Maritime

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–2.5

  A layer of surface hoar that was buried on 12/22 (12/22 BSH) has been identified in the Intermountain and Continental zone up to 3800' on all aspects.  This layer has not been found yet in terrain that is close to the Thompson Pass gap probably due to pre-storm wind that would have disturbed it.  It is still unclear as to whether this layer exists in the Maritime zone.  12/22 BSH is now 10-18 inches beneath the surface after this weeks gradual snowfall, with another 1-3 inches expected today.  

  The 12/22 layer is a concern because it is known for being weak in shear and can persist for long periods.  This layer has not yet shown signs of propagation in Extended Column Tests.  This is likely due to the surface snow being very uncohesive (no slab) and new snowfall amounts not yet reaching a critical amount.  This puts us in a period where snowpack assesment will be more of a challenge due to several factors.

  At the current state of the snowpack, instability in the upper snowpack is not expected to be widespread even though a weak layer may be present.  This is because the overlying snow in many locations, has very little slab charachter currently.  Slab charachter is a variable that can change with time as snow naturally settles and with location as areas with wind affect will have stiffer snow that is more prone to failure.  Not only is this a challenge  because slab character can vary along a single slope but also because identifying the weak layer can be a challenge even if you take time to dig a snowpit.  

  After a clear day on Friday light snowfall has returned to our area with a couple inches expected today.  North winds are expected to increase to 30mph in wind channeled terrain which will add to cohesion and increase sensitivity where wind distribution is occurring.

    Watch for signs of instability, such as shooting cracks that could indicate the presence of a weak layer.  Pay attention to the amount of new snow that exists and the stiffness of the slab.  The stiffer the surface snow is, the more propagation potential there is.  Using small inconsequential test slopes can be a good tool today especially if you can find areas where the wind has stiffened the surface.  Be suspicious of hand shear results when performed in areas with light unconsolidated snow at the surface.

  The hazard is expected to increase today as north winds increase the cohesion of the surface snow in places, along with a couple inches of additional snowfall.

   

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  North wind is expected to increase today along wind channeled terrain, but is not expected to be widespread.  There is plenty of loose snow available for transport which will form fresh wind slabs up to 2 feet in depth in specific areas.  Wind slabs may be forming on buried surface hoar, making these areas very sensitive (see problem above).  Even where BSH does not exist, like areas close to the Thompson Pass Gap or high elevation ridge lines the likelihood of human triggered avalanches will increase where wind reditribution is occurring.  This type of avalanche problem is most sensitive when it is forming or directly after.

  Wind slabs can have pillowed look or a rippled surface texture.  Watch for signs of instability such as shooting cracks, collapsing, recent avalanche activity, and active wind loading that would indicate the possibility of unstable wind slabs.