERY

Sastrugi on Point Six
Sastrugi on Point Six
Rattlesnake-persistent-weak-layers-01_18_2021
Rattlesnake-persistent-weak-layers-01_18_2021
Lost Horse 01:18:2021
Lost Horse 01:18:2021
Crown Skier Triggered Slide Saddle Mountain01:18:2021
Crown Skier Triggered Slide Saddle Mountain01:18:2021
Lost Horse Headwaters-18-Jan.jpg
Lost Horse Headwaters-18-Jan.jpg
20210118LostTrailBC-18-Jan
20210118LostTrailBC-18-Jan
20210116_Lolo_E_Profile
20210116_Lolo_E_Profile

Observations 01/18/2021

FORECAST & OUTLOOK

Thursday afternoon to Friday is the next chance for significant snowfall. See the forecast.

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.