Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–2.5

  Strong North winds were observed yesterday through Thompson Pass Gap and were visible along high elevation ridge lines.  This wind event didn't last a long time but was sufficient to redistribute new snow into wind slabs in specific locations.   Lower Girls Mountain/ Bald boy still held protected snow. However, wind effect was visible on the Butresses across the road, and the gullies were cross loading.  Wind slabs were experienced in isolated spots on the lower and mid elevations of Girls and were reactive with shooting cracks. The distribution of wind affected snow likely increases with elevation.  Expect to encounter wind slabs near wind channeled terrain on the lee side of terrain features, cross loaded gullies, and high elevation ridges 1-2 feet in depth that are reactive to human and machine triggers. Watch for signs of wind slab formation such as rippled snow texture, fresh wind lips, and stiffer snow at the surface.  Shooting cracks and collapsing are direct indicators that wind slabs have the potential to produce an avalanche on slopes steeper than 30°.

  In areas that are not being affected by wind, there is also a storm slab concern in the Maritime zone.  Incremental snowfall since 2/14 has reached the point (> 18" above 2500') where instabilities within various pulses of snowfall are now a concern near the coast where snowfall totals are higher.  This is less of a concern in the Intermountain and continental zone, wherever the snow remains wind protected, settled storm snow since the 14th averages 12" deep and uncohesive.  

  Good bonding continues to be observed at the 2/14 wind affected layer that exists above 3000'.  However, lower elevations are harboring buried surface hoar that is now under a sufficient slab to be an issue between 1500-2500' and poor structure exists below 1500' in the top meter of the snowpack (see problem 2)

  Low elevation terrain is expected to see less of a wind slab issue as winds will likely be lighter, and warm temps have created denser snow that will be more resistant to redistribution.

   

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–2

  There are 2 persistent weak layers of current concern.  The first is the Valentine's buried surface hoar.  This layer only exists in a narrow elevation band ~1500-2500' and is now sufficiently buried to be an issue in steep terrain that falls within this elevation band.  Even within this band, the distribution of this layer is spotty, which will make assessment challenging.  This BSH layer has been identified as recently as 2/25 in the Port of Valdez at 2500', down 20 inches.  Slabs overlying this weak layer are closer to 10-14 inches deep on Thompson Pass.  We have not yet seen this layer produce any avalanche activity, but with continued incremental loading of snowfall forecasted in the future, it will be important to remember this layer when traveling between 1500-2500'.  Choosing conservative routes through this elevation band will be the best way to avoid this problem, as the threshold strength of this layer is unclear and distribution varies.  Wind loading on this layer is a concern. However, the formation of this grain was most pronounced in wind protected areas, making it less likely for wind loading to occur in areas where BSH is the most pronounced since this wind event is expected to be short and relatively mild.

  The second persistent weak layer of concern is faceted snow that exists below a decomposing rain crust from late January down 12-16 inches in the snowpack.  This poor structure exists below 1500' and was producing large collapses across connected terrain as recently as 2/25 in the Maritime zone with moderate ECT scores producing propagation in multiple locations over the last week.  Same with the Valentines BSH this layer exists in a specific elevation band.  Steep low elevation canyons are the most likely areas to see this layer be reactive.  Continued snowfall this week will put additional stress on this weak layer.  Once again the best travel advice is to recognize where this layer exists and choose conservative terrain to move through.

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Temperatures are above freezing at the coast as of 6am and rain is possible during the day today.  Wet loose will become possible today depending on the amount of rain we receive and how high daytime temps get.  The surface snow has already encountered above freezing temperatures which will dull the reaction the surface snow has today.  However a small to moderate amount of wet loose activity can be expected.

  When assessing wet loose hazards, it is essential to take the hazard in relation to the terrain you are exposing yourself to.  Are there terrain traps? Are there consequences if I were to be swept off my feet by even a small avalanche? 

  The size and shape of the terrain above you along with the amount of loose snow available to slide, directly correlates to the size of the wet loose activity that can occur. Terrain traps and exposure below you compound consequences.