Good morning. This is Mark Staples with the Gallatin National Forest Avalanche Advisory issued on Thursday, December 6 at 7:30 a.m. Today’s advisory is sponsored by the Yellowstone Club Community Foundation in partnership with the Friends of the Avalanche Center. Bridger Bowl opens Friday and is conducting avalanche control, so be careful traveling inside the ski area boundaries. Our next advisory will be issued tomorrow morning.
Yesterday rain fell in the valleys and snow fell in the mountains. Near Bozeman and Big Sky 1-3 inches of dense snow fell. Near West Yellowstone and Cooke City 3-5 inches of dense snow fell. Overnight temperatures dropped to the high teens and low 20s F. Winds eased a little since yesterday and were blowing 15-25 mph from the WSW this morning. Both winds and temperatures shouldn’t change much today, and snowfall will return sometime this afternoon or early evening. By tomorrow morning most areas will receive 3-5 inches of snow with more coming Friday night followed by much colder temperatures for Saturday.
Southern Madison Range Southern Gallatin Range Cooke City
Lionhead area near West Yellowstone
For seven of the last eight days, snow has fallen in the southern mountains. This fact alone is a red flag. In the last 48 hours, SNOTEL sites recorded 0.9 to 1.7 inches of SWE. If temperatures had been colder, this precipitation would have translated to 12-24 inches of snow, but it has resulted in 6-12 inches of warm, dense snow.
There are two avalanche problems:
- The combination of plentiful new snow and wind created wind slabs. Fresh wind slabs can almost always create avalanches. Fortunately the warm temperatures have helped wind slabs and storm snow bond well but they should not be trusted in the near future.
- Although the snowpack lacks widespread weak layers, they do exist on some slopes. Yesterday in the Taylor Fork area on a NE facing slope, I found surface hoar buried 2ft deep that produced unstable results in my stability tests (photo, video). A skier near Cooke City on Scotch Bonnet Mountain on a S facing slope also found a thin layer of facets on an ice crust buried 2ft deep that produced unstable results in his stability tests. Because these layers are not widespread and we have limited data, their exact distribution is unknown. Either dig snowpits or assume they exist on every slope.
For today with so much new snow, the avalanche danger is rated CONSIDERABLE on all wind-loaded terrain. All other slopes will have a MODERATE avalanche danger.
Bridger Range Northern Madison Range Northern Gallatin Range
This time of year we pay close attention to avalanche activity at nearby ski areas because their snowpack mirrors the backcountry snowpack. The good news is that most avalanche activity at Moonlight Basin, Big Sky and Bridger has been confined to storm snow and wind slabs. Big Sky triggered some avalanches breaking in deeper, older layers in the snowpack on northerly facing, higher elevation slopes that harbored snow from October and early November. At lower elevations or on southerly facing slopes, this old snow either melted or warmed enough that it did not facet and become weak. Last Sunday, a snowmobiler on Buck Ridge just south of Big Sky remotely triggered a 700ft wide avalanche on this old snow (video, pictures, snowpit). Fortunately many other slopes have stronger, stable snow. In the Bridger Range, the Bridger Bowl Ski Patrol has not triggered avalanches on these old layers of snow, but they have found very sensitive cornices breaking closer to the ridge than anticipated.
It’s still possible to trigger avalanches in wind-loaded terrain where today the avalanche danger is rated MODERATE. On slopes without a wind-load, the danger is rated LOW.
I 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. Thank you.
Snowmobiler Introduction to Avalanches at Greenup Performance in Black Eagle near Great Falls, MT, December 8, 9 a.m. to 2 p.m. FREE. No sign up required!
Snowmobiler Introduction to Avalanches with Field Course in Billings at Hi-Tech Motor Sports on December 12 and 13, 6-9 p.m. and the field course on January 20 in Cooke City. Sign up for this class HERE
Snowmobiler Introduction to Avalanches with Field Course in West Yellowstone on December 20 and 21. Sign up for this class HERE.
FREE 1 hour lectures:
December 12, REI in Bozeman, 6:30 p.m.