Good morning. This is Eric Knoff with the Gallatin National Forest Avalanche Advisory issued on Monday, December 17 at 7:30 a.m. Today’s advisory is sponsored by Yellowstone Club Community Foundation and Bridger Bowl in partnership with the Friends of the Avalanche Center.
Over the past 24 hours the mountains around Cooke City and West Yellowstone including the southern Madison Range picked up 4-5 inches of snow. The mountains around Bozeman and Big Sky picked up 1-2 inches. Today, an additional 3-4 inches will fall in the southern mountains including the northern Madison Range while the Bridger Range and Northern Gallatin Range will see 1-2 inches. Currently, temperatures are in the mid-teens to low twenties F and winds are blowing 20-30 mph out of the WSW with gusts over 50 mph in Hyalite and Big Sky. Today will be the warmest day of the week with highs reaching close to 30 degrees F. Winds will remain strong out of WSW blowing 20-30 mph with upper elevation gusts reaching 50 mph. Temperatures will drop this evening as conditions begin to clear and tomorrow will be a colder and drier day.
Bridger Range Madison Range Gallatin Range
Lionhead area near West Yellowstone Cooke City
Is the weather contributing to instability? Without leaving the office, I can tell you the answer is yes. This has been confirmed by recent avalanche activity in the mountains around Big Sky.
Yesterday, the Moonlight Basin Ski Patrol triggered a large slide during control work. The slide initiated within the new snow, but stepped down to weaker snow near the ground. The resulting debris field was 100 meters wide, 200 meters long and upwards of 15 deep. This event is a good reminder that our snowpack still harbors the potential for producing large and dangerous avalanches. The Big Sky Ski Patrol also triggered slides yesterday during control work. Here, nothing broke into older snow, but fresh wind slabs were still breaking 1-2 feet deep. Recent avalanche activity was also reported by snowmobilers on Buck Ridge south of Big Sky.
These reports indicate the snowpack is changing. The past 10 days of abundant snowfall, light winds and overall good stability has come to an end. Today, wind slabs will become more widespread as new snow and strong WSW winds continue to load leeward slopes. Areas below ridgelines or the lee side of cross loaded gullies will be likely places to find wind deposited snow. Wind drifts will increase in size, coverage and sensitivity today as the storm continues. Identifying and avoiding wind loaded slopes will best way to stay out of trouble.
On slopes that have not been wind loaded, stability is generally good. The lack of prolonged dry spells along with consistent snowfall has kept the formation of persistent weak layers to a minimum. On slopes that do have buried weak layers, triggering a slide will require finding just the right spot. Steep, upper elevation slopes, especially those with rocky margins and a shallow snowpack will be the most suspect (photo). If you do decide to commit to steep terrain today, digging a snowpit on a representative slope to assess structure and stability is a wise idea (video).
Today, plenty of fresh snow and strong winds will make human triggered avalanches likely on wind loaded slopes which have a CONSIDERABLE avalanche danger. Non-wind loaded slopes have a MODERATE avalanche danger.
Doug will issue the next advisory tomorrow morning at 7:30 a.m. If you have any snowpack or avalanche observations drop us a line at mtavalanche@gmail.com or call us at 587-6984.
Interested in more snowpack data? Check out the snowpit page for recent profiles from around the area. Snowpit Profiles
Snowmobiler Introduction to Avalanches with Field Course in West Yellowstone on December 20 and 21. Sign up for this class HERE.
Free 1-hour Avalanche Awareness lecture at Bridger Bowl at 1 p.m. on the 4th floor of the Saddle Peak Lodge on December 23. No registration necessary.