Island Park

The rain-snow line was above 8000’ elevation last night. Dangerous conditions exist on upper-elevation slopes, where new and wind-drifted snow is loading an unstable snowpack, and on low-elevation slopes, where rain has further weakened the snow. Avoid slopes steeper than 30 degrees and the runout zones below. Play on slopes less than 30 degrees in steepness with nothing steeper immediately above you.

Dangerous avalanche conditions exist in the mountains around Big Sky, West Yellowstone, Island Park, and Cooke City. Yesterday, Ian and his partner remotely triggered an avalanche that broke 6 feet deep in the Taylor Fork while they were riding to investigate several different avalanches in the S. Madison Range (video). On Buck Ridge in the N. Madison, a deep avalanche broke either naturally or was triggered from a distance (details and photo). On Tuesday, an avalanche broke one-quarter mile wide and up to 12 feet deep on Scotch Bonnet Mountain in Cooke City (details and photo). Persistent slab avalanches breaking widely across slopes on deeply buried weak layers are the primary concern throughout the forecast area.

Wind slab avalanches at upper elevations where last night’s precipitation fell as snow will break on slopes with fresh drifts. Wind-loaded slopes are often those immediately below ridgelines and in upper-elevation gullies, with instability indicated by avalanches and shooting cracks. For example, look at Monday’s slide on Henderson Mountain in Cooke City.

Rain on snow weakens the snowpack and could result in wet loose avalanches. Stay off steep slopes with an unsupportable, wet, and gross snowpack. This will be most prominent in the mountains around West Yellowstone and Island Park. 

Regional Differences:

• Recent Snowfall: Cooke City, Madison, S. Gallatin, Lionhead, and Island Park received the most recent snow and are the most hazardous (weather log).
• Snowpack Depth (potential avalanche size): Deepest in Cooke City, followed by the Madison and Southern Gallatin Ranges (weather stations and SNOTEL Sites).
• Recent Avalanches: Most frequent and largest in Cooke City and the Southern Madison Range (avalanche activity list).

Danger Ratings:

• HIGH: on upper-elevation slopes in the Southern Madison and Southern Gallatin Ranges where the 3-8” of snow fell last night, and southwest winds are drifting snow onto an already unstable snowpack.
  • Travel Advice: Avoid steep terrain and runout zones.

• CONSIDERABLE: Island Park, West Yellowstone, Northern Madison, and Cooke City. New snow, rain, wind-loading, and deep weak layers make human-triggered avalanches likely.
  • Travel Advice: Play in terrain with slope angles less than 30 degrees and stay out from under steeper slopes.

• MODERATE: Bridger Range and Northern Gallatin Range, where less recent snow and activity exist. Persistent weak layers and wind-loaded slopes require careful consideration.
  • Travel Advice: Choose terrain sheltered from wind-loading and evaluate for buried weak layers before committing to steeper features. Select lower-consequence, less-steep slopes with fewer terrain traps.