Cooke City

Relentless snowfall (11.1 inches of water in just over 2 weeks) continues to test the strength of deeper layers in the snowpack. Everything has a breaking point, and large persistent slab avalanches breaking near the ground remain likely.

The challenge is that you probably won’t see any obvious signs of instability - the main warning sign is the heavy snowfall over the last two weeks. For today avoid all avalanche terrain including avalanche runout zones. Ride slopes less than 30 degrees in steepness with nothing steeper above you.

Alex said it best last night on the phone - around Dec 7th with the first big storm, the snowpack was tested and failed, ie - it produced many avalanches. The snowpack is being tested now and will likely fail again.

Near Cooke City, Island Park, and West Yellowstone, an AVALANCHE WARNING and a HIGH avalanche danger are in place for two different reasons with persistent slab avalanches and storm slab avalanches likely to occur.

• Near Cooke City, relentless snowfall has increased the snowpack by 176% over the last 17 days, gaining 11.1” of water during that time. In the long run, this will make a strong snowpack. For now, this snowfall is creating dangerous avalanche conditions. Old weak layers near the ground are groaning under this crushing load.
• Near Island Park and West Yellowstone, the fledgling snowpack is being tested with the first major load of snowfall. Baby snowpacks always fail their first test.

The avalanche danger is CONSIDERABLE in the Gallatin and Madison Ranges which means dangerous conditions as well. The snowpack has obvious weak layers near the ground that can produce persistent slab avalanches as Dave and I found yesterday in the Taylor Fork of the southern Madison Range. These weak layers are being tested again and I don’t trust them. 

The new snow is warm and dense. It may produce storm slab avalanches as there is a layer of slightly weaker/softer/faceted snow that was the snow surface during a brief period of clear weather (Dec 13-17th) as Alex shows in this video yesterday near Big Sky. Storm slab avalanches will be even more likely near ridgetops in areas with wind loading..

Conditions in the Bridger Range are both similar and different to other areas. The difference is that lower elevations mean warmer temperatures which has helped stability. The main areas to watch out for are wind loaded spots under ridges where wind slabs can pop which have a CONSIDERABLE avalanche danger.

A persistent slab avalanche problem in the Bridger Range hasn’t reared up yet. The layer of old facets responsible for this problem exists at upper elevations or northerly facing mid elevations (any place that harbored old snow this fall). This problem is most likely to cause an avalanche where winds have drifted snow and added extra weight to the snowpack. Seek out slopes sheltered from wind loading to avoid both this problem and wind slab problems.