Continental

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–3

  Significant snowfall yesterday has elevated the avalanche hazard.  Human triggered avalanches 1-3 feet in depth are likely today.  Storm slab avalanches may be able to step down to a layer of buried surface in hoar in some locations and create very large avalanches (see problem 2).  Wind loaded slopes will be the most likely areas to trigger an avalanche, although all slopes steeper than 30° should be considered suspect.  Conservative terrain choices are recommended today.

  23" of new snowfall has been reported on Thompson Pass over the last 24 hours.  Winds during the storm were light to moderate beginning out of the north and eventually swinging around to the south.  Changing wind directions will have loaded multiple aspects.  Temperatures did not warm as significantly as forecasted although a slow increase in temperatures during the storm created slightly top heavy storm slabs, especially at low elevations.  This was observed yesterday as shooting cracks and collapsing were present at low elevations where temps were warmer, and went away with a gain of elevation and a decrease in temperature.  Temperatures are expected to slowly increase today creating unstable conditions at the surface.  

  Allow the snowpack time to accept the weight of the new snowfall.  Stability is expected to slowly increase in most locations in a couple days once skies clear and precipitation stops.  In the meantime, dangerous avalanche conditions exist.  The caveat to this is where a layer of surface hoar exists that is now buried under 2-3 feet of snow (see problem 2),   dangerous avalanche conditions are expected to persist longer than storm snow instabilities.

   

   

   

  

• 

  Persistent Slabs

  • All elevations

  Likelihood

  Likely

  Size

  2–3

  A layer of surface hoar that was buried on 3/9 has been identified in the Intermountain and Continental zones on the northerly half of the compass above 3000'.  Prior to the most recent storm this layer was reactive to skiers and produced multiple D1 avalanches.  These D1 avalanches will now be D2's or even 3's as the mass above the weak layer has significantly increased.  This weak layer is just above the very hard wind damaged snow created in late February.  This wind damaged snow will act as an effective bed surface and may allow avalanches that are initiated to have very wide propagations.  

  Moving forward the presence of this layer will be very important in determining stability.  This setup is dangerous as not only does it have the potential to produce dangerous avalanches but identifying this layer can be challenging even for experienced users.  Surface hoar is notorious for being spotty in its distribution meaning that good stability can be observed in one location and an adjacent slope may have unstable conditions.  As stated above this layer has been identified north of Thompson Pass on protected northerly slopes.  Snow pits are a good tool to tell you where not to ski or travel.  Don't assume that negative results in a stability test mean a slope is safe.