Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–3

  North winds are forecasted to continue today with diminishing strength expected through the day.  A continuation of wind slab development is expected to be confined to gaps, passess and lee of high elevation ridges.  

  On 2/1 strong wind was observed to be affecting mostly high elevation terrain and exposed passess.  Low and mid elevation terrain was reported and observed to be more protected yesterday than previously forecasted.  As north wind begins to slowly diminish in strength and coverage new wind slab sensitivity will slowly decrease.  Today it remains possible to trigger a windslab avalanche 1-5 feet in depth.  

  Pay attention to the surface snow and what effect recent north wind has had upon it. Recently formed wind slabs can be identified by hard snow over soft and may have a pillowed look to them.  Watch for the usual signs of instability including shooting cracks, collapsing and avalanche activity. Areas receiving active wind loading will be the most suspect.

  Areas along the road corridor from Thompson Pass and north have been found to have upside down storm snow.  This was created by strong wind and rising temperatures during the late January storm and can be confused with recent north wind effect (see problem 2).

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–2.5

  The composition of the 1/28 storm snow has been found to vary depending upon location.  Areas in close proximity to Thompson Pass and north received significant storm wind from multiple directions along with rising temperatures.  This has created a top heavy persistent storm slab in some locations.  Even though it has been 4 days since the brunt of precipitation human triggered avalanches remain possible due to the poor structure near the surface.  This avalanche problem may easily be confused with recently formed wind slab as they both will have a slabby feel near the surface.  The difference will be sensitivity,  newly formed wind slabs will likely be more sensitive than upside down persistent slabs.  Older slabs may be identified by having an inch or two of undisturbed snow on them

  In areas further from the road and south of Thompson Pass, recent storm snow has bee found to be more right side up.  Instabilities have been identified 1 and 2 feet down although these have not shown propagation in stability tests and failure is slowly becoming more stubborn.  

  There is a lot of spatial variability out there currently.  Upper snowpack assesment will be necessary and actual stability may vary from place to place.  Hand shears are difficult due to the depth of storm snow, test slopes are becoming a reasonable tool as storm snow settles and instabilities may exist closer to the surface.  Snow pits are a good tool as well, but can result in confusing or misleading results.

• 

  Glide Cracks

  • All elevations

  Likelihood

  Unlikely

  Size

  2.5–3

  The sensitivity of Glide avalanches is not possible to predict.  These release independent of snowpack stability and are tied more to a complicated relationship between the underside of the snowpack and the friction at the ground.  This interaction is not well understood.  What is well understood is that this avalanche problem produces deadly avalanches that include the entire snowpack and can release spontaneously.

  Travel advice is very simple: avoid spending time under glide cracks.  Glide cracks are generally easy to identify as they look like crevasses, but exist in non glaciated terrain.  New snow and wind will make identifying these more difficult, as they may have been covered.