December 23, 2011 Avalanche Advisory
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 facets and surface hoar 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 crust developed during the recent dry period and the new snow from Wednesday is failing on that crust as well.
Yesterday Steve and I skied a short distance into the Rattlesnakes. We felt and heard collapsing in the snowpack and observed the snow cracking in front of our skis. Steve triggered a 16 inch slab when approaching an east slope and we noted debris from a slide on a similar aspect. Observers near Lost Trail Pass also noted frequent collapsing of the snowpack on southerly aspects. All observers are seeing the buried surface hoar; but the load deposited on the weak snow in the Rattlesnakes has not occurred throughout the advisory area.
Adding the extra weight of wind-loading 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 layers (surface hoar and facets) 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 hoar formation, buried surface hoar, fracture propagation and the wind-loading 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 firstname.lastname@example.org with any observations or questions.