Avalanche Activity

During our field work today (12/29) we noted 3 avalanches:

1. On the north of the ridge before the Yellowmule Weather Station. It was small sized, wind-loaded and broke on facets. Guessing R2, D1.5.

2. 3rd Yellowmule we saw another small slide (R1, D1.5) on a wind-loaded slope that broke on facets.

3. We saw our third slide as soon as we turned down the hill from 3rd YM to MacAtee Basin. It was small (R1, D1) and broke to the ground on facets. It was a steep hill.

All three slides were likely triggered from the bottom on Sunday (12/27), were small and did not propagate far.

From email: "Went for a tour today on specimen ridge in yellowstone park. Stuck to terrain well below 30 degrees. Widespread whumphs throughout tour. Cracking and collapsing snowpack on small convexities. Observed multiple small avalanches on windloaded slopes. I believe they were triggered remotely by game."

A solo skier was doing an annual run from the M to Bridger Bowl but could not enter the resort as normal due to limited terrain openings etc. The skier read the avalanche advisory on the 24th but not on the 25th, the day of the traverse. The skier observed recent avalanches on Bridger Peak and in Argentina Bowl, immediately south of Saddle Peak. He did not observe other signs of instability or dig pits but mentioned that travel was generally on non-snow-covered ground. Arriving at the ski area boundary, he was concerned about the avalanche danger and decent route but decided that going down the Bitter End run at the Bridger Bowl Boundary and getting on to the lower angle prow on the ski area boundary would be the best option. He made jump turns down to the steep convexity 50’ below the ridge. The slope cracked at his feet and the avalanche initiated 20’ away.

The avalanche propagated at least 500’ wide into the Football Field adjacent to Bridger Bowl's south boundary, and ran down below the cliff band mid-slope. The avalanche was 2’ deep and was a hard slab that failed on depth hoar (SS-ASu-R3-D2.5-O). The skier descended the track and looked for any recent ski or skin tracks that would indicate other skiers in the area. Seeing no recent tracks, the skier went to Bridger Bowl Ski Patrol to report the incident.

From email: "At approximately 8600", east of Maid of the Mist Creek, I started skinning from the tree edge slightly into the north-east facing meadow. Immediately after leaving tree-line, I heard a large "whump" and noticed a faint crack that shot far into the meadow (I could only see it for 20-30 feet it was so faint, and did not show up in my photo, but it is likely it went farther from how loud the collapse was). We decided to turn around there and carefully re-traced our tracks back into the safety of the dense forest and descend back to our cars that route."

From email: "Today we saw a small avalanche that came out of the rocks lookers left of the 4th of July Couloir, too far away to give any details, just a small debris pile. I don't think it was there yesterday, but I could have missed it. We also saw some very small point releases and roller balls on steep south facing slopes."

Observed multiple natural avalanches on Bridger Peak around 1200 on 12/24/20. Broke within last 24 hours. These appeared to be the recent storm snow breaking on weak depth hoar, 1-3 feet deep. Showing good propagation, and entraining weak snow in the track. Storm on 12/22-12/23 totaled 8-12" (.3-.5" SWE).

Several collapses and whumpfs reported by skiers in low elevation meadows in the Republic Creek area and around Mt. Fox. 

Riding north of Cooke City we saw numerous natural wind slab avalanches on Sheep Mountain, Miller Mountain, Wolverine Peak, Mt. Abundance and near Goose Lake. These avalanches mostly appears to have have broken during the storm, but the slide on Miller appeared to be fresh this morning.,

All the slides broke underneath newly wind drifted snow. Most of these slides were small, but several broke 300-500 ft wide and ran 500-1000 vertical ft. Crowns were generally 1-3ft deep, with a couple breaking into much deeper drifts.

Multiple fresh avalanches observed on the drive into Cooke City. From email: "The recent dense snowfall seems to be avalanching wherever it's getting loaded by a heady SW wind."

A skier triggered a low angled slope (29-30 deg). It was new snow that failed on a layer of graupel. Crown was 6” to 2’ at its deepest. Broke 400’ wide, maybe 200’ vert. The skier triggered the slide on descent and was not aware of the avalanche until he stopped at the bottom of the run. Skier had a partner, rescue gear and was not caught.

We observed a large slab avalanche that likely failed during the storm on the 22nd on an east-facing slope in the Divide Cirque. I suspect it failed on the depth hoard that we saw failing in ECTs in our pits. 

Multiple fresh avalanches observed on the drive into Cooke City. From email: "The recent dense snowfall seems to be avalanching wherever it's getting loaded by a heady SW wind."

Multiple fresh avalanches observed on the drive into Cooke City. From email: "The recent dense snowfall seems to be avalanching wherever it's getting loaded by a heady SW wind. This was in the region below Republic, NW facing."

From email, "Natural avalanche on the road sledding down from Colter pass at noon yesterday on a steep north-facing wind loaded slope near the Daisy Pass turnoff. Small but propagated."

From obs.: "Kicked off a larger than expected cornice as we skinned up the ridge on the W side of Beehive basin. Wind was blowing 20-ish across the opening, cornice had grown pretty good-sized for so little snow. I was making a turn on skins to get back away from the edge when the cornice gave way, probably pulled 25 feet end to end and broke a good 5 feet back from the lee edge of the cornice. I was able to get off the cornice and avoid going over the edge. No activity on the slope below after cornice drop,... although there was a decent hard slab that pulled out just below the ridge when the cornice dropped onto it.... Hard slab on the edges was about 10" deep, some big chunks of cornice on the slope for any future riders..."

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.

From obs: "We were ice climbing below Divide Peak today. I heard and saw a natural avalanche across the basin on the west facing slopes below Overlook Mountain, I also observed recent avalanche debris below “The G Spot” which is in a couloir below Overlook. Even though there isn’t any recent snow, the snow that is there is getting actively blown around."

From email: "I heard and saw a natural avalanche across the basin on the west-facing slopes below Overlook Mountain, I also observed recent avalanche debris below “The G Spot” which is in a couloir below Overlook. Even though there isn’t any recent snow, the snow that is there is getting actively blown around."

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"

From email: "Noted one natural crown off Elephant, NW aspect, 9,600', D2 HS, estimated crown depth around 2,' appeared several days old."

Natural slide off lower Hyalite Peak.

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.

From email: "We observed a couple more recent slab avalanches today on the east face of Barronette too.  Each slide had similar characteristics to the avalanche photos from yesterday:  about 1' deep, and 50' wide." 

Small slides were triggered on a road cut by the groomer along the Big Sky trail on Thursday night.

Several other small snowmobile triggered slides were seen along the Big Sky trail in Cabin Creek on Friday (12/18/20).

Snow plow triggered slide near Big Sky on 12/18/20.

A snowbiker reported to GNFAC forecaster, while in the field near Cooke City on 12/18, that he had triggered a few slides on the east face of Sheep mountain, north of Round Lake.

From observation: "the 12" or so of facets on the ground have no cohesion  .. there is 12-18" of fairly high moisture content snow on the facets ... On several steeper slopes there was considerable collapsing and shooting cracks but no whumpfing. I am sure these would have slid if the slope angle was steep enough."

I went on a ski around the Bighorn/Fawn Pass loop today, and had some unfortunate, but unsurprising observations. There was about 30-60 cm of snow on the ground, which consisted of a low-density slab sitting on top of unconsolidated, sugary facets. Each step forward on skis gave little resistance as I sank all the way to the ground. I observed one small, recent avalanche on a wind-loaded north aspect at 7800’, which clearly failed at the new snow/facet interface (the base is so rotten here there was an obvious difference in tone between the layers). Only one Meadow gave an audible whumph for me, but I imagine that will become more common as we get a little more SWE in the coming days. Light snowfall and moderate winds were creating drifts surprisingly quickly.

Road cut south facing slope of 42 degrees, triggered intentionally, shows about 12 inches of windblown soft slab on top of a faceted layer (in this case more extreme due to aspect and inclination toward the sun) but was triggered only by chopping a hand into the snow a few times. This same faceted layer (the uppermost layer of a thick faceted layer of 1-2 fee) exists throughout the snowpack. Also, the bottom-most layers of snow are very faceted and conditions have not occurred yet to set the snow for a base. Walking around your feet go right to the ground even in areas where snow is 2+ feet deep. There are many rock/tree hazards lurking under the soft snow because of this, two people in our group damaged their a-arms.

5 slab avalanches breaking on facets reported west of Cooke City. Likely broke on 12/16.  

From email on 12/18: "The avalanches looked to be about 36 hrs old, and were found on N/NE and SE aspects.  About 5 avalanches were observed in total.  Very minimal wind effect in regards to the avalanches, and the landscape in general.  The avalanches were all about 30-40cms deep, and a couple were over 100' wide.... The weak layer was a layer of 2mm facets that formed beneath a m/f crust."

Small, loose snow avalanche in the Divide Cirque in Hyalite Canyon. Released naturally sometime during the weekend's storm. These could push a skier into/ off obstacles such as trees and cliffs.

We observed a cluster of relatively small, loose snow avalanches in the Tyler's Slide area of Beehive Basin. We don't have the recipe for slab avalanches... yet. Wind-loading and/ or new snow could tip the scales. Both are coming this week and next, so stay heads-up!

Natural and skier triggered dry loose snow avalanches reported near Bridger Bowl, Beehive and Cooke City. Some of these entraining faceted old snow.

From IG post: "Weak and thin snowpack. A little sluff was all it took to entrain the whole season’s snowpack. S. Madison ~9000’ east facing" 

We saw three small natural wind slabs. One on east-northeast facing Miller Ridge in to Miller creek. One on north facing Miller into Abundance, and one on East end of Scotch Bonnet.

A natural avalanche released in Sluice Box mid morning on 12/3/20 - Crown estimated at 2 Feet.

From obs. on 12/2/2020: "...a skier triggered avalanche... at Bridger Bowl (Schlasmans Lift) from this afternoon. I was skinning up the N subridge to the lookers right of the lift and some skiers above me triggered a wind slab on faceted snow that came down almost right on top of me. I had to run on my skins to get out of the way. I dug 3 pits yesterday and got ect1 every time so was up skiing and skinning conservatively which allowed me to get out of the way of the slide in time to avoid being caught and most likely buried. The toe of the slide was all of 6’ deep....

"...There is about a foot of fresh sitting on top of faceted crust that is sitting on top of more facets and another crust, and obvious red flags from wind affect (big ripples, obviously wind loaded slopes, obvious slabs forming that you can break through with your skis)..."

From skiers at Bridger Bowl: "Small avalanche running from Northwest Passage about 1/3rd of the way down the Apron. 20ft wide, unknown length or start location. R1 D1 involving primarily new snow with minor entrainment of old snow."

Skiers in the Northern Bridgers also reported a small slab avalanche in Frazier Basin. 

Skiers triggered small loose snow avalanches (sluffs) of cohesionless facets in middle/beehive Basins on 11/30. These sluffs were 1ft to 18 inches deep and involved the whole, thin, snowpack.

A skier observed a small slab avalanche observed on 11/28 near Cooke City. East aspect, 9,000' elevation.

On 11/12, two small slides were reported in the Northern Bridger Range near Fairy Lake, at least one of which was skier triggered.

The skier triggered slide was "a wind slab approximately 50 X 50’ with a 6” crown. The slab itself was only about 4” thick and only slid about 5” down the slope before stopping, and I was able to stop and then turn around and ski off of the slab without it moving any further. The 4” of dry slab was sitting on about 3” of sugary snow that was on a very firm icy bed surface."

We don't have details on the other slide, but it was reported as a wind slab avalanche on the NE side of Pomp Peak.

After the storm from Oct 13-24, a strong southwest wind event on Oct 30 drifted the recent snow into slabs that avalanched naturally. Activity was mostly on NE aspects. 

Skier triggered slide on 10/26 at Bridger Bowl near the top of Pierre's Knob lift. Reported via IG story/message.

Cracking wind drifts reported in Sheep Creek Basin on 10/24.

From an observation:

...we were out in Frazier Basin today and noticed some activity. While skinning up a south face we found a couple isolated wind pockets that propagated the top 2-3”... nothing slid. We did also found some couloirs that had storm slabs that had ran in the last few days, both south and north aspects.

From another observer:

Increasing wind speeds overnight and during the morning were easily transporting snow and building thin, stiff slabs. Slab formation was most intense along ridgelines but there were some thin, mid-slope slabs present on cross-loaded terrain features. Lots of loose issues, particularly as the sun was warming things up.

From Doug Chabot:

Today I turned around on the approach to an ice climb on the north face of the Sphinx. Cracking in the windblown snow was a sign of things to come. At the first wind-loaded gully I crawled on my hands and knees over to the slope and whacked it with my ice axe. It avalanched. Although it was only 4" thick, it propagated 75 feet above and scraped the snow over a cliff. We opted to not tempt any more slopes and went home.

Today I skinned in and skied around the E side of the N ridges coming down off of Blackmore. The snow is about 2’ deep off of the ridge down to the bottom of the bowl on the east faces, with variations where the wind has loaded/stripped the snow. The wind was gusting out of the N/NW/W.  I observed some natural loose dry sluffs on the N face and the E face off the N ridge. They look like they came down in the last 24 hours with this new snow and wind; they were still fairly defined when I saw them.

A skier saw widespread small dry loose avalanches in Hyalite on 10/22.

Two parties skiing near Fairy Lake on October 19th observed several natural avalanches that likely broke the day before (Sunday, Oct 18th). The avalanches broke in areas of wind drifted snow.