20-21

Natural Avalanche, Sunlight Basin

Sunlight Basin
Southern Madison
Code
SS-N-R2-D2-O
Elevation
9450
Aspect Range
ESE-N
Latitude
44.97310
Longitude
-111.31000
Notes

We observed two natural avalanches that likely failed 2-3 days ago in the Sunlight Basin in the Taylor Fork. They broke ~150-250 feet wide and ran 250 vertical feet.

Number caught
0
Number buried
0
Avalanche Type
Soft slab avalanche
Trigger
Natural trigger
R size
2
D size
2
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness
24.0 inches
Vertical Fall
250ft
Slab Width
250.00ft
Weak Layer Grain type
Depth Hoar
Weak Layer grain size
3.00mm
Weak Layer Hardness
F
Images
Natural Avalanche, Sunlight Basin 2
Natural Avalanche, Sunlight Basin 1
Attached Videos
More Weak Facets, More Avalanches - 21 December 2020
Slab Thickness units
inches
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year
20-21

North Shoulder of Blackmore

Date
Activity
Skiing

Went for a quick lap on the Blackmore shoulder to check out conditions up higher in Hyalite. Temps hung in the upper 30s/low 40s with overcast skies and light gusting moderate SW winds throughout our tour.

- Dug a quick pit @9650' on a 27deg SE slope (65cm total depth) and found a mixed bag of well-developed fist-hard facets with a couple of pencil-hard melt-freeze crusts mixed in with ~10cm of decomposing new snow on the surface. The faceted layer from 20-40cm contains grains that are striated and 2-4mm in size, with some forming small chains. Test results: ECTN13@53cm and CT12PC@53cm on facets below a pronounced 4cm thick MF crust.
- Took a peek on the northerly aspect as well and found far more spatially variable snow depths, and more wind-effected surface conditions. The stratigraphy was relatively more simple with soft-to-hard slabs (hardness and thickness depending on location) sitting atop well-developed fist hard facets.
- Spatial variability seems to be the name of the game with snow depths and wind slab formation, but the lower ~40cm of well-developed facets seems pretty darn widespread... bummer!

Region
Northern Gallatin
Location (from list)
Mt Blackmore
Observer Name
Zachary Stephen Miller

GNFAC Avalanche Forecast for Mon Dec 21, 2020

Snowpack and Avalanche Discussion

<p>New snow and wind drifted snow loading a weak foundation of sugary facets has created dangerous avalanche conditions in the mountains around West Yellowstone. Doug and I rode to Ski Hill yesterday and triggered two avalanches by walking across the top of the slope. These avalanches released 30-75' below us, propagated and ran well into the trees below (<strong><a href="https://www.youtube.com/watch?v=u0Wqce1UvKY&amp;list=PLXu5151nmAvQDzKmH…;, <strong><a href="https://mtavalanche.com/images/20/remote-triggered-avalanche-2-ski-hill…;, <strong><a href="https://mtavalanche.com/images/20/remote-triggered-avalanche-1-ski-hill…;). Yesterday's 3" of new snow (0.3" of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a><u> </u>- SWE) will keep the snowpack at its breaking point. Stay off and out from under steep slopes being extra cautious around drifted slopes that will be hair-trigger today.</p>

<p>The danger is rated HIGH on wind-loaded slopes and CONSIDERABLE on all other slopes.</p>

<p>Cooke City received 6" of new snow (0.4" SWE) in the last 24 hours bring the storm total to 10" (0.7" SWE). Skiers and riders are reporting more snow at higher elevations with avalanches breaking within the new snow. Yesterday, a natural avalanche released 18" deep across Astral Lake from a group assessing the snowpack (<strong><a href="https://mtavalanche.com/images/20/astral-lake-natural-avalanche">photo<…;). This avalanche told the group what they needed to know about the stability and should inform your decisions as well. Cooke City does not have widespread weak layers and avalanches breaking deeper in the snowpack are unlikely (<a href="https://www.youtube.com/watch?v=71V0bipTxfY"><strong>video</strong></a&…;). However, avalanches involving the new snow alone are large enough to bury or kill a skier or rider. Route selection designed to avoid steep terrain and thorough snowpack assessment is vital today as human triggered avalanches are likely and natural avalanches are possible and the avalanche danger is rated CONSIDERABLE.</p>

<p>The Gallatin and Madison Ranges haven’t received much new snow, but strong winds formed unstable snowdrifts on top of a weak layer of faceted snow (<a href="https://www.youtube.com/watch?v=eAZKRKDi-pg&amp;list=PLXu5151nmAvQDzKmH…;). A skier in Beehive had a scare when a cornice collapsed unexpectedly, triggering the slope below (<strong><a href="https://mtavalanche.com/images/20/skier-triggered-large-cornice-beehive…;, <strong><a href="https://mtavalanche.com/images/20/skier-triggered-large-cornice-beehive…;). In Hyalite, climbers and skiers reported small natural avalanches on the west-facing slope of Overlook Mountain and near Hyalite Peak with wind-blown snow continuing to drift (<strong><a href="https://mtavalanche.com/images/20/small-slide-hyalite-peak">photo</a></…;). A group of riders at Buck Ridge got consistently unstable test results on wind-drifted slopes. Watch for cracking around your skis or sleds and avoid steep wind-drifts where human-triggered avalanches are likely.</p>

<p>The danger is rated CONSIDERABLE on wind-loaded slopes and MODERATE on all other slopes.</p>

<p>Recent storms have missed the Bridger Range, but strong winds transported the remaining soft snow into stiff drifts where human-triggered avalanches are possible in steep terrain (<a href="https://www.mtavalanche.com/images/20/snowpit-bridger-29-nov"><strong>p…;, <a href="https://www.youtube.com/watch?v=JrjrgTLRff0&amp;feature=youtu.be"><stro…;). These drifts will take longer to stabilize because the snowpack's foundation is a weak layer of sugary facets that have demonstrated their inability to support a load in other ranges. Assess slopes for wind-drifting by feeling for a hollow or drum-like upper layer and watching for shooting cracks or active drifting. Turn around if you find them and search for softer snow.</p>

<p>Today the avalanche danger is MODERATE on wind-loaded slopes and LOW on other slopes.</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>website<…;, email (<a href="mailto:mtavalanche@gmail.com"><strong>mtavalanche@gmail.com</strong></a…;), 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:

Two skier triggered slides at Lionhead

Ski Hill
Lionhead Range
Code
SS-ASc-R1-D2-I
Elevation
8000
Aspect
NE
Latitude
44.70160
Longitude
-111.29300
Notes

Dave and Doug of the GNFAC intentionally triggered 2 slides on Ski Hill on Lionhead.

We parked sleds and Doug took off along the rollover to skiers right and triggered a slide from 75’ away. The slides broke under the 8-12" new(er) snow that was on top of the 1' deep layer of sugary facets/depth hoar. Size was about 100’ across and 150’ slope distance. Whumpfing, cracks. ECTP3 in our pit. Dave then took the lead over to the skiers left slope and got the far end of it to go with a whumpf. Same depth, same story, but this was bigger: estimated about 200’ wide and 3-400’ slope distance. ECTP2.

Number of slides
2
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
2
Bed Surface
I - Interface between new and old snow
Problem Type
Persistent Weak Layer
Slab Thickness
30.0 centimeters
Vertical Fall
200ft
Slab Width
150.00ft
Weak Layer Grain type
Depth Hoar
Weak Layer grain size
4.00mm
Weak Layer Hardness
F
Slab Layer Grain Type
Precipitation Particles
Slab Layer Grain Size
2.00mm
Slab Layer Hardness
F+
Images
Remote Triggered Avalanche 2 - Ski Hill
Remote Triggered Avalanche 1 - Ski Hill
Ski Hill North
Ski Hill South
Attached Videos
We Triggered Avalanches, So Will You - Lionhead - 20 December 2020
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year
20-21

This avalanche released while we were across Astral Lake digging snow pits today. South aspect 9500'. Approximately 18" storm total this weekend by yurt.  Only found new snow instabilities in our stability tests this weekend. HS around yurt zone 160-180 cm. Photo: B. Zavora

Avalanche Details: Astral Lake, Natural Avalanche
Forecast link: GNFAC Avalanche Forecast for Mon Dec 21, 2020
Cooke City, 2020-12-20
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    • Astral Lake, Natural Avalanche

    Mt. Zimmer
    Cooke City
    Code
    SS-N-R1-D1.5-I
    Elevation
    9500
    Aspect
    SW
    Latitude
    45.09440
    Longitude
    -109.90000
    Notes

    From email: "This avalanche released while we were across Astral Lake digging snow pits today. South aspect 9500'. Approximately 18" storm total this weekend by yurt.  Only found new snow instabilities in our stability tests this weekend. HS around yurt zone 160-180 cm"

    Number of slides
    1
    Number caught
    0
    Number buried
    0
    Avalanche Type
    Soft slab avalanche
    Trigger
    Natural trigger
    R size
    1
    D size
    1.5
    Bed Surface
    I - Interface between new and old snow
    Problem Type
    New Snow
    Slab Thickness
    18.0 inches
    Vertical Fall
    150ft
    Slab Width
    100.00ft
    Weak Layer Grain type
    Precipitation Particles
    Images
    Astral Lake, Natural Avalanche
    Slab Thickness units
    inches
    Single / Multiple / Red Flag
    Single Avalanche
    Advisory Year
    20-21
    Subscribe to 20-21