GNFAC Avalanche Forecast for Sun Dec 17, 2023

Not the Current Forecast

Good morning. This is Alex Marienthal with the Gallatin National Forest Avalanche Forecast on Sunday, December 17th at 6:45 a.m. This information is sponsored by World Boards and Mystery Ranch. This forecast does not apply to operating ski areas.

Mountain Weather

This morning there is no new snow, temperatures are teens to low 30s F, and wind has been westerly at 10-25 mph with gusts of 25-40 mph. Today, temperatures will reach 30s to low 40s F, and wind will be west-northwest at 5-15 mph with gusts to 20 mph. No snow is expected over the next few days with a slight chance for snow during the second half of the week.

Snowpack and Avalanche Discussion

All Regions

A person can trigger a large, dangerous avalanche that breaks on weak snow in the lower half of the 1-3 foot deep snowpack. These avalanches can break wide, and can be triggered on slopes steeper than 30 degrees or from lower angle terrain that is connected to steep slopes. 

On Friday at Big Sky, ski patrol reported two avalanches thought to be naturally triggered during the day, and yesterday they confirmed at least one was triggered by a mountain goat. Based on the goat’s tracks, it was carried over 1000’ vertical and walked away from the debris apparently uninjured (details, photo). Additionally, over the last three days we continued to receive reports of collapsing and whumphing of the snowpack, a clear sign avalanches can be triggered on steeper slopes. This red flag was experienced by skiers in the Bridger Range (observation), Cooke City (observation1, observation2), Lionhead (observation), and Beehive Basin (observation). 

The poor snowpack structure is responsible for a long list of avalanches over the last week and a half (avalanche log), both natural and human-triggered. On Friday Ian saw a couple recent natural avalanches at Lionhead which he discusses in a video (observation). On Tuesday a skier triggered a large avalanche on Mt. Blackmore in Hyalite, and he generously shared a video from his POV (video). These videos are good examples of the type of avalanche you could trigger today.

The snowpack is slowly becoming more stable since the last storm, but recent avalanches and collapsing of the snowpack are warnings that dangerous human-triggered avalanches are possible. Choose terrain carefully. Consider the consequences of an avalanche if you travel on or below slopes steeper than 30 degrees, and thoughtfully evaluate the snowpack for the potential of an avalanche. Clear signs that you should stay off steep slopes include cracking, collapsing, recent avalanches, or poor stability test scores.

Heightened avalanche conditions exist, human-triggered avalanches are possible, and the avalanche danger is rated MODERATE.

If you venture out, please fill an observation form. It does not need to be technical. Did you see any avalanches? How much snow is on the ground? Was the wind moving snow? Simple observations are incredibly valuable. You can also contact us via email (, phone (406-587-6984), or Instagram (#gnfacobs).

Upcoming Avalanche Education and Events

Our education calendar is full of awareness lectures and field courses. Check it out: Events and Education Calendar.

We offer Avalanche Fundamentals with Field Session courses targeted towards non-motorized users in December and January and one geared towards motorized users in January. Sign up early before they fill up.

Loss in the Outdoors is a support group for those affected by loss and grief related to outdoor pursuits. Check out the link for more information.

The Last Word

Listen to GNFAC Forecaster Dave Zinn on the Hoary Marmot Podcast for some extracurricular avalanche talk (link to episode).

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