December 17, 2010 Avalanche Advisory

Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with a backcountry avalanche advisory for December 17, 2010.  This is the first of our regular avalanche advisories for this season.  Starting today, we will post avalanche advisories on Friday and Monday mornings until the first week in April.

Current Avalanche Danger

We are rating conditions as MODERATE for the entire area. Natural avalanches are unlikely, human triggered avalanches are possible.  Small avalanches are possible in specific areas or large avalanches in isolated areas.

Weather and Snowpack Analysis

Observers were out in the Saddle Mountain, Lolo Pass and Brushy Fork area, Lookout Pass, the Rattlesnake and in the Upper Deer Creek area near Seeley Lake.

The surface hoar that was buried in early December is not as widespread as expected. It appears to be most prominent in the Rattlesnake and the higher elevations of the Bitterroot mountains. Isolated columns were failing and propagating in the Rattlesnake with moderate force (ECTP14-15 with Q1 shear planes). Failures were 40cm deep at the level of the buried surface hoar but not all our pits were showing this layer nor demonstrating any weakness. We also noted several collapse noises while touring the ridges, a good indicator of instability.

Observers in other areas either did not find the buried surface hoar or it had gained significant strength during the past few days.  Surface Hoar was evident in snow pits in the Bitterroot Mountains near Gash Point and in the Brushy Fork area but the snowpack was much stronger than found in the Rattlesnake with ECTP22 and Q1 shear in the Brushy Fork.  The Gash Point area was not propagating fractures in the pits and compression tests were in the moderate range (CTM15 Q2).

At the far ends of the range at Lookout and Lost Trail the snowpack appears mostly stable.

Our primary concern is the spotty nature of the buried surface hoar and its ability to propagate fractures in some areas. As demonstrated in our YouTube video, it is a good idea to dig a few pits to see if this weakness is present on the slope you want to ride on. 

httpv://www.youtube.com/watch?v=1v5SjUeKtA0

Weather Forecast and Avalanche Outlook

A high pressure ridge is building over the area the next 24 hours. Expect dry and cold conditions with little chance for snow until Saturday or Sunday when a short wave with warmer air aloft overruns colder valley air which is favorable for light precipitation development.  We’ll have more information on this situation for Mondays advisory.

Expect current avalanche conditions to remain the same and slowly strengthen over time.