GNFAC Avalanche Forecast for Mon Dec 18, 2023

Not the Current Forecast

Good morning. This is Alex Marienthal with the Gallatin National Forest Avalanche Forecast on Monday, December 18th at 6:30 a.m. This information is sponsored by Yellowstone Ski Tours, Cooke City Motorsports and Stronghold Fabrication. This forecast does not apply to operating ski areas.

Mountain Weather

This morning there is no new snow, temperatures are mid-20s to 30s F, and wind has been westerly at 10-20 mph with gusts of 25-30 mph. Today, under partly sunny skies, temperatures will reach mid-30s to low 40s F, and wind will be west-southwest at 5-10 mph with gusts to 20 mph. No snow is expected today, and there is a slight chance for an inch tomorrow night near West Yellowstone and Island Park.

Snowpack and Avalanche Discussion

All Regions

The snowpack has a poor structure which makes human-triggered avalanches possible. Weak, sugary facets are buried below 1-2 feet of snow that fell last week. The likelihood of triggering an avalanche is slowly decreasing, but recent avalanches and red flags tell us the possibility remains. On nine of the last ten days we received reports of avalanches or signs of an unstable snowpack, such as collapsing and whumphing (weather and avalanche log).

Yesterday a skier triggered an avalanche at Lionhead and shared a video showing they triggered it from far upslope, or remotely triggered. I was north of Cooke City yesterday and experienced collapses of the snowpack and saw a snow structure similar to what is responsible for unstable conditions throughout the forecast area (video). Also yesterday, multiple groups of skiers reported collapsing and loud “whumphs” in Beehive Basin (details) and the Bridger Range (details1, details2).

Other notable signs of instability from the last week include: Big Sky ski patrol reported two avalanches thought to be naturally triggered during the day on Friday, and later they confirmed at least one was triggered by a mountain goat (details, photo). Collapsing and whumphing of the snowpack in nearly every region of our forecast area (avalanche activity). Last Friday Ian saw a couple recent natural avalanches at Lionhead (video), and last Tuesday a skier triggered a large avalanche on Mt. Blackmore in Hyalite (video), both are good examples of the type of avalanche you can still trigger. See our avalanche activity page for a comprehensive look at recent avalanches and red flags.

Avalanches can break wide, and can be triggered on slopes steeper than 30 degrees or from lower angle terrain connected to steep slopes. 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. Signs to stay off steep slopes include cracking, collapsing, recent avalanches, and poor stability test scores.

Human-triggered avalanches are possible, and the avalanche danger is 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.

Tuesday, December 18th, 6:00 pm, Free 1 hr Avalanche Awareness talk  at REI, Bozeman.

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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