GNFAC Avalanche Forecast for Fri Nov 19, 2021

Snowpack and Avalanche Discussion

<p>The Gallatin National Forest Avalanche Center is issuing an avalanche warning for the Centennial Mountains outside Island Park, Idaho. The avalanche danger is HIGH on all slopes. Natural and human-triggered slides are very likely.&nbsp;A large amount of new snow with strong wind are creating unstable conditions over a wide area.&nbsp;Very dangerous avalanche conditions. Backcountry travelers should stay off of steep slopes and stay away from the bottom of steep slopes. Avalanches can be triggered from flat terrain. Steep slopes can avalanche far above and run into lower angled terrain. <a href="http://www.mtavalanche.com/forecast/centennials"><u>www.mtavalanche.com…; for more detailed information.</p>

<p>This week’s storm has a bull’s eye painted on the West Yellowstone area with Lionhead, and the Southern Madison and Southern Gallatin Ranges feeling the effects with 21” equal to 1.8-2” of snow water equivalent (SWE) since Saturday. Strong winds and another blast of snow today are creating dangerous avalanche conditions on all slopes. When we rode Lionhead on Monday, we found heavily wind-drifted slopes already struggling with the weight of the new snow. One large drift expressed this instability with a loud collapse or “whumph” (<a href="https://www.youtube.com/watch?v=Vo9-92FngRA&amp;list=PLXu5151nmAvT1nrM2…;). At the same time, skiers in the Taylor Fork triggered wind drifts on a test slope that broke as a small, but thick slab avalanche (<a href="https://www.mtavalanche.com/images/21/thick-wind-slabs-taylor-fork"><st…;). Continued snowfall throughout the day will increase the danger.&nbsp;</p>

<p>Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential. The danger is rated CONSIDERABLE on all slopes.</p>

<p>Cooke City has received 15” of snow since Saturday equal to 1.5” of SWE with strong winds stripping some slopes and loading others. Last week, Alex and Ian found weak layers buried deeply in the snowpack (<a href="https://www.youtube.com/watch?v=1Nt5LOMI1PQ"><strong><u>Cooke City video 1</u></strong></a>, <a href="https://youtu.be/O29XGDZBL8c"><strong><u>Cooke City video 2</u></strong></a>). These weak layers have not produced avalanches or propagated in unstable test scores yet, and our primary concern revolves around steep slopes with the added weight of wind-loaded snow. Avoid wind-loaded slopes and dig down to assess whether deeper weak layers are nearing their breaking point as continued snowfall adds weight and stress to the snowpack.&nbsp;</p>

<p>Heightened avalanche conditions exist on wind-loaded slopes. On non-wind-loaded slopes, slab avalanches are unlikely. Watch for signs of decreasing stability as today’s snow adds up. The danger is rated MODERATE on wind-loaded slopes and LOW on all others.</p>

<p>With the exception of slopes loaded by the recent wind (<a href="https://www.youtube.com/watch?v=lyl9dBUt0u4&amp;list=PLXu5151nmAvT1nrM2… video</u></strong></a>), the snowpack in the mountains around Bozeman and Big Sky is generally stable. Yesterday in the Bridger Range, we found some slopes stripped down to the dirt and small areas with drum-like wind-slabs. This weekend, climbers in the gullies of Hyalite observed shooting cracks in localized drifts and had them collapse under their feet (<a href="https://www.mtavalanche.com/images/21/propagating-cracks-along-top-and-…;, <a href="https://www.mtavalanche.com/images/21/cracking-drifts-hyalite"><strong>…;, <a href="https://www.youtube.com/watch?v=065JmnekjVs&amp;list=PLXu5151nmAvT1nrM2… Hyalite gullies video</u></strong></a>). We are finding weak facets on some slopes, but they will not be a concern until they are loaded with significant new snow (<a href="https://www.mtavalanche.com/images/21/thin-snowpack-hyalite"><strong><u… of weak snow in Hyalite</u></strong></a>, <a href="https://www.youtube.com/watch?v=8yJ1-BzM8Eg&amp;list=PLXu5151nmAvT1nrM2… video</u></strong></a>). The 5-7” in the forecast today is unlikely to tip the scales but watch for signs of decreasing stability as today’s snow adds up.&nbsp;</p>

<p>The avalanche danger is MODERATE on wind-loaded slopes and LOW on all others.</p>

<p>If you get out, please send us your observations no matter how brief. You can submit them via our <a href="https://www.mtavalanche.com/node/add/snow_observation"><strong><u>websi…;, email (<a href="mailto:mtavalanche@gmail.com"><strong><u>mtavalanche@gmail.com</u></str…;), phone (406-587-6984), or Instagram (#gnfacobs).</p>

attached image

Big 'Un

Dillon Area

The Ice Fall was cubes into a whiskey glass at an Irish pub in Butte. R1 D1

Number of slides
Number caught
Number buried
Number killed
Avalanche Type
Loose-snow avalanche
Ice fall
Trigger Modifier
u-An unintentional release
R size
D size
Bed Surface
U - Unknown
Slab Thickness
2.0 centimeters
Vertical Fall
Slab Width
Slab Thickness units
Single / Multiple / Red Flag
Single Avalanche
Advisory Year