Posted:
Feb 20, 2021 @ 6:16 am

The avalanche danger in the west central Montana backcountry is CONSIDERABLE on wind loaded slopes and moderate on other slopes. Winds and new snow are creating dangerous avalanche conditions on wind loaded slopes.

Good Morning. This is Travis Craft with the West Central Montana Avalanche Center advisory on Saturday, February 20, 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 17 degrees to 26 degrees F this morning. Snotels are reporting between .3 and .7 SWE for the last 24 hours. Winds are predicted today to be out of the west and range 12 mph gusting into the 30’s.

Winds were loading leeward slopes at higher elevations yesterday. We saw active snow transport in the southern Bitterroot, central Bitterroot, and southern Missions yesterday. Overnight winds have increased and created dangerous large slabs that will be easily triggered on leeward slopes. There is plenty of new snow available to transport. Yesterday our sled tracks were completely erased in 2 hours on the ridge in the southern Missions.

New snow was sluffing easily and quickly in steep terrain yesterday in the southern Bitterroot. Yesterday in the southern Missions we found a storm slab in terrain that was slightly wind blown. The new snow was not bonding well and propagating in our tests. While small, these sluffs could knock a skier off their feet. Terrain traps increase the consequences of these slides. In many areas, the new snow is denser than the underlying snow and as it consolidates soft storm slab avalanches are possible, convexities close to 38º are the most likely places for these. Use hand pits and small test pits to assess how the new snow is bonding to the old snow.

Frequent snowpack investigation is required to determine the presence of facets. Snow pits, probing with ski poles, and quick hand pits will all aid assessment. Where facets are found, avoid slopes over 30º. Previously shallow snowpacks at mid-elevations and wind swept areas have multiple faceted layers deep in the snowpack that are failing in stability tests and collapsing during travel. The persistent weak layers in our snowpack are slowly gaining strength but should not be discounted yet.If you get off your machine or skis and punch down deep into weak snow, this is a sign of poor snowpack structure and facets. Dig a pit, see if there is a shallow snowpack.

The Bottom Line

Today choose sheltered slopes. Avoid wind loaded terrain. Use hand pits and small test slopes to see how the new snow is bonding to the old snow. Dig a pit see if the snowpack is shallow 3 to 4 feet deep. Manage your terrain to your advantage. Mid elevations and north aspects are the most likely places to find persistent weak layers. Loose snow and storm slab is an issue on sheltered slopes over 38º. Frequent terrain and snowpack assessment are essential for safe travel.

Carry a beacon, shovel, and probe. Reassess conditions throughout the day and stay alert for signs of instability. Dig pits. 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.

Special Announcements
 
You’ve prayed, hoped, and danced for snow all winter, and it’s finally here! Western Montana’s mountains are busier than ever before, with new and veteran backcountry users enjoying the new snow. The West Central Montana Avalanche Foundation wants to ensure everyone has the information needed to enjoy the mountains and return home safely. To celebrate the return of winter and support west-central Montana’s avalanche forecasting and education programs, we bring you Loving La Niña! This virtual fundraising event includes two outstanding gear packages and two ways to enter to win. Please click HERE for more information about this exciting event. Together, we can save lives and continue creating the most fun, safe, and responsible backcountry community possible. Spread the word, and spread the love for La Niña!

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 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-5

    Likely

    The likelihood of an avalanche resulting from this problem.

Winds have loaded leeward slopes. These slabs will be large and easily triggered by riders today. Avoid wind loaded slopes today.

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 - New Snow

  • TYPE

    storm-slabs

    Storm Slabs

    Release of a soft cohesive layer (a slab) of new snow which breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

New snow is not yet consolidated and was readily sluffing on steep slopes yesterday.

These are small slides but could knock a rider off their feet. Terrain traps such as gullies can concentrate the depth and strength of these slides, obstacles such as cliffs or rocks will increase the likelihood of trauma.

In other areas the new snow is slabbing up and propagating in pit tests. The new is sitting on a cold layer of old snow and not bonding well.

Problem 3 - 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-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

Mid elevations and areas that were shallow prior to the storm have weak faceted snow as the base. These have been collapsing under the weight of skiers and we are seeing propagation in tests. 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.

Where depth hoar, basal, or near surface facets exist, avoid slopes over 30º and provide wide margins below and adjacent to avalanche slopes.

While stability has improved in many areas we still 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 these surfaces.

Frequent snowpack investigation is required to determine the presence of facets. Snow pits, probing with ski poles, and quick hand pits will all aid assessment. Where facets are found, avoid slopes over 30º.

GALLERY

Lolo wind transport, 02:19:2021
Lolo wind transport, 02:19:2021
899EDE23-80BE-49F2-9D17-E0A121CF61E4
899EDE23-80BE-49F2-9D17-E0A121CF61E4
Sluffs in steep Terrain Lost Trail
Sluffs in steep Terrain Lost Trail
Wind Loading from the South Lost Trail
Wind Loading from the South Lost Trail
83886171-8F3D-4131-BF9C-0B47AC848D16
83886171-8F3D-4131-BF9C-0B47AC848D16

Observations 02/19/2021

FORECAST & OUTLOOK

Winds will increase today and through the weekend. Light snow today. Sunday winds increase and 1.5 to 2.0 SWE to Monday. The avalanche danger will rise with new snow and wind. 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.