Sunday, December 23rd, 2007 Good Morning. This is Scott Schmidt with the Gallatin National Forest Avalanche Advisory issued on Sunday, December 23rd, at 7:30 a.m. The Big Sky Snowriders of Livingston, in cooperation with the Friends of the Avalanche Center, sponsor todays advisory. This advisory does not apply to operating ski areas. MOUNTAIN WEATHER The anticipated northwest flow did not materialize and as a result neither did my forecasted snow. One to three inches of snow has fallen in most of the mountains of southwest Montana since yesterday. Winds have been 15 30 mph from the northwest and mountain temperatures are in the low teens. Winds will be westerly today, blowing15 30 mph, with temperatures in the upper-teens at 9000 feet. Another system moves into our area tonight with a chance for 1 3 inches of snow favoring the southern mountains. SNOWPACK AND AVALANCHE DISCUSSION The southern Gallatin and southern Madison Ranges, the Lionhead area near West Yellowstone, the mountains around Cooke City, and the Washburn Range: The ACE Powder Guides from West Yellowstone rode into the area behind Lionhead Mountain yesterday. They saw 6 7 large natural avalanches that released near the ridgeline and felt the snow settle under their sleds every time they came to a stop. A group skiing in the Bacon Rind drainage reported similar snowpack collapsing as they toured up to the ridge. Two snowpack formations are producing signs of instability in the southern mountains. Weak snow that formed near the ground in November is struggling to support last weeks new snow load. The added weight of a ski or snowmachiner exceeds the load bearing capacity of these weak facets; resulting in the snowpack collapsing that is being reported. This weak layer was responsible for an avalanche cycle that occurred in the southern mountains the first week in December. As a result, many steep slopes in this area shed this weak snow when they avalanched and are now stable. A group skiing near Hebgan Lake Friday dug numerous snowpits and report good stability on steep slopes. From past observations, we know these slopes avalanched during the early December cycle and are now stable. If you know a slopes history or if you are willing to spend some time digging in the snow and making a stability assessment many non-wind-loaded slopes in the southern mountains can safely accessed for your recreational enjoyment. If youre not willing to do the work to ascertain a slopes stability then you should avoid all steep terrain in the southern mountains today. The second problematic snowpack formation is at the interface between the old snow surface and the new snow that has fallen in the past week. Since last Sunday, 2 4 feet of snow has fallen in the mountains around West Yellowstone and Cooke City. In many areas, this new snow has been deposited on a thin layer of faceted snow that formed on the surface the second week of December. Karl reported this layer in the Lionhead area last week and I found it on slopes in the Bacon Rind drainage Friday. This layer is failing on slopes where wind is depositing additional load in the form of wind slabs. It is these recently wind-loaded areas that are producing most of the reported natural avalanche activity. Today, for the southern Gallatin and southern Madison Ranges, the Lionhead area near West Yellowstone, the mountains around Cooke City, and the Washburn Range, wind-loaded slopes steeper then 35 degrees have a HIGH avalanche danger and should be avoided. Wind-loaded slopes that are less steep and non-wind-loaded slopes steeper than 35 degrees have a CONSIDERABLE avalanche danger and should be approached with caution until you determine their stability. A MODERATE avalanche danger exists on all other slopes in these mountains. The northern Gallatin and northern Madison Range: The Big Sky patrol reported a skier-triggered avalanche that released 1 2 feet deep on a wind affected slope near tree line yesterday. A skier was caught and partially buried on a similar slope in Middle Basin just north of Big Sky. Two other groups touring in this area also reported ski-trigging avalanches. The Big Sky patrol also released two avalanches in the alpine that ran in the new snow and then stepped down into older layers. Remember that wind moves snow through trees near ridgelines. Any signs of wind deposited snow, such as small cornices and wind pillows are an indication you are in wind-loaded terrain, even if the slope in question has trees on it. Today, the avalanche danger in the northern Gallatin and Madison Ranges is CONSIDERABLE on recently wind-loaded slopes steeper then 35 degrees. Wind-loaded slopes that are less steep, and non-wind-loaded slopes steeper than 35 degrees have a MODERATE avalanche danger. A LOW avalanche danger exists on all other slopes in these mountains. The Bridger Range: The Bridger Bowl patrol used explosives to released several avalanche on wind-loaded slopes near the ridge crest yesterday, and a skier on Saddle Peak triggered a small wind-slab that fractured about 75 feet wide. Winds have been blowing 30 40 mph in the Bridger Range for the past 24 hours and steep wind-loaded slopes should be avoided. The avalanche danger in the Bridger range is CONSIDERABLE on recently wind-loaded slopes steeper then 35 degrees. Wind-loaded slopes that are less steep have a MODERATE avalanche danger. On non-wind-loaded slopes, the avalanche danger is LOW.