GNFAC Avalanche Advisory for Sat Dec 19, 2015

Not Current Advisory 

Good morning. This is Alex Marienthal with the Gallatin National Forest Avalanche Advisory issued on 
Saturday, December 19, at 7:30 a.m. This advisory does not apply to operating ski areas.

AVALANCHE WARNING

The Gallatin National Forest Avalanche Center is issuing a Backcountry Avalanche Warning for the 
mountains near Cooke City, the Madison Range, the Gallatin Range, the Lionhead area near 
West Yellowstone, and the Bridger Range. Continuous snowfall and strong winds have added weight to 
a weak and unstable snowpack. Strong wind this morning will continue to load slopes. The avalanche 
danger is rated HIGH on all slopes. Natural and human triggered slides are likely. Avalanche terrain 
and avalanche runout zones should be avoided.

Mountain Weather: 

At 4 a.m. mountain temperatures are in the 20s F. Since yesterday morning, the mountains around 
Bozeman received 3-4” of snow, and the mountains near Big Sky and West Yellowstone received 6-8”. 
SNOTEL data is still lagging, but reports of heavy snow in Cooke City suggest close to a foot of 
snow fell in the mountains. Overnight, southwest winds reached gusts of 50-60 mph, except in the 
Bridger Range where gusts reached 30 mph. Today, temperatures will be near 30 F and will cool to 
the 20s F this afternoon. Wind will continue to blow 30-40 mph out of the southwest. Snowfall will 
begin this afternoon, and I expect 5-7” by morning.

Snowpack and Avalanche Discussion: 

Bridger Range   Madison Range   Gallatin Range  

Lionhead area nearWest Yellowstone    Cooke City  

In the last 24 hours, snowfall in the mountains totaled .3 to .9” of SWE. Strong winds last night 
transported this snow and added weight to an already unstable snowpack. Yesterday’s snow was not a 
lot of weight in some areas. However, weak faceted snow throughout our advisory area has been 
producing avalanches under loads from the past week’s snow and wind. Avalanches, shooting cracks, 
and collapsing of the snowpack are sure signs that the snowpack is unstable. 

We have observed and received reports of unstable snow in all areas. Snowmobilers triggered and were 
caught in an avalanche near Cooke City on Tuesday. Yesterday, I observed a large natural avalanche 
in the backcountry south of Bridger Bowl and two smaller avalanches on the west side, behind 
Bridger Bowl. 

All of these failed on weak faceted snow. The Big Sky and Yellowstone Club ski patrols continued to 
release avalanches on this weak faceted snow during avalanche mitigation work this week. On Thursday, 
snowmobilers triggered avalanches on this weak snow in Cabin Creek and Buck Ridge in the Madison Range. 

Unstable snow is everywhere in the mountains throughout our advisory area.

Today, snowfall and strong winds have loaded an unstable snowpack to make the avalanche danger HIGH 
on all slopes. Avalanche terrain and avalanche runout zones should be avoided.

SNOTEL sites in SW Montana are not working which impedes our ability to report snowfall amounts. 
NRCS is working hard to fix this issue soon.