GNFAC Avalanche Advisory for Fri Nov 19, 2010

Not the Current Forecast

Good Morning. This is Mark Staples with an early season avalanche information bulletin issued on Friday, November 19 at 7:30 a.m. This bulletin is sponsored by Montana Ale Works, in partnership with the Friends of the Avalanche Center. Please remember that uphill traffic is not allowed at Big Sky or Moonlight Basin Ski Areas. Bridger Bowl is not open and has backcountry snow conditions.

Mountain Weather

Overnight most areas received 5-7 inches of new snow except near Big Sky where up to 10 inches has fallen and near West Yellowstone where only 2 inches has fallen. At 5 a.m. winds were blowing 15-30 mph from the WSW, but yesterday they blew 60-80 mph. Temperatures dropped as snow started falling last night into the low teens F and will only climb to the high teens today under cloudy skies. Winds should calm to around 15 mph, and another 10-12 inches of snow should accumulate by tomorrow morning.

Snowpack and Avalanche Discussion

Yesterday 60-80 mph winds (video) scoured some slopes and formed stiff, meaty slabs on others. Two avalanches were observed at the north end of Bridger Bowl (which is not open, not patrolled, and has backcountry conditions) as they ripped down Hidden Gully onto the Apron below. These appeared to be skier triggered and confined to recently formed wind slabs. With new snow overnight, these wind slabs will remain sensitive to human triggers today and be our primary concern.

On Wednesday the Big Sky Ski Patrol triggered two large avalanches. We pay close attention to avalanches triggered by ski patrols this time of year because the snowpack at ski areas is essentially a backcountry snowpack. These avalanches were on heavily wind loaded slopes but also involved old snow. They broke on an ice crust at the interface between snow from October and snow from November (photo). Eric and I skied in Beehive Basin yesterday to assess the strength of this interface. It was easy to find and seemed well bonded. There you have it, two data points. In one place this interface was strong and well bonded, while only a short distance away it produced big avalanches. Until we get more data, assess the bonding at this interface. More importantly watch for recently formed wind slabs which remain the primary concern today.
 
The danger scale was revised this year and should be more helpful. Read more about it in Doug’s article in Carve Magazine by clicking here and view the new scale here.
 
I will issue another Avalanche Information Bulletin tomorrow morning and Doug will issue one on Sunday. We’ll start daily advisories by next weekend if not sooner. Stay tuned. If you have any snowpack or avalanche observations, drop us a line at mtavalanche@gmail.com or call us at 587-6984.
 

Upcoming Avalanche Education

 
Basic Avalanche Awareness – Wed & Thurs, December 1 & 2, 7:00pm – 9:30pm at SUB Ballroom B&C; 12/4- Field day at Bridger Bowl (more information) (PrePay $25 fee)
 

Avalanche Awareness for Snowmobilers – Wed & Thur, December 1 & 2, 7pm – 9:30pm at Team Bozeman, 2595 Simmental Way (more information)

ADvisory SElect Date Block