Bitterroot

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–1.5

  2 to 4 inches of snow is forecast to fall today with moderate west winds. The wind will drift the fresh snow into small wind slabs on leeward aspects at upper elevations. The wind slabs could be sensitive and slide easily on the slick crusts below. 

  Wind slabs often form along ridgelines, in gullies, and below convex rollovers. Their smooth, rounded appearance can often identify them. If you see recent avalanches, shooting cracks, or collapsing, avoid steep, wind loaded terrain. Small avalanches can still knock you off your feet and lead to consequences. 

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Wet snow avalanches remain possible today. After several days of sunshine and warm temperatures and a widespread natural wet loose avalanche cycle, the snowpack still contains a lot of heat and liquid water. Recent observations found wet snow down to the ground on sunny aspects. We are entering a period of cooler weather, but it takes time for the snowpack to fully refreeze. 

  There is uncertainty about how well-frozen the snowpack will be today. Last night, temperatures only dropped slightly below freezing, and cloud cover may have prevented effective cooling. Today, temperatures will climb above freezing again in many areas, but cloud cover and moderate winds will help keep things cool. 

  You could find crusty snow at the top of a slope and mushy, wet snow at the bottom. Additionally, even if the old snow surface is crusted, it doesn't mean that the snowpack is fully frozen and stable underneath. The snowpack could still be very wet below the crust, creating significant uncertainty about stability. Although unlikely, you could still trigger a larger wet slab avalanche due to water percolating down through the snowpack and pooling on crusts and weak layers. Observers reported large natural wet slab avalanches in the Swan Range earlier this week. 

  If you find yourself in wet, mushy snow, or find a still wet snowpack under a crust, consider stepping back to lower angle terrain.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1.5–2.5

  A mostly dry snowpack with poor structure still lingers on some upper elevation north facing terrain. The primary layer of concern is weak snow surrounding a crust in the middle of the snowpack. This problem has been unreactive and had not produced avalanches in weeks, until last week when a group of skiers triggered a persistent slab avalanche in the southern Bitterroot in an area where the snow above the crust is especially weak. Tragically, this resulted in an avalanche fatality. You can read the preliminary accident report here. 

  Today, you are unlikely to trigger a persistent slab avalanche, except on specific slopes where this problem is more developed. You will most likely trigger a persistent slab avalanche on convexities, rollovers, or near rock bands. Before triggering, you are unlikely to see obvious signs of instability, such as cracking and collapsing. 

  Identify this problem by digging in the snowpack. Look for a crust buried by 2-4 feet of snow with a thin layer of weak snow above or below the crust. If you find this, use slope angle to avoid the problem by sticking to slopes under 30 degrees in slope angle. If you choose to ride in terrain with this persistent weak layer, reduce exposure by choosing a smooth planar slope free of terrain traps or trees in the runout.