Silver Valley & Bitterroot Mountains

Problèmes avalancheux

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Likely

  Taille

  1.5–2.5

  Consecutive days of reported human-triggered avalanches involving storm snow is keeping this problem at the forefront. (Avalanche Observation 1), (Avalanche Observation 2),  and (Avalanche Observation 3). Across all of the forecast zones, we are nearing the 2 foot mark for storm totals since it started snowing a week ago. In many places these thick slabs are resting above a fragile layer of surface hoar that developed prior to Valentines Day. The behavior of the recent human-triggered avalanches is showing us that this is not your run of the mill storm stab in places where this weak layer is present.

   These slabs gain both in cohesion and thickness as you climb in elevation, and in turn increase in their sensitivity, showing more signs of instability like cracking around your skis and audible collapses. Exposed to wind, recently loaded by wind, and steeper than 30 degrees, are all examples of characteristics that have been present at the sites of recent avalanches. These Storm Slab avalanches are especially worrisome right now due to the fact that many slopes have fragile weak layers deeper in the snowpack that are also waiting to be provoked. For the safest riding experience, we recommend that you avoid being on or below all avalanche terrain, and favor slopes less than 30 degrees that are sheltered from the wind.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  2–3

  New snowfall in the month of February continues to incrementally add stress to buried weak layers that formed during January’s dry spell. The character and distribution of these weak layers vary between our northern and southern mountains, and subtle differences even exist within the same range. The differences reflect variations in rain line elevations, storm totals, aspect, and periods of warm, sunny weather. With each passing storm, the overlying slabs have gained thickness and cohesion, steadily increasing the likelihood for both human triggered and naturally occurring avalanches. Now in the middle of February, the weak layers can be found up to 3 feet below the surface. This is a prime depth for disturbance by a human or machine, resulting in large dangerous avalanches that would be difficult to escape and survive.

   The best strategy we can offer for navigating this problem is to favor southerly slopes that are below 30 degrees in steepness. Very warm weather that occurred the first week of February saturated, softened, and later refroze the surface into a hard supportable crust on our southerly aspects. Where the deeper weak layers are still intact and the now buried crust is thin or non-existent, we urge you to move through the mountains with heightened caution. The current structure is particularly problematic because the main evidence of instability we’ve observed is ongoing propagating failures in tests, while obvious warning signs like recent avalanches, cracking, or collapsing have been less frequent. Travel with a conservative mindset and favor slopes around 30 degrees that have a clean runout and aren’t connected to steeper slopes above.