19-20
GNFAC Avalanche Forecast for Wed Dec 4, 2019
Snowpack and Avalanche Discussion
<p>The mountains south of Big Sky to West Yellowstone have the weakest snow in our forecast area. Near the ground a layer of sugary, faceted snow (called depth hoar) struggles to support the snow from the weekend. Dave and I had this layer collapse with an audible “whumph” many times in Lionhead, and Ian and I had the same yesterday in Taylor Fork. A whumph is an avalanche on flat terrain and bullseye information the snow is unstable. Without new snow in the forecast this weak layer will become less reactive to triggering; however, like a bad case of diaper rash, it will be problematic for weeks to come.</p>
<ul>
<li>Watch our videos from <strong><a href="https://youtu.be/mAbxNKrJb2U">Lionhead</a></strong> and <strong><a href="https://youtu.be/tlOyL-hbwE4">Taylor Fork</a></strong> and learn about this layer.</li>
<li>Check out these 2 photographs of depth hoar (<strong><a href="https://www.mtavalanche.com/images/19/large-and-weak-depth-hoar">pic1,<…; <strong><a href="https://www.mtavalanche.com/images/19/depth-hoar-taylor-fork">pic2</a><…;).</li>
</ul>
<p>For today, the avalanche danger is rated<strong> MODERATE</strong> since we are still seeing signs of instability and it is possible to trigger avalanches.</p>
<p>Yesterday the Bridger Bowl Ski Patrol triggered large avalanches on a few heavily wind-loaded slopes at the ridgeline. These were hard slabs of wind-packed snow that traveled 1000’ downhill (<strong><a href="https://www.mtavalanche.com/images/19/large-avalanche-mundys-bowl-bridg…;, <strong><a href="https://www.mtavalanche.com/images/19/large-avalanche-wind-loaded-slope…;). They broke 2-4 feet deep and ran on a crust that formed early in the season. The size of these slides caught us off guard. We were not expecting them to run that deep. Patrollers triggered the slides as they cut and shoveled cornices. Although these were triggered in the Bridger Range, the Yellowstone Club Ski Patrol and Big Sky Ski Patrol are also finding wind-loaded slopes to be problematic and I have no reason to think otherwise in the northern Gallatin Range or around Cooke City. Heavily wind-loaded terrain is avalanche prone. Without a wind-load slopes are generally stable, yet isolated areas could harbor weak snow. Dave and Alex found this in the northern Bridger Range which they detected with a stability test (<strong><a href="https://youtu.be/5U2vv30wHmw">video</a></strong>).</p>
<p>For today, the avalanche danger is rated <strong>MODERATE</strong> on all wind-loaded slopes and <strong>LOW</strong> on all others.</p>
<p>If you get out, please send us your observations no matter how brief. You can fill out an <u><strong><a href="https://www.mtavalanche.com/node/add/snow_observation">observation form</a></strong></u>, email us (<u><strong><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></strong></u>), leave a VM at 406-587-6984, or Instagram (#gnfacobs).</p>
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This is a close-up of the angular Depth Hoar grains found near the ground in the mountains south of Big Sky to West Yellowstone. These are weak and cannot support a lot of snow weight. Avalanches will break near the ground on these. Photo: GNFAC
Forecast link: GNFAC Avalanche Forecast for Wed Dec 4, 2019
At the bottom of the snowpack is a thick layer of weak and sugary, faceted snow called Depth Hoar. These large grains do not bond well and form an unstable foundation of the snowpack. Avalanches will break at this layer. Photo: GNFAC
The Bridger Bowl Ski Patrol triggered this slide on a heavily wind-loaded east facing slope at the ridgeline. They wrote, "With a shovel push the vertical cornice/pillow broke 15''-36'' deep on a steep and rounded bulge just south of Lee's Leap running on the crust. The debris terminated well below Pat's Chute." Its code is HS-AC-R2-D2-O-TR. Photo: BBSP
This large avalanche was triggered intentionally by the Bridger Bowl Ski Patrol at the ridgeline above little Slushman's Ravine. They were cutting a cornice and it broke 7 feet back from the edge, 4 feet deep and 150 feet wide. It was a hard slab avalanche and ran 1150 feet. It was east facing and broke on a crust formed in the early season. It's code is HS-AC-R2-D3-O-TL. Photo: BBSP
We put up the Taylor Fork weather station today (12/03) and it is churning out hourly data. Check it out here: https://www.mtavalanche.com/weather/stations/taylor-fork
We partnered with the Gallatin Valley Snowmobile Association to put in two new beacon checkers at the Taylor Fork and Buck Ridge trailheads. They are working great!
Thanks GVSA! Photo: GNFAC
Forecast link: GNFAC Avalanche Forecast for Thu Dec 5, 2019