Continental

Problemi valanghivi

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  2–3

  Multiple potential weak layers exist in the mid snowpack.  These layers are of most concern in the mid elevation band, but some may exist at upper elevations as well in isolated areas.

  First and most concerning is the 12/5-10 rain crust sandwich. This layer consists of 2 rain crusts that are separated by about 4 inches with faceted snow in between.  The upper rain crust has lost most of its strength and propagation in stability tests has been observed multiple times this week failing between the crusts.  This layer exists below 3500' and is on average nearly 3 feet deep in the snowpack.

  Second is a layer of surface hoar that was recently buried on 1/4.  Despite rain on snow, this layer was insulated by dry snow that fell at the beginning of the storm, and has been found intact and reactive in stability tests as recently as 1/8.  The distribution of this layer is less continuous than the rain crust sandwich as an uptick in north wind prior to the storm destroyed this layer in many locations above brush line.  This layer continues to exist below brush line and in isolated wind-protected alpine areas.

  Lastly is the 12/22 BSH which was found to be unreactive prior to this round of storms.  This is still mentioned simply because the stress of above this layer has increased and buried surface hoar can lay dormant, and then activate once a significant load is applied.  Reactivity of this layer is unceratain.

  Digging snowpits is the best way to identify persistent weak layers, and ECT and PST tests are the best way to evaluate their sensitivity.

  

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  2–3

  Storm slabs continue to build and will vary in depth and composition depending on elevation.  Thompson Pass received another 6 inches in the last 24 hours, 4 of which fell overnight.  Winds increased to moderate overnight and are expected to taper through the day along with precipitation.  An increase in the wind will be creating deeper pockets of storm snow that will be more sensitive to human triggers.  

  Below 1500' storm slabs are not a concern as mostly rain has occurred at low elevations.

  Between 1500-3000'- There is a stout rain crust that is in the middle of nearly 2 feet of settled storm snow.  This rain crust will limit the snow available for transport to what is above the crust, which was observed to be 6-12 inches depending on previous wind exposure.  The snow beneath the rain crust is being insulated by the snow above and remains moist.  Propagation was observed at the interface of new/old snow and below the mid storm rain crust on 1/8.  

  Above 3000'- We dont have any observations since the latest round of storms began.  Storm snow depths are expected to be greater, and without the mid storm rain crust the snow is available for transport will be significantly greater allowing deeper windloaded pockets to form.

  Light snowfall will occur today through at least the morning hours which will continue to put a stress on an overloaded snowpack.  The mid-storm rain crust at mid elevations will disperse a person or machines weight to some extent, but building storm slabs and propagation in stability tests point to continued elevated hazard.  Human triggered avalanches remain likely today up to 4 feet in depth and may be able to step down or fail at persistent weak layers in the snowpack (see problem 2).  Conservative terrain choices and cautious route finding are recommended today.