Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2.5

  Strong north winds have been occurring over the last 24 hours with gusts to 80 mph reported on Thompson Pass by DOT last night.  These winds are expected to continue through the day, but may slowly decrease in strength.  Wind slabs will continue to form that are expected to be sensitive to triggers 1-2+ feet in depth.  Areas seeing the highest wind speeds (60+ mph) are seeing a lot of stripping and sublimation, ( snow moving from a solid to gas state) with less deposition than areas that are seeing more moderate speeds.

  Expect to find wind slab deposits on the lee side of catching terrain features such as cross loaded gullies.  At upper elevations, wind slabs that form on the lee side of ridges may exist further down from ridgelines than what is typical.  Wind slabs will likely have a hollow drum like feel and may have a rippled and/or pillowed look to them on the surface.

  Facets exist 8-18 inches beneath the surface in specific locations that will increase the potential for natural and human triggered avalanches today. ( see problem 2)

  Avoid slopes steeper than 30° that are being actively loaded by wind, or where signs of instability such as shooting cracks, collapsing and natural avalanche activity.

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1–2

  Faceted snow exists in specific locations about a foot down in all three forecast zones that was created by the mostly dry weather since 12/25.   These facets have been found to be rounding to some extent and mixed with other grain types, although remain a concern.  The distribution of this layer is not widespread due to previous north wind on 1/9 that removed it from windward slopes in wind exposed terrain.  It is likely that the strong winds that are forecasted today will remove/blow away this weak layer in more places.  

  In multiple locations, this weak layer exists underneath wind slabs formed by the moderate 1/9 outflow event.  The major questions moving forward will be:

  1) Is the wind slab above these facets strong enough to withstand the damage that is going to occur with strong winds and leave faceted snow preserved?

  2) Where could this layer of facets possibly stay preserved.

  The first question we will have to wait and see the full effect of the current outflow event.  As for the second, facets will be most likely to remain in protected areas.  These can exist as you move further away from the Thompson Pass/ Lowe river valley corridors, where winds are less severe.  Digging snowpits will be the best way to identify the presence of this layer in protected areas moving forward.  This process can be complicated and often misleading as this layer can be difficult to identify and spatially variable

  The presence of this layer will increase the odds and potential propagation length of human triggered and natural avalanches today.

  

• 

  Glide Cracks

  • All elevations

  Likelihood

  Unlikely

  Size

  2.5–3

   Glide avalanches are a strange beast that act differntly than other avalanche problems.  Full depth failure can occur independent of snowpack stability.  Or in other words, they can release even when a stable snowpack is in place.

  Since it is not possible to time their sensitivity the only way to mitigate this hazard is by avoidance.  First identify where glide cracks exist and avoid spending time beneath them.  Glide cracks are easy to identify.  They look like crevasses, but exist in non glaciated terrain.

  These do not produce small avalanches,  but rather failure will include the entire snowpack.