GNFAC Avalanche Forecast for Sat Nov 18, 2017

The storm that ended yesterday morning left 1-2 feet of dense snow in the mountains around Cooke City and West Yellowstone. This new snow was equal to 1.5 to 2” of SWE (snow water equivalent) and was accompanied by strong west to southwest winds. This rapid, heavy load created very unstable conditions. Snowmobilers near Cooke City witnessed a natural avalanche on the east side of Mt. Abundance that covered their tracks from earlier in the day (photo). Strong wind last night continued to load slopes, and today avalanches will remain easy to trigger or could run naturally. Carefully evaluate terrain and avoid traveling on or underneath large, steep slopes.

The 2-4” of new snow in the mountains near Bozeman and Big Sky does not create as much instability as the 1-2 feet in the southern ranges. However, strong west-southwest wind drifted recent snow into wind slabs and large cornices (video, photo). Fresh wind slabs and cornices are mostly confined to ridgelines (photo, photo), and will be possible to trigger this weekend. The best strategy is to travel on non-wind loaded slopes, and stay far back from the edge of cornices while travelling along ridgelines.

On non-wind loaded slopes, snow from November has bonded mostly well to the older snow. Avalanches breaking on deeper layers are still possible, as Eric and I found earlier this week in the northern Bridgers (video, photo), and as shown by a large natural avalanche near Cooke City last week (photo). Many slopes are stable while some are not. Dig a snowpit and do a stability test to assess the relationship between all layers in the snowpack before traveling in steep terrain.

We are still collecting snowpack data from around the advisory area and will being issuing danger ratings when we start daily advisories a week from today.

If you get out and have any avalanche or snowpack observations to share, drop a line via our website, email (mtavalanche@gmail.com), phone (406-587-6984), or Instagram (#gnfacobs).