Bitterroot

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Overnight, most areas received 5-6 inches of new snow, with the exception of the Seeley zone, which received less snow. Wind speeds were light overnight but shifted around the compass, with periods of wind in the northwest, southwest, and east-southeast. Today, another 1-4 inches of snow is expected with west winds.  D1 wind slabs are likely today, and D2 wind slabs are possible.

  Expect wind slabs to be most prevalent on east aspects, but with the shifting winds, all aspects are included because of the potential for cross-loading and terrain-driven winds. Wind slab size will be dependent on how much new snow is in the area, and if you find 6 inches of new snow, be on the lookout for slabs up to a foot in depth.

  Wind slabs can often be identified by their smooth, rounded appearance. If you encounter firm snow that feels hollow underneath or see shooting cracks around your skis or snowmobile, step out of avalanche terrain and stick to slopes under 35 degrees.  Keep in mind that new snow could hide previous wind slabs, making red flags like cracking harder to identify.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1.5–2.5

  Persistent slab avalanches remain unlikely, but high consequence if triggered. The primary layer of concern is weak snow surrounding a crust in the middle of the snowpack. This problem has been unreactive and not producing avalanches, except for one avalanche in the southern Bitterroot that resulted in a fatality.

  You will not see red flags like cracking, collapsing or whumfing before triggering an avalanche. Slopes with previous ski tracks are still suspect, and it is possible that the second or third skier on the slope could trigger the avalanche. The only way to identify this problem is to dig in the snowpack. If you find a crust with a thin (1cm - 3 inches thick) layer of weaker snow above or below it and a slab above, recognize the potential for a larger avalanche. If you want to avoid the issue completely, avalanches are unlikely on slopes under 30 degrees in slope angle.

• 

  Wet Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1.5–2.5

  Continued cooler temperatures have helped alleviate wet slab concerns today, but especially at mid elevation, there are areas with wet snow still in the snowpack. Even if you are on top of a supportable crust, the snow under the crust can still be wet and have low strength.  This week, a skier had a close call with a wet slab avalanche on Lolo Pass. Wet slab avalanches are difficult to forecast or manage and have an inherent uncertainty surrounding them.

  The best way to identify the problem is to dig. You don't need a full snowpit profile for this problem. Simply dig down or reach with your hand in the snowpack and look for wet snow. If you find wet snow, even if it is below a supportable crust, recognize the potential for a wet slab avalanche and move to slopes under 30 degrees in slope angle. 

  Be alert to changing conditions as you make your way down a slope or wrap around aspects in a bowl or cirque, as the snow can become wetter in more solar-affected areas.