Lawinenlagebericht

Lizard-Flathead

Feb 21 – 22, 2021Avalanche Canada
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Lawinengefahr

Tue
Wed
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Kernpunkte

Reactive wind slabs have formed in lee features and avalanche hazard will likely increase through the day. Smaller wind slabs could easily step down and trigger larger persistent weak layers. Seek out sheltered and low angle terrain for the best and safest riding.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Strong southwest wind will continue to form touchy wind slabs in lee terrain features in the alpine and exposed treeline. As winds increase, slabs may form lower on slopes, into tree line areas, or in other unusual places. Older wind slabs formed under recent variable winds, be mindful that the upper snowpack could have a few layers of wind slab.

    These wind slabs are slow to bond where they sit over cold sugary facets. There is also potential for wind slabs to step down to the late January persistent weak layer. 

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible - Likely
    Größe
    1.5–2.5

    This layer continues to be easily triggered by skiers and riders. Small terrain features are producing small avalanche results, bigger features produce larger more dangerous avalanches. 40-70 cm of snow sits above a buried weak layer consisting of surface hoar, facets, and a crust (depending on elevation and aspect). 

    The surface hoar interface is the biggest repeat offender, and things have been most reactive on northerly and easterly aspects at treeline and the treeline/alpine interface, but don't let your guard down elsewhere! These kinds of avalanches can happen in surprisingly mellow terrain and can be triggered from a distance away. This MIN report from Wednesday is a good illustration. 

Terrain and Travel

  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

SATURDAY NIGHT: Flurries, up to 5 cm, moderate southwest wind, temperature low -11 C.

SUNDAY: Cloudy, 5-10 cm, strong southwest wind, temperature high -2 C, freezing level rising to 1500 m.

MONDAY: Cloudy, 20-30 cm with rain at lower elevations, strong southwest wind, temperature high 0 C, freezing level at 1700 m.

TUESDAY: Cloudy with clear periods, trace of new snow, moderate northwest wind, temperature high -8 C, freezing level dropping to valley bottom.

Avalanche Summary

On Friday, skiers remote triggered a size 2 persistent slab avalanche on a north east aspect at 1550 m. A reactive wind slab also developed, small (size 1-1.5) avalanches were easily triggered by by skiers in lee features, a size 2 natural wind slab avalanche was also reported on a northeast aspect.

A small persistent slab avalanche was triggered remotely by a rider below treeline (see this MIN report) on Thursday. Whumpfing and shooting cracks were reported by several parties. 

More reports of easily triggered persistent slab avalanches came in on Wednesday, including these large avalanches at treeline on Mt. Fernie. There were also several small loose dry avalanches which were triggered by skiers. 

On Tuesday, a natural cornice failure triggered a small avalanche on the slope below in the alpine. 

On Monday, skiers triggered size 1 avalanches at treeline and in the alpine on northeast aspects. One avalanche was triggered from a short distance away (see this MIN report). On Sunday skiers triggered avalanches up to size 2.5 on open features at the treeline/alpine interface in the Lizard Range and Tunnel Creek. These failed on the late January persistent surface hoar layer (see this MIN as an example). 

Reports on human and remote triggered persistent slab avalanches up to size 2.5 continue since last week. Locations like Mount Fernie, 2000, Liverwurst and McDermid are a few location examples. Some avalanches were triggered from a distance away. 

Snowpack Summary

Southwesterly winds have redistributed loose snow and formed slabs in lee features. Fresh snow now covers old wind slab, a thin sun crust, or layers of faceted snow. Below 1600 m a hard melt-freeze crust is underneath 20-40 cm of recent snow. 

A persistent weak layer lurks 40-70 cm below the surface. In some places it consists of surface hoar, in other places just facets, or crust/facet combinations. This weak interface has been responsible for the majority of recent avalanches. 

A solid mid-pack sits above a deeply buried crust and facet layers near the bottom of the snowpack (150-200 cm deep), which is currently unreactive. 

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