20-21

Crown Butte, Snowmobile triggered Avalanche

Crown Butte
Cooke City
Code
SS-AMu-R2-D2-O
Elevation
9800
Aspect
SE
Latitude
45.05250
Longitude
-109.96200
Notes

A snowmobile triggered avalanche broke 1-2' deep on a layer of near surface facets sandwiched between two crusts. 

Number of slides
1
Number caught
0
Number buried
0
Number killed
0
Avalanche Type
Soft slab avalanche
Trigger
Snowmobile
Trigger Modifier
u-An unintentional release
R size
2
D size
2
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness
60.0 centimeters
Vertical Fall
400ft
Slab Width
150.00ft
Weak Layer Grain type
Near surface faceted particles
Weak Layer grain size
1.00mm
Weak Layer Hardness
F
Slab Layer Grain Type
Decomposing and Fragmented precipitation particles
Slab Layer Hardness
4F
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

Telemark Meadows area E. aspect ~8,000ft.

Date
Activity
Snowboarding

Widespread ice crust on all aspects buried underneath fresh snow. Facets directly underneath the crust, small layer(few inches) of more consolidated snow, then more weak/sugary snow all the way to the ground. Widespread collapsing and cracking observed while stomping around the slope doing beacon practice

Observer Name
Jeffrey Mann

GNFAC Avalanche Forecast for Wed Dec 30, 2020

Snowpack and Avalanche Discussion

<p>The snowpack in all our ranges except Cooke City is weak. Snow that fell in October rotted into sugary facets from exposure to clear skies and little snowfall. These grains do not bond well and they fall out of a fist like sand. These grains are weak and cannot support much weight. This layer is 1-2 feet thick and blankets the mountains from West Yellowstone to the Bridger Range, as well as most of the western US. Small snowstorms are slowly building a slab of meatier snow which the facets are struggling to support. Whumpfs, cracks, small avalanches, and unstable test scores are warnings from Mother Nature that the snowpack structure is poor. These signs have been reported <a href="https://www.mtavalanche.com/weather/wx-avalanche-log"><strong><u>13 of the last 14 days</u></strong></a>. Yesterday, Ian and I rode into Buck Ridge and saw recent avalanches, had collapsing and cracking of the snowpack when we post-holed over to dig a snowpit, and got unstable test results. <a href="https://www.mtavalanche.com/photos"><strong><u>3 pictures</u></strong></a>, one <a href="https://www.mtavalanche.com/images/20/3rd-yellowmule"><strong><u>snowpi…; </u></a>and a <a href="https://youtu.be/7jzopnikTNE"><strong><u>video</u></strong></a&gt; document the problem through all these ranges.&nbsp;</p>

<p>It is not an issue of <u>if</u> avalanches will occur, but an issue of <u>when</u>. We need more snow to tip the scales towards widespread instability. Without the weight of new or wind blown snow being added to slopes the chances of triggering avalanches is decreasing, yet still possible. It is difficult to determine which slope will slide and which will not because the snow structure is uniformly bad everywhere. The only defense against this threat is to treat every slope with suspicion. It’s like driving on icy roads with bald tires; you can do it, but it might not end well.&nbsp;</p>

<p>For today the avalanche danger is rated MODERATE on all slopes since our snow structure is poor and triggering slides is possible.</p>

<p>Cooke City has a deep snowpack and only a few isolated instabilities. Without snowfall actively loading slopes, today’s concerns are limited to areas where the snowpack is thin and harboring weaker snow (i.e. in valley bottoms or near steep, craggy areas). Collapses (whumpfs) and cracks are a sign you found it. These mountains do not have widespread weak layers which makes it even more important to hunt for instability by digging and testing the snowpack before committing to avalanche terrain. Alex is in the area and reported that stability has improved from our visit last week. In the absence of new snow and widespread/persistent weak layers, the avalanche danger is rated LOW since natural and human triggered slides are unlikely.&nbsp;&nbsp;</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>

Upcoming Avalanche Education and Events

See our education calendar for an up to date list of all local classes. Here are a few select upcoming events and opportunities to check out:

North West of Gardiner Montana

Date
Activity
Skiing

Ski toured towards sheep mountain above Gardiner MT to a spot I have skied frequently the last 4 years fall, winter and spring. Never had I had so many cracks or whumphs up there. 2 very large whumphs in different locations where the whole meadow (25 degrees) settled ~2cm and cracks shot out as well. Dug a pit on a 28 degree east facing slope at 8800 feet. Broke on ectp2 directly above 8cm of facets (fist) with rounding grains above it (1 finger) shovel shear broke on the same level as well as the Column test (ctp7)

Region
Out of Advisory Area
Observer Name
Oscar Dalling

Snowmobile triggered avalanche in the 3rd Yellowmule off Buck Ridge. Appears to have been triggered from low on the slope on Sunday, Dec 27th. Broke on weak snow at the ground.

Photo: GNFAC

Northern Madison, 2020-12-29