Posted:
Feb 18, 2021 @ 6:47 am

The avalanche danger in the west central Montana backcountry is Moderate. Heightened avalanche conditions exist on specific terrain features. Evaluate snow and terrain carefully.

Good Morning. This is Jeff Carty with the West Central Montana Avalanche Center advisory on Thursday, February 18, 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.

Snowpack and Weather

Mountain temperatures range from 8º to 16º and will rise up to 30º today. Between 0.2″ and 1.4″ SWE has fallen, depending on location, in the past 48 hrs. Winds have been moderate and westerly with strong gusts and will be light out of the southwest today. The next round of snow arrives this evening.

Variability continues to be the theme throughout the forecast area with avalanche conditions differing broadly by aspect and elevation. As a result, careful terrain and snowpack assessment are essential for safe travel. 

Overall conditions are improving, with many areas showing good bonding at problem layers. Exceptions to this are areas of:

Persistent weak layers continue to be an issue in many zones. 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. It is possible to trigger large destructive avalanches in these areas from above, below, and adjacent to slopes. North aspects were scoured by north winds last week and rapidly faceted by the frigid cold. Very weak snow underlies the new snow. As this consolidates, larger avalanches will become more likely. 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.

Winds were loading leeward slopes in the Rattlesnake and central Bitterroot yesterday. Wind slabs were cracking and sliding in some places. Avoid wind-loaded start zones over 35º in upper elevations. 

New snow was sluffing easily and quickly in steep terrain yesterday. 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.

The Bottom Line

Heightened avalanche conditions exist on specific terrain features. It is possible to trigger an avalanche on persistent weak layers, windslab, storm slab, and in loose snow today.

Mid elevations and north aspects are the most likely places to find persistent weak layers. Wind slabs are found at upper elevations over 35º. Loose snow and storm slab is an issue on 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.

  • February 24th, 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

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 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

    2-3

    1 (Small)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-4

    Possible/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

West winds were loading easterly aspects yesterday. It is possible to trigger wind slab in leeward terrain over 35º.

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. 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.

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.

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

Here is a video.

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

Problem 3 - New snow

  • TYPE

    loose-dry

    Loose Dry

    Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose-Dry Avalanches,they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose-wet avalanches can trigger slab avalanches that break into deeper snow layers.

  • SIZE

    1-2

    < 1 (Small)

    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

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.

As new snow consolidates the risk of storm slab avalanches increases. Steep convexities are the most likely place to trigger these.

Slope management will be needed on steep, >38º slopes today. In specific locations, sluffs and storm slab avalanches could trigger slab avalanches on the layers discussed above.

VIDEO

GALLERY

St. Joe facets #1, 02/17/2021
St. Joe facets #1, 02/17/2021
St. Joe Facets #2, 02/17/2021
St. Joe Facets #2, 02/17/2021
St. Joe facets #3, 02/17/2021
St. Joe facets #3, 02/17/2021
Rattlesnake facets, 02:17:2021
Rattlesnake facets, 02:17:2021
Lolo wind slab, 02/17/2021
Lolo wind slab, 02/17/2021
Mt. Fuji snowpack, 02/17/2021
Mt. Fuji snowpack, 02/17/2021
02_17_21_Central_Bitterroot snow profile
02_17_21_Central_Bitterroot snow profile
Rattlesnake north aspects, 02_15_2021
Rattlesnake north aspects, 02_15_2021

FORECAST & OUTLOOK

Fair weather is expected today with high pressure. Widespread snow returns this evening into Friday morning with 0.4″ to 0.8″ SWE for the Bitterroot, Rattlesnake, and southern Mission ranges. Periodic snow events are expected through early next week. Snow levels are anticipated to rise to around 5000 feet by Monday with a notable increase in ridgetop winds beginning Sunday.

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.