24-25

Wind slabs, cornice formation on Mt. Blackmore

Date
Activity
Skiing

2/17 Immediately noticed signs of wind loading and wind slabs on NE-E aspects once in the basin. New cornices have formed in the last couple days along the North ridge to the summit. Cornice collapse on the summit triggered a small wind slab on an isolated slope. 

Region
Northern Gallatin
Location (from list)
Mt Blackmore

Small storm slabs Bridger Ridge

Date
Activity
Skiing

3 small storm slabs observed on steep terrain of Bridger Ridge (E aspect observed from wolverine woods) Attached is a photo of the deepest likely ~12in crown

No observed wind slab development below 8100ft. No propagating test results in pits dug Lower ramp, and wolverine woods.

Observer Name
Lael Butler

GNFAC Avalanche Forecast for Tue Feb 18, 2025

Snowpack and Avalanche Discussion

<p>There has been a lot of snow recently, and dangerous avalanche conditions exist across the forecast area. Two things have kept us from going to an avalanche warning. First, snow has fallen with little wind. Second, we have a stronger-than-average snowpack this season (see Mark’s <a href="https://www.youtube.com/shorts/O6qlZ9VHs3M"><span>video</span></a&gt;). But make no mistake: if you ski or ride in steep terrain, you will <em>likely</em> trigger an avalanche (<a href="https://www.mtavalanche.com/avalanche-activity"><span>list of recent avalanche activity</span></a>), and that avalanche will <em>likely </em>be large enough to carry, injure or kill you.</p>

<p>Across the forecast area, <strong>wind slab avalanches </strong>on steep wind-loaded slopes are sensitive to human triggers (you). Thankfully, without much wind so far (knock on wood), these unstable wind drifts will be in relatively predictable locations on higher elevation slopes, often above treeline, near ridgelines and below cornices. <em>Avoid all steep slopes</em> you think might be wind-loaded and be watching for signs of instability telling you to back off.</p>

<p>Careful snowpack assessment, cautious route-finding and conservative decision-making are essential to safer travel in non-wind-loaded terrain. During my fieldwork the last two days, I continually evaluated for shooting cracks, scanned hillsides for avalanche activity and dug down a couple of feet below the new snow to assess and test for instability related to buried weak layers (<a href="https://www.youtube.com/shorts/hf1nmcVcC_s"><strong><span>Beehive video</span></strong></a>). Evaluate the potential for <strong>storm slab avalanches </strong>resulting from inconsistencies within the recent snow and <strong>loose snow avalanches </strong>(or sluffs) on steep slopes where the new snow isn’t acting cohesively. Yesterday, skiers north of Bridger Bowl and in Cooke City did a great job recognizing instability and adjusting travel plans to match conditions (<a href="https://www.mtavalanche.com/node/34219"><strong><span>Ramp observation</span></strong></a>, <a href="https://mtavalanche.com/node/34236"><strong><span>Throne observation</span></strong></a>, <a href="https://www.mtavalanche.com/node/34222"><strong><span>Republic Creek observation</span></strong></a>). Even with good evaluation, instability can surprise you as it did with skiers in Hyalite and riders on Buck Ridge (<a href="https://www.mtavalanche.com/node/34210"><strong><span>Elephant Mountain photo</span></strong></a>, <a href="https://www.mtavalanche.com/images/25/snowmobile-triggered-avalanche-bu… photo</span></strong></a>).</p>

<p><strong>Persistent slab avalanches</strong> could break on weak layers buried 2-4 feet deep in the Southern Gallatin and Southern Madison Ranges and the mountains around West Yellowstone, Cooke City and Island Park. Last week in Taylor Fork Alex and I saw four avalanches that broke 1-2 feet deep on layers of sugary facets and feathery surface hoar. This weekend, a rider triggered a small wind slab avalanche at Lionhead that subsequently triggered a deeper persistent slab avalanche (<a href="https://www.mtavalanche.com/node/34208"><strong><span>report</span></st…;). Continued snowfall and wind-loading increase the likelihood of these larger slides<span>&nbsp; </span>(<a href="https://www.mtavalanche.com/node/34156"><strong><span>Lionhead photo</span></strong></a>, <a href="https://www.mtavalanche.com/images/25/cornice-triggered-avalanche-miner… City photo</span></strong></a>,<strong> </strong><a href="https://www.youtube.com/watch?v=wZ3k35z5Ej8&amp;embeds_referring_euri=h… Fork video</span></strong></a>). Test for instability on these buried weak layers before considering any steeper terrain.</p>

<p><span>If you are unsure about the terrain and snowpack assessments described above, or they seem like too much work, avoid terrain steeper than 30 degrees because the danger is rated CONSIDERABLE on all slopes. </span></p>

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Small Wind Slab Avalanche On Middle Peak

Middle Basin
Northern Madison
Code
SS-ASc-R1-D1-S
Aspect
E
Latitude
45.33740
Longitude
-111.38100
Notes

Cohesive wind slabs roughly 1 foot deep were triggered between middle peak and the going home chute on the northeast aspect. Upon skiers weight shooting cracks traveled roughly 100 feet to trigger a small avalanche. The size was small as only the top 50 feet of the slope slid but the snow from the avalanche carried down the entire face.

Number of slides
1
Number caught
0
Number buried
0
Avalanche Type
Soft slab avalanche
Trigger
Skier
Trigger Modifier
c-A controlled or intentional release by the indicated trigger
R size
1
D size
1
Bed Surface
S - Avalanche released within new snow
Problem Type
Storm Slab
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

Natural storm slabs northern Bridgers

The Throne
Bridger Range
Code
R2-D2-S
Elevation
8500
Aspect
E
Latitude
45.88220
Longitude
-110.95200
Notes

From IG: 3 separate natural slides viewed south of the throne today. All east facing. This was the biggest. 
If this is R3 others were more R2

Number of slides
3
Number caught
0
Number buried
0
R size
2
D size
2
Bed Surface
S - Avalanche released within new snow
Problem Type
Storm Slab
Snow Observation Source
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year