Continental

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–3

  Storm slabs continue to build along with an uptick in already strong southerly winds.  DOT estimated 8" of new snow overnight at 6 am with heavy snow and southerly wind continuing.  Storm totals at the top of Thompson Pass have been estimated at 20", and 15" recorded at Nicks Snotel.  Both of these sites tend to under report snowfall when strong winds coincide with snowfall.  Storm totals are likely higher in protected terrain above 2000'.

  In addition to continuing heavy snow and wind, temperatures are falling across our forecast area with the freezing line moving close to sea level.  Valdez remains a mix of precipitation types as of 6 am.

  On 2/21 snowfall depths were found to be variable (6-18+") on Catchers Mitt due to moderate to strong active wind redistribution.  Instabilities such as shooting cracks and failures in test slopes were found on wind loaded terrain features.  Expect for these instabilities to increase today in depth, distribution, and sensitivity as more snow and wind add to the problem.  Human triggered avalanches are likely today, especially in wind loaded terrain 1-3+ feet in depth.

  Pay attention to new snow depth and watch for clues on the surface such as fresh cornice development to determine wind direction and which slopes are loaded.  Pole probing along your route is a good way to determine how much wind redistribution has occurred.  Hand pits and small test slopes help identify instabilities.  Careful snowpack assessment and conservative terrain choices will be critical for safe travel in avalanche terrain today.

   

• 

  Wet Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–3

  Dangerous avalanche conditions exist in Keystone Canyon or other areas that are typically very protected from outflow winds at low elevations.  Multiple large natural avalanches have occurred in Keystone over the last 48 hours with at least 3 wet slabs observed.  One of these wet slabs buried the Richardson Highway near the upstream end of the canyon with ~10' of debris for about 100' on 2/21.  It is unclear as to whether this avalanche originated as a wet loose that stepped down or initially failed as a wet slab.  The failure plane existed of near surface facets that were created during the January drought and were protected from strong winds during the period.

   This weak layer is isolated in its existence in the snowpack through the region due to strong January winds in the majority of locations that broke down this weak layer.  Be aware that this weak layer may exist in other steep river canyons.  Very dangerous human triggered wet slab avalanches are possible in isolated areas until a thorough freeze of the upper snowpack occurs.  Even once temperatures go below 32° snow can remain moist below the surface.  Digging beneath the surface to see if you can make a snowball is a good way to test the moisture content of the snow.  Once a solid refreeze occurs the snow beneath the surface will be very hard as free water in the upper snowpack freezes.  As mentioned above this process can take much longer than you expect.

  Avoid steep terrain in Keystone Canyon until a thorough refreeze of the upper snowpack occurs.

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Mixed precipitation continues to occur at the coast, with the freezing line descending overnight to near sea level.  This will lower the likelihood of natural wet loose activity from likely to possible, although human triggered wet loose will remain likely as the upper snowpack is saturated near sea level and will take a while to refreeze with fresh snowfall insulating the wet snow.  Travel is not advised in steep terrain (>35°) below 2000'.  Wet loose avalanches may be able to step down and produce wet slab activity in isolated locations (see problem 2).

  Avoid traveling on or exposing yourself to steep terrain below 2000'. A lot of natural wet loose activity has already occurred, although additional human triggered activity remains likely.  Realize that even small wet loose activity can be dangerous as it is very dense and can cause injury.  Consequences are also much higher anywhere a terrain trap is involved.