On slopes that are steeper than 35 degrees and wind-loaded above 7000 feet there is a CONSIDERABLE avalanche danger in the West Central Montana backcountry. On all slopes above 5000 feet that are steeper than 35 degrees there is a MODERATE avalanche danger .  In the Rattlesnake Mountains north of Missoula there is HIGH avalanche danger on wind-loaded slopes steeper than 35 degrees. We are not issuing an avalanche warning for the entire area; but a foot of new snow fell on a weak snow surface in the Rattlesnakes Wednesday.    

Good morning. This is Dudley Improta with the West Central Montana Avalanche Center’s advisory for December 23, 2011. We just had a weather event that shows how varied our advisory area can be. Overall, we have Moderate avalanche conditions with pockets of Considerable avalanche danger on steep wind-loaded slopes above 7000 feet. There is High avalanche danger on steep wind-loaded slopes in the Rattlesnakes.

Weather and Snowpack Analysis

On Wednesday we got a nice shot of snow in the region; the Rattlesnakes, in particular, had a localized storm producing over a foot of new snow. The Southern Missions got about 7 inches and the rest of the area received between 2 to 4 inches.

This new snow fell on a weak snow surface of facetsAngular snow with poor bonding created from large temperature gradients within the snowpack. and surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried. that developed during the last couple of weeks. This video of an east slope in the Rattlesnakes tells the story. 

On sun exposed slopes a crustA crust is a hard layer of snow where liquid water has refrozen into grain fabric. Crusts usually result from sun, rain or wind. developed during the recent dry period and the new snow from Wednesday is failing on that crustA crust is a hard layer of snow where liquid water has refrozen into grain fabric. Crusts usually result from sun, rain or wind. as well.  

Yesterday Steve and I skied a short distance into the Rattlesnakes. We felt and heard collapsingWhumpf has actually been adopted as a technical avalanche term to describe the sound of a collapsing snowpack when you cross the snow. A sign of instability. in the snowpack and observed the snow cracking in front of our skis. Steve triggered a 16 inch slabA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers.A relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. when approaching an east slope and we noted debris from a slideA mass of snow sliding, tumbling, or flowing down an inclined surface. Same as avalanche. on a similar aspectThe compass direction a slope faces i.e. North, South, East, West.. Observers near Lost Trail Pass also noted frequent collapsingWhumpf has actually been adopted as a technical avalanche term to describe the sound of a collapsing snowpack when you cross the snow. A sign of instability. of the snowpack on southerly aspects. All observers are seeing the buried surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried.; but the load deposited on the weak snow in the Rattlesnakes has not occurred throughout the advisory area.

Adding the extra weight of wind-loadingThe addition of weight on top of a snowpack, usually from precipitation, wind drifting, or a person. above 7000 feet ( west and northwest winds blowing steadily at 25mph were recorded Thursday at 8000’) makes most steep slopes suspect.

 Weather Forecast and Avalanche Outlook

A fairly dry pattern is predicted under the influence of a high pressure ridge. This will keep mountain temperatures mild to moderate. The mild temperatures may help stabilize the snowpack over time; but the weak layersA snowpack layer with less strength than adjacent layers. Often the layer in the snowpack where an avalanche fractures.A snowpack layer with less strength than adjacent layers. Often the layer in the snowpack where an avalanche fractures. (surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried. and facetsAngular snow with poor bonding created from large temperature gradients within the snowpack.) that are buried are the type that can persist and cause problems in the future.  The next chance for snow showers will be Christmas evening into Monday.

There are photos of the surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried. formation, buried surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried., fractureThe physical separation of the slab from the bed surface and surrounding snow during the initiation of a slab avalanche. propagationThe spreading of a fractureThe physical separation of the slab from the bed surface and surrounding snow during the initiation of a slab avalanche. or crackThe spreading of a fractureThe physical separation of the slab from the bed surface and surrounding snow during the initiation of a slab avalanche. or crack within the snowpack. within the snowpack. and the wind-loadingThe addition of weight on top of a snowpack, usually from precipitation, wind drifting, or a person. in the 2012 season folder in our photo section.

For those of interested in looking at snow profiles from this season we are posting them in a folder of 2012 snow profiles in our photo section also.

Due to the Christmas holiday, I will issue the next advisory on Tuesday December, 27 .

Ski and ride safe, have a great holiday.

If you get out and see avalanche activity or want to send us quick snow observations please use our public observations form on the home page missoulaavalanche.org or write us at info@missoulaavalanche.org with any observations or questions.