GNFAC Avalanche Advisory for Tue Feb 16, 2010

The Bridger, northern Madison and northern Gallatin Ranges, the mountains around Cooke City and the Washburn Range:

We are still feeling the effects of instability from the weekend's snowfall.  A lot of weight (almost 3 inches of water/ 24+ inches of snow) was added in the Bridger Range with half that in the northern Madison and Gallatin mountains and Cooke City.  Both natural and human triggered avalanches were rampant.  On Sunday, separate groups of backcountry skiers triggered a large avalanche on the SW face of Lone Peak and a big slide in Beehive Basin.  On Buck Ridge a snowmobiler triggered a small avalanche which was on the heels of a lot of natural activity Saturday.  Eric explored Buck Ridge on Sunday (video) and saw fresh slides on many aspects with a wind load.  A large natural avalanche near Brackett Creek in the Bridger Range on Saturday broke near the ground 3-5 feet deep on facets from early December.  On slopes above Truman Gulch a skier got the new snow to avalanche too.  Not to be left out, even far south in Cooke City there were two natural slides on Crown Butte, the second time one of these slopes has run.  Our photos page is filling up with great pictures documenting the action. 

Most slides are breaking under Friday's snowfall on small-grained facets, but some slopes are avalanching even deeper. For today, the avalanche danger is rated CONSIDERABLE on all wind-loaded slopes steeper than 35 degrees and MODERATE on all other terrain.  Although the winds have been calm the past few days I'm still concerned with slopes near the ridgelines that got loaded from the weekend's new snow and strong winds.

The southern Madison, southern Gallatin Ranges and the Lionhead area near West Yellowstone:

The southern mountains from Big Sky to West Yellowstone have a buried layer of surface hoar which is breaking clean 1.5 to 2 feet deep.  Near Hebgen Lake on Sunday a skier released an east facing wind-loaded slope (photo).  In addition to the avalanche he also got several large collapses and cracks. Being prudent they skied slopes no steeper than the low 30s.  Ace Powder Guides called in a second hand report of a snowmobiler triggering and getting buried in an avalanche in the Lionhead area yesterday.  A piece of the rider's orange helmet was showing in the debris and it took about 45 minutes to dig him out.  He was miraculously uninjured. 

Although the southern half of our advisory area only picked up 6-8 inches of snow this weekend, the buried surface hoar is straining to support this small load. Slopes that were drifted by the wind will continue to be unstable too. And if that's not enough to worry about, be aware that facets near the ground are occasionally breaking clean in our stability tests.  The difference between the surface hoar and depth hoar is the difference between a two foot and five foot crown line.

For today, the avalanche danger is rated:

CONSIDERABLE on all wind-loaded slopes;

CONSIDERABLE on all slopes steeper than 35 degrees;

MODERATE on slopes less than 35 degrees that were not affected by the wind.      

I will issue the next advisory tomorrow morning at 7:30 a.m. If you get out in the backcountry let us know what you find.  You can reach us at 587-6984 or email us at mtavalanche@gmail.com.

Avalanche Education

1.Bozeman: Lindley Center

Join the Friends of the Avalanche Center for a FREE 2-hour beacon practice from 6 p.m. to 8 p.m. on Thursday, February 18^th.  All ages welcome.  

2. Bridger Bowl

AAI Level 1 Avalanche Course - Friday, February 19^th to Sunday, February 21^st  

www.americanavalancheinstitute.com

3. Bridger Bowl

AAI Level 2 Avalanche Course - Monday, February 22^nd to Thursday, February 25^th

www.americanavalancheinstitute.com

4. Moonlight Basin

Comprehensive avalanche awareness class - Thursday, March 4^th to Saturday, March 6^th

events@moonlight.com or 406-993-6026