22-23

Sacajawea 4/03 AM

Date

Saw this yesterday morning,  thought I sent it.

Location (from list)
Sacajewea Peak
Observer Name
Robert Sheridan

GNFAC Avalanche Forecast for Tue Apr 4, 2023

Snowpack and Avalanche Discussion

<p>Strong winds gusting to 75 mph from the east to the north are blowing in the Bridger Range, drifting snow into unstable slabs where triggering an avalanche is likely. Winds are scouring slopes that are commonly wind-loaded and depositing the snow in unusual patterns. Watch for mounds of drifted snow, a stiffening of the snow surface or shooting cracks to help identify areas of concern. Recent natural avalanches breaking 1-3’ deep on wind-loaded slopes in the Bridger Range are good examples of the problem (<a href="https://www.mtavalanche.com/node/28928"><strong><span>Battle Ridge</span></strong></a>, <a href="https://www.mtavalanche.com/node/28930"><strong><span>Naya Nuki</span></strong></a>).</p>

<p>Avalanches breaking on weak layers deep in the snowpack are less likely, but a natural avalanche above Ainger Lake that broke 3-4 feet deep and 500 feet wide this weekend is a clear indicator that they remain possible (<a href="https://www.mtavalanche.com/node/28900"><strong><span>details and photos</span></strong></a>).</p>

<p>Take a step back and seek out more conservative terrain that is sheltered from the wind. The avalanche danger is CONSIDERABLE on wind-loaded slopes and MODERATE on all others.</p>

<p>Human-triggered avalanches breaking 1-2’ deep are possible within slabs of recently wind-drifted snow and 4-6’ deep where persistent weak layers are buried in the snowpack. The more likely of the two problems is triggering an avalanche within the wind-drifted snow, and the more dangerous is a deep slab avalanche that could propagate long distances across slopes.</p>

<p>Concern about deep slab avalanches should drive decision-making. Reckon with the consequence of getting caught if you choose steep terrain, especially in higher elevation wind-loaded terrain. Riding in the Lionhead area yesterday, we saw evidence of a natural avalanche cycle that occurred Saturday night through Sunday and included a slide that broke 4-6’ deep and ran to the creek below (<a href="https://www.mtavalanche.com/node/28949"><strong><span>observations, video, photos</span></strong></a>). Similar deep slab avalanches near Cooke City and in the Southern Madison Range broke over 1000’ wide during this weekend’s storm (<a href="https://www.mtavalanche.com/node/28924%5C"><strong><span>Woody Ridge avalanche</span></strong></a>, <a href="https://www.mtavalanche.com/node/28911"><strong><span>Hilgard avalanche + video</span></strong></a>). The best mitigation strategy is patience and conservative terrain selection. Choose lower-angle slopes sheltered from the wind and with fewer terrain features to exacerbate the consequences of getting caught.</p>

<p>Wind-slab avalanches breaking within recently drifted snow are more likely. Recent examples include a natural avalanche on Woody Ridge (<a href="https://www.mtavalanche.com/node/28936"><strong><span>photos and details</span></strong></a>), a skier triggered a slide on Mineral Mountain (<a href="https://www.mtavalanche.com/node/28943"><strong><span>photo and details</span></strong></a>), six natural avalanches above Beaver Creek (<a href="https://www.youtube.com/watch?v=WCGTZlvG5-E"><strong><span>video</span>…;), and several slides below the cornice on Lionhead Ridge (<a href="https://www.youtube.com/watch?v=URlpahpvzJA"><strong><span>video</span>…;). Avoid recently and commonly wind-loaded slopes, dig down three feet to test the upper snowpack and watch for signs of instability, such as shooting cracks or stiffening of the snow surface, that indicate problem areas.</p>

<p>The danger is MODERATE.</p>

<p>Please share avalanche, snowpack or weather observations via our website, email (mtavalanche@gmail.com), phone (406-587-6984), or Instagram (#gnfacobs).</p>

<p>Human-triggered avalanches breaking 1-2’ deep are possible within slabs of recently wind-drifted snow and 4-6’ deep where persistent weak layers are buried in the snowpack. The more likely of the two problems is triggering an avalanche within the wind-drifted snow, and the more dangerous is a deep slab avalanche. Choose lower-angle slopes sheltered from the wind and with fewer terrain features that would exacerbate the consequences of getting caught. Dig down three feet to test the snowpack and watch for signs that indicate problem areas, such as shooting cracks or stiffening of the snow surface.</p>

<p>Skiers in the Hellroaring Creek drainage reported a widespread avalanche cycle around Nemesis Mountain during this weekend’s storm. The larger avalanches leveled many trees (<a href="https://www.mtavalanche.com/node/28948"><strong><span>observation and photos</span></strong></a>).</p>

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Natural slides on all aspects of Nemesis Mountain

Hellroaring Creek
Island Park
Code
N-D3
Latitude
44.54900
Longitude
-111.47500
Notes

From obs: "Our party of 6 stayed at the Hellroaring Creek hut over the weekend. The winds and snowfall were intense, with the snow turning more graupelly by mid-Saturday. We dug a pit on a southerly aspect of Mount Nemesis, near the hut around 8000.’ We noted the complex layering, including a weak layer about 20” down that failed on the 23rd shovel hit. 

On Sunday morning, the sun broke through for a bit and allowed us to view the widespread avalanche cycle that occurred overnight on all aspects. Across the creek, we saw a large crown (approx 3’ in depth) in the meadows. 

An avalanche that occurred on Nemesis’s south face ran from mid-mountain all the way into Hellroaring Creek, running over our old skinner. 

The natural slides on Nemesis’s north face were some of the biggest we have seen in any slide, knocking out a lot of trees towards the bottom."

Number of slides
1
Number caught
0
Number buried
0
Trigger
Natural trigger
D size
3
Slab Thickness
36.0 inches
Slab Thickness units
inches
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

From obs: "On Sunday morning, the sun broke through for a bit and allowed us to view the widespread avalanche cycle that occurred overnight on all aspects. Across the creek, we saw a large crown (approx 3’ in depth) in the meadows. 

An avalanche that occurred on Nemesis’s south face ran from mid-mountain all the way into Hellroaring Creek, running over our old skin track." Photo: Eric and Melissa

Island Park, 2023-04-04

Natural Avalanche Cycle at Lionhead

LIONHEAD AREA
Lionhead Range
Code
N-R2-D2
Aspect Range
E - N
Latitude
44.72920
Longitude
-111.32300
Notes

We saw three more avalanches that broke at least a few feet deep and several storm slab avalanches that failed within the new and wind-drifted snow. No other observed avalanches were as large as the one on Targhee Peak. 

We stayed in terrain less than 30 degrees steep, with minor exceptions on small slopes. We avoided all large, wind-loaded slopes and crossed below them cautiously. The snowpack maintains the characteristics of mid-winter, and deeply buried persistent weak layers remain a concern. Going forward, we will continue to assess the upper few feet of the snowpack for instability before considering any steep slopes. We will continue to manage the deep slab avalanche problem through avoidance (of avalanche terrain) and consequence minimization (by selecting non-wind-loaded slopes that are smaller and free of terrain traps). 

Number of slides
5
Number caught
0
Number buried
0
Trigger
Natural trigger
R size
2
D size
2
Snow Observation Source
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year

Deep Slab Avalanche, Bald Peak - Lionhead

LIONHEAD AREA
Lionhead Range
Code
HS-N-R3-D2.5-O
Elevation
9500
Aspect
E
Latitude
44.72920
Longitude
-111.32300
Notes

There was a large natural avalanche that broke yesterday on Bald Peak it was bounded laterally by the terrain, so it only broke a three hundred feet across, but it appeared to be 4-6' deep and ran the full available vertical into the creek below. This avalanche was the key piece of information for the day and answered the question of whether or not the snowpack is continuing to produce deep slab avalanches with every storm, it is.

Number of slides
1
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Natural trigger
R size
3
D size
2.5
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness
54.0 inches
Vertical Fall
1200ft
Slab Width
300.00ft
Snow Observation Source
Slab Thickness units
inches
Single / Multiple / Red Flag
Single Avalanche
Advisory Year