Avalanche forecast

Swan Range

Flathead Avalanche Center
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Avalanche danger

Sun
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Yesterday, riders triggered avalanches in wind-affected snow and possibly on a buried weak layer. Extended periods of sunshine may develop wet snow instabilities. Seek out areas sheltered from the wind for safer and softer snow while staying off of and beneath steep sunny slopes as they heat up.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    A rider triggered a Wind Slab avalanche in the Swan Range yesterday. Southwest winds have been at speeds capable of transporting snow for most of the week and have drifted recent low-density snow into fresh cornices and slabs on leeward slopes at both mid and upper elevations. Today, you may encounter slabs of wind-drifted snow reactive to a rider's weight near ridgelines or prominent terrain features where the wind decelerates and deposits snow.

    Slabs will increase in thickness and become more dangerous as you gain elevation. A rippled surface or dune shape formation may identify these wind features. Older, harder slabs may feel and sound hollow like a drum. Slabs are generally denser than the surrounding snow and may keep you or your machine from sinking into the snow.

    Cracking in the snow surface beneath you is a sign directing you to seek out sheltered terrain for a safer, higher-quality experience.

    Windward terrain or areas sheltered from recent wind offer slopes without a wind slab problem. If you plan on riding steep leeward terrain, choose runouts free of surface obstacles which can magnify the consequences of a slide or terrain traps where the snow can pile up deep.  

     

    (2/4/2023)

    A well-developed mid-elevation cornice on a forested ridgeline.

    Photo: Dundas

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–3

    A motorized party triggered what we believed to be a Persistent Slab in the Swan Range yesterday. This slide fractured above a rider and was around 40 inches deep. Observers continue to find buried facets and surface hoar in snowpits. A Level 1 motorized class had propagating results in 4 of 4 snowpits in the Swan Range Thursday. These weak layers are gaining strength slowly but continue to be a concern. This layer is 1-2 feet below the surface in the Flathead Range and Southern Glacier Park and 2-4 feet below the surface in the Swan Range. This layer has been found in most aspects but is most developed on NW-NE-SE slopes.

    The only way to identify this hazard is to dig down and see if this weak layer is present. Where you find it, choose lower-angle terrain free of overhead hazards and convexities. Small avalanches have the potential to step down to buried weak layers.   Slopes with clean runouts are an excellent way to build a margin of safety into your day.

    (2/8/2023)

    Poor upper snowpack at middle elevations in the Flathead Range. These weak layers are sitting ducks waiting for a slab.

    Photo: Cam

Avalanche Discussion

A motorized party in the South Fork of the Flathead triggered two slides yesterday. One appears to have been a wind slab that ran on top of a crust in a wind-loaded gulley. The other may have been a Persistent Slab that ran on the same crust, which they believe is a sun crust. This second slab cracked 30 feet in front and about 30 feet above the rider, with a crown depth of 40 inches. This slide may have failed on a surface hoar layer buried in mid-January which continues to show propagation in snowpit tests.  The photo of this slide confirms that it occurred in an older burned area open to the sky, which is required for surface hoar development.  A slab that propagates above a rider may be a hard Wind Slab, as the group in Skiumah triggered last weekend.  Or, the slab fails on buried weak snow and is considered a Persistent Slab. 

We have listed Wind Slab and Persistent Slab almost daily since January 1, and we worry about user fatigue or complacency with these avalanche problems. However, these slides confirm our continued listing of Wind Slab and Persistent Slab in the daily forecast. Winds capable of transporting snow have been recorded at mid and upper-elevation weather stations for most of the work week. As an observer noted in the Flathead Range yesterday, there remains enough loose snow available for transport. A Level 1 motorized avalanche class had propagation on all four extended column tests performed at mid-elevations in the Swan Range Thursday. Combined with yesterday's triggered slide, these results are bulls-eye data that buried weak layers still pose a concern. 

Yesterday, an observer noted the sun was out until around 11:00, felt strong, and developed rollerballs and a thin crust on sunny aspects. Temperatures this morning were warmer at all elevations than this time yesterday. Today's forecast calls for a bit more sun than yesterday, and we expect the snow surface to moisten as the day progresses. Stay in front of the sun or head to the north side of the compass to avoid this issue. Tree bombs and snow peeling off rock faces are signs of warming. Rollerballs or pinwheels indicate that the snow surface is becoming unstable.

Check out Rob's excellent weekly summary in the tab above. 

Today is the last day of Flathead Avalanche Friends Fest, a four-day festival from Feb 8-11. Today is hot dog day at WMR, followed by apres and a Big-A$$ Raffle at the Tap House. Learn more here. We thank everyone who came out last night to enjoy an engaging and entertaining weather talk by LeeAnn Allegretto from the National Weather Service in Missoula. 

 

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