GNFAC Avalanche Forecast for Mon Jan 20, 2025

Southern Madison

Southern Gallatin

Lionhead Range

Cooke City

Avalanches that break 2-4 deep on weak snow near the bottom of the snowpack are possible for a person to trigger. These persistent slab avalanches are slowly becoming less likely, but can break hundreds of feet wide and have big consequences. Yesterday a rider in the southern Madison Range triggered long shooting cracks across a slope (photos) which shows this problem is still out there. 

You are more likely to trigger persistent slab avalanches on slopes with snow depths less than 3-4 ft deep. On slopes where snow depth varies you can trigger one of these slides from a thinner area and it can break wide into a deeper part of the slope. A few persistent slab avalanches from last week show some types of terrain where these are more likely to occur (Red Mountain, Woody Ridge 1, Woody Ridge 2).

Additionally, wind slab avalanches up to 1-2 feet deep can be triggered where recent snow was drifted into stiffer or thicker slabs.

Choose slopes that are non-wind-loaded and with a relatively deep and more uniform snow depth, or stay on slopes less than 30 degrees steep to keep it simple. Human-triggered avalanches are possible and the avalanche danger is MODERATE in Cooke City, West Yellowstone and the southern Madison and southern Gallatin Ranges.

Bridger Range

Northern Gallatin

Northern Madison

Island Park

Near Bozeman, Big Sky and Island Park wind slab avalanches can be triggered where recent snow was drifted into slabs a few inches to a couple feet deep. Yesterday, we toured in the Bridger Range and triggered a thin, 3-4” deep, soft wind slab on a small test slope (video, observation), and the Big Sky and Bridger ski patrols reported triggering recently formed soft and hard slabs.

Many of the recent slabs formed on either a thin layer of facets, low density snow, or a firm crust which can make them easy to trigger for a couple more days (or more). Winds increased overnight and will easily drift recent snow and grow larger slabs. Older drifts that formed at the end of last week will be more stubborn, but worth watching out for. Note that strong winds last week formed some drifts in less typical locations like lower elevations.

Watch for cracking across the snow surface (photo, photo) as a sign of potential unstable wind-slabs, and look for rounded pillows of snow or a wavy textured snow surface to identify drifts. Steer clear of drifts on steep slopes, and keep in mind that any size slide can be hazardous on slopes with terrain traps like trees, cliffs or confined gullies.

Persistent Slab Avalanches breaking on weak layers near the bottom of the snowpack are unlikely in these parts of our forecast area. However, buried weak layers do exist, so it is worth choosing slopes with a deeper, more uniform snow depth. Yesterday, in the northern Bridger Range, skiers found a poor snowpack structure and had multiple collapses in a relatively shallower snowpack (less than 3 feet deep)(observation). 

Before committing to steep slopes, carefully assess the snowpack for buried weak layers and unstable drifts. In the mountains around Bozeman, Big Sky, and Island Park the avalanche danger is MODERATE on wind-loaded slopes and LOW on other slopes.