Posted:
Mar 26, 2020 @ 6:45 am

The avalanche danger is Low  in the west central Montana backcountry. Use normal caution when traveling today to assess the snowpack and avalanche danger.

Good morning, this is Jeff Carty with the West Central Montana avalanche advisory for March 26, 2020. Today’s advisory is sponsored by onX Maps. 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 13ºF to 25ºF and are forecast to reach into the low 30s above 6000’ today. Winds are forecast to be light with moderate to strong gusts out of the west.

In the last 48 hours, we’ve received 0.3-0.7″ of snow water equivalent (SWE) or about 3-7 inches of new snow throughout the forecast area. The Gash had about 4” of fresh at upper elevations yesterday and the area around Chief Joseph Pass had 4-8”. Cooler temps, cloudy skies, and snow showers throughout the day made it feel like winter in the mountains again.

Southern aspects didn’t get much sun the past two days, but it’s been enough to consolidate and begin bonding the new snow. Loose wet avalanche potential could increase with solar warming today, if the clouds part for long enough, but they should be small and manageable. 

North faces have dry new snow that is overlying a variety of surfaces including surface hoar, near surface facets and crusts. The surface hoar under the new snow, where present, is well preserved and will likely persist as a weak layer for a while. Yesterday loose dry avalanches were running fast and far indicating how slippery these surfaces are. The new snow is shallow, has not consolidated and overall slab avalanches are not yet a concern. However, we did kick off a very small 4” deep slab on a steep NE aspect off the Gash indicating there is isolated potential. With further loading by wind or snowfall more consequential slabs could form. Isolated areas may have received enough snow to form thicker slabs and winds may have loaded slopes at ridge top. Pay attention to snow depth and consistency as you tour and check for the presence of a low density band of surface hoar under the new snow. Even small slides and loose dry avalanches can be consequential with terrain traps. 

In the southern Bitterroot yesterday, Travis and Andrew found the depth hoar that formed earlier this season is still persisting at the base of the snowpack and still propagating in snowpit tests. It is unlikely for a skier or rider to trigger an avalanche on this layer, however, a rain on snow event or significant loading could cause it to fail (video).

Large cornices are still lurking out there, and recent cornice fall was observed this past weekend. Avoid traveling below cornices and stay well away from the edge when above.

Glide cracks are opening in isolated areas of the Bitterroot and Missions. These are the result of the entire snowpack sliding on a wet ground surface and they can slowly open for weeks at a time. They can also fail suddenly and unpredictably producing avalanches the full depth of the snowpack. Stay well away from slopes where glide cracks are present.

Bottom Line

Generally stable avalanche conditions exist throughout the forecast area. However, low avalanche danger does not mean no avalanche danger. The main concern is loose dry avalanches on northern aspects. These could entrain enough snow to knock you off your feet and push you into hazards and may require management. Small soft slabs may be present in isolated areas. Loose wet avalanches are possible on southern aspects. Continue to practice safe travel protocols in case you find an exception to a generally stable snowpack. Travel one at a time in avalanche terrain, carry a beacon, shovel, and probe, and stay alert for signs of instability. Dig a pit. Look for red flags

Please take a moment to read this message from Missoula Avalanche, Greater than the Sum of Our Parts.

Your observations are extremely helpful! If you get out, please take a minute to fill out the observation form, or shoot us a quick email at [email protected]

Ski and ride safe.

 

READ FULL ADVISORY  

Problem 1 - Loose 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

    2-3

    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

On northern aspects, loose dry avalanches are possible on steep terrain. These could entrain enough snow to knock you off your feet and push you into hazards. Terrain traps will increase consequences.

On southern aspects wet loose avalanches are possible. Look for rollerballs and pinwheels. They are clues that the snowpack is changing. Move to lower angle slopes or more shaded aspects.

Problem 2 - Surface Hoar

  • 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

    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
    NW - Northwest

Northern aspects throughout the forecast area have a layer of surface hoar buried under the new snow. The distribution is patchy as warm temps and wind knocked some of it down in the past week. In isolated areas, it may be possible to trigger small soft slab avalanches on this layer.  Steep convexities on protected north slopes are the most likely place for these to occur. It is not currently much of a concern. However, as we receive more snow and winds load slopes it could become more of a problem. This is something to keep in mind for the weekend with snow in the forecast.

VIDEO

FORECAST & OUTLOOK

Temperatures to the low 30s, winds light with moderate gusts out of the west. Partial cloud cover. Minimal shower activity is expected today with perhaps a few weak convective cells developing very late in the afternoon or early evening. A slightly stronger disturbance is forecast for Friday and thus more widespread shower activity is likely throughout the terrain. Accumulations will be incredibly diverse with some areas seeing little, if any snow while other ranges experience several inches.

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.