Weather and Avalanche Log for Tue Mar 18, 2025
AVALANCHE WARNING: Island Park, Lionhead, Cooke City
AVALANCHE WARNING: Island Park, Lionhead, Cooke City
From IG: Nature or cornice triggered slides in Arrowhead and Hardscrabble Bowls.
We skied north of Cooke today. The storm snow is upside down with a wind slab sitting on top. There is also a new dust layer that came in last night with the extreme winds. We had two collapses while skinning and ECTP13 down 60cm at the new/old interface on top of a crust, SE aspect at 9700'. Winds were L-M out of of the W and S2 snowfall all day with 8" new during the day.
Today I triggered a D1 dry loose avalanche on a E facing 35 degree slope, 9300 ft on Mt Henderson. The new snow is very low density and is not bonding well to the old interface. I also observed some more dry loose activity/small wind slabs on Sheep Mt, NE facing, 10400 ft.
We traversed the ridge south from the Bridger Bowl ski area boundary to Bridger Peak...
Dry Loose avalanches (sluffs) ended up being the biggest hazard we encountered. The new snow (approximately 8") was easily sluffing over last weeks crust and was entraining significant volume and running far in steep terrain. It had plenty of power to knock you over or into trees/rocks.
Found 5-8” of fresh low-medium density snow. Triggered multiple small soft storm slabs/sluff which occurred on steeper slopes (40+ degrees).
<p>Very dangerous avalanche conditions exist in the mountains of Island Park, West Yellowstone and Cooke City. Natural and human-triggered avalanches are likely. Avoid slopes steeper than 30 degrees and lower-angle terrain in the runout zones of avalanche paths.</p>
<p>Slides will fail within the new and wind-drifted snow as <strong>wind slab </strong><span>and</span><strong> storm slab avalanches. </strong>Since Thursday, the mountains in Island Park have received 40” of snow, equal to 4.1” <a href="https://avalanche.org/avalanche-encyclopedia/#snow-water-equivalent-swe… water equivalent (SWE)</span></a>. During the same period, the Lionhead area received 30” of snow and Cooke City received 24” of snow. Yesterday, hurricane-force gusts up to 100 mph transported snow into unstable slabs at all elevations (the strongest gusts were recorded on Lionhead Ridge).</p>
<p>In the Lionhead area, <strong>persistent slab avalanches </strong>breaking 3-5 feet deep on the weak snow buried in late January are possible. Slides on buried weak layers tapered off a couple of weeks ago, but heavy loading from recent snow could wake these layers up, resulting in large avalanches failing deep within the snowpack.</p>
<p>The specifics of weak layers and types of avalanches are irrelevant today. All can be large enough to injure or kill riders and skiers. Avoidance is critical.</p>
<p><span>The avalanche danger is HIGH on all slopes.<span> </span></span></p>
<p>Dangerous avalanche conditions exist in the Bridger, Southern Madison and Southern Gallatin Ranges, where 20-24” of snow has fallen since Thursday, and strong winds built unstable slabs of drifted snow. Human-triggered <strong>wind slab avalanches</strong> breaking 2-3 feet deep are likely. Yesterday, Alex and I avoided the steep terrain in Tepee Basin, where we measured two feet of recent snow and saw failures within and below the new snow (<a href="https://www.youtube.com/watch?v=VpFFY3Gug0w"><strong><span>video</span>…;). A warming trend yesterday is likely to result in increased instability within layers of the storm snow and <strong>storm slab avalanches</strong>.</p>
<p><strong>Persistent slab avalanches</strong> breaking 3-5 feet deep on the weak snow buried in late January are possible in the Southern Gallatin and Southern Madison Ranges. On Friday, a snowmobiler triggered a 2-3 foot deep hard slab on a steep slope with a relatively shallow snowpack in the Taylor Fork (<a href="https://www.mtavalanche.com/node/34610"><strong><span>video, photo and obs</span></strong></a>). With the heavy loading from recent snow, we could see an increase in larger avalanches.</p>
<p><span>To enjoy a safe day in the mountains, cautious route-finding that largely avoids steep slopes and runout zones is essential. The avalanche danger is CONSIDERABLE on all slopes.<span> </span></span></p>
<p>Since Thursday, the Northern Madison and Northern Gallatin Ranges have received 11-19” of snow (0.8-1.1” SWE). Strong winds scoured some slopes and loaded others, where <strong>wind slab avalanches </strong>breaking 1-2 feet deep are the primary concern. Yesterday, Mark and his partner triggered small drifts on test slopes (<a href="https://www.mtavalanche.com/node/34624"><strong><span>photos and details</span></strong></a>). Seek out slopes sheltered from the wind for better snow and safer avalanche conditions.</p>
<p>On non-wind-loaded slopes, <strong>loose snow avalanches</strong> are possible. While relatively harmless in open terrain, they can be dangerous above cliffs, rocks, or trees.</p>
<p><span>Identify and avoid unstable wind-loaded terrain by watching for a textured or pillowy snow surface, cracking or a stiffening of the snow surface. The avalanche danger is MODERATE. </span></p>
Warming trend in the south - upside-down snow. Peak Gust: 100 mph on LH Ridge
AVALANCHE WARNING: Island Park, Lionhead, Cooke City