19-20

GNFAC Avalanche Forecast for Fri Dec 6, 2019

Snowpack and Avalanche Discussion

<p>The snowpack in the Madison Range, Southern Gallatin Range, and Lionhead area has a weak foundation of sugary, faceted snow (<strong><a href="https://www.mtavalanche.com/images/19/poor-snow-structure-buck-ridge">p…;, <strong><a href="https://www.youtube.com/watch?v=AbtyS3X7glA&amp;list=PLXu5151nmAvSbcbVf…;, <strong><a href="https://www.youtube.com/watch?v=tlOyL-hbwE4&amp;list=PLXu5151nmAvSbcbVf…;, <strong><a href="https://www.youtube.com/watch?v=mAbxNKrJb2U&amp;list=PLXu5151nmAvSbcbVf…;). As each additional day passes since the last snowfall, avalanches are slowly becoming less likely. However, triggering an avalanche remains possible on slopes with a cohesive slab capping the weak snow at the ground. Before getting onto a steep slope quickly dig down to see if you have this worrisome combination of slab and weak layer. If you find this combo, evaluate it carefully or stick to lower angled terrain.</p>

<p>The avalanche danger is <strong>MODERATE</strong> today.</p>

<p>Triggering avalanches remains possible on heavily wind loaded slopes in the Bridger Range, Northern Gallatin Range and around Cooke City. Yesterday, Bridger Bowl Ski Patrol triggered several large avalanches breaking under hard slabs of wind drifted snow near the ridgeline. Alex went up to take a look at these slides and found they were breaking on a thin layer of weak snow low in the snowpack (<strong><a href="https://www.youtube.com/watch?v=mbm3kJ652Fk&amp;list=PLXu5151nmAvSbcbVf…;). You can trigger similar slides today on high elevation slopes with thick drifts of windloaded snow (e.g. Saddle Peak). Hard slabs are a difficult problem to manage, you will likely not see any signs of instability until you hit just the wrong spot and trigger a large slide.</p>

<p>Slopes without thick winddrifts are generally stable. However, digging to evaluate the snowpack for yourself on a particular slope is the best way to avoid an unpleasant surprise. &nbsp;</p>

<p>Today, avalanches are possible on wind loaded slopes where avalanche danger is <strong>MODERATE</strong>. On non-wind loaded slopes avalanche danger is <strong>LOW</strong>.</p>

<p>If you get out, please send us your observations no matter how brief. You can fill out an&nbsp;<strong><a href="https://www.mtavalanche.com/node/add/snow_observation">observation form</a></strong>, email us (<strong><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></strong&gt;), leave a VM at 406-587-6984, or Instagram (#gnfacobs).</p>

Backcountry Barriers Contest

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Unstable Hard Slabs on Saddle

We went to look at the snowpack on Saddle Peak in the backcountry south of Bridger Bowl (12/5/19). Heavily wind loaded slopes have hard slabs over weak facets. Unfortunately this poor snowpack structure will be with us for a while in the backcountry. It will not get compacted by skier traffic or show obvious signs of instability. SKi patrol triggered this slide just inside their boundary. This is the first time they have done control this season, so it is a glimpse of what we might find in the uncontrolled backcountry snowpack. Photo: GNFAC

Bridger Range, 2019-12-05

Collapses in s. Gallatin

Specimen Creek
Southern Gallatin
Code
Elevation
8300
Aspect
S
Latitude
45.02650
Longitude
-111.04800
Notes

From e-mail: "Skied up to 8300 feet on Meldrum Mountain until I succumbed to the deadfall and turned around. Poor structure with facets at the ground and low coverage. Collapsing on southerly aspects where there was enough snow. Northerly aspects were unsupportable."

Number of slides
0
Number caught
0
Number buried
0
Problem Type
Persistent Weak Layer
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Red Flag
Advisory Year

GNFAC Avalanche Forecast for Thu Dec 5, 2019

Snowpack and Avalanche Discussion

<p>Yesterday I went to Buck Ridge near Big Sky and found a poor snowpack structure with a foundation of sugary, weak facets (<strong><a href="https://www.mtavalanche.com/images/19/poor-snow-structure-buck-ridge">p…;, <strong><a href="https://www.youtube.com/watch?v=AbtyS3X7glA&amp;list=PLXu5151nmAvSbcbVf…;). This structure is similar to what Doug, Ian and Dave found in the southern Madison Range (<strong><a href="https://www.youtube.com/watch?v=tlOyL-hbwE4&amp;list=PLXu5151nmAvSbcbVf…;) and mountains near West Yellowstone (<strong><a href="https://www.youtube.com/watch?v=mAbxNKrJb2U&amp;list=PLXu5151nmAvSbcbVf…;). It is becoming more stable without new snow and wind to add load, but the poor structure makes triggering an avalanche possible if you find the right spot on a slope.</p>

<p>Signs of instability such as collapsing and “whumphing” of the snowpack are becoming less frequent and may not present obvious clues that the snowpack is unstable. Carefully evaluate the snowpack and terrain, and avoid steep slopes if you find sugary weak snow below a cohesive slab. Today, avalanches large enough to bury a person are possible to trigger, and avalanche danger is <strong>MODERATE</strong>.</p>

<p>The Bridger Range, Hyalite and mountains near Cooke City have a snowpack that lacks widepsread buried weak layers. Without recent snow and wind the snowpack is generally stable, but this does not mean avalanches are impossible to trigger. Slopes that were heavily loaded by wind after last weekend’s storm are where you might get unlucky. On Tuesday, Bridger ski patrol triggered a couple large slides when they cut large cornices along the ridgeline (<strong><a href="https://www.mtavalanche.com/images/19/large-avalanche-mundys-bowl-bridg… href="https://www.mtavalanche.com/images/19/large-avalanche-wind-loaded-slope…;, <strong>photo2</strong></a>). This was their first time doing control work on these slopes, so the snowpack there is similar to the backcountry, like what you might find on Saddle Peak.</p>

<p>It is early season and we still have limited data, but so far evidence favors a mostly stable snowpack near Bozeman and Cooke City. Even with a mostly stable snowpack, it is important to carefully evaluate consequences of terrain and the stability of each slope. Today, avalanches are possible on wind loaded slopes where avalanche danger is <strong>MODERATE</strong>. On non-wind loaded slopes avalanche danger is <strong>LOW</strong>.</p>

<p>If you get out, please send us your observations no matter how brief. You can fill out an&nbsp;<strong><u><a href="https://www.mtavalanche.com/node/add/snow_observation">observation form</a></u></strong>, email us (<strong><u><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></u></strong&gt;), leave a VM at 406-587-6984, or Instagram (#gnfacobs).</p>

Backcountry Barriers Contest

Click here to learn more about the Backcountry Barriers Contest: 1.shortstack.com/NLfNvh

Bridger patrol triggered large slides on heavily wind loaded slopes

Bridger Bowl
Bridger Range
Code
HS-ACc-R2-D3-O
Elevation
8500
Aspect
E
Latitude
45.81560
Longitude
-110.92300
Notes

Ski patrol triggered two large hard slab avalanches, one in Slushman's Ravine and the other in Mundy's Bowl, by knocking large cornices down on the slope. One was 4' deep, AC-HS-R2-D3-O, ran 1150' vertical. The other was 2.5' deep, AC-HS-R2-D2-O, ran 650' vertical. This was their first control in this area for the season, so the snowpack is similar to backcountry.

Number of slides
2
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Cornice fall triggered by human or explosive action
Trigger Modifier
c-A controlled or intentional release by the indicated trigger
R size
2
D size
3
Bed Surface
O - Old snow
Problem Type
Wind-Drifted Snow
Slab Thickness
36.0 inches
Vertical Fall
1000ft
Slab Thickness units
inches
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year

Poor snow structure at Buck Ridge

We sledded into Buck Ridge (12/4/2019) for the first time this season. The riding was supportable and soft, but we sank to the ground when we stepped off our sleds. This poor snowpack structure makes avalanches possible to trigger, and will create bad avalanche conditions when more snow loads on top. Enjoy the powder in low angle terrain. Photo: GNFAC

Northern Madison, 2019-12-04