Avalanche Activity

Two natural avalanches near Lionhead Ridge likely broke during the storm on Christmas Eve. One slide appears to have broken on a weak layer in the middle of the snowpack, while the other only involved the new snow. Both were less than 100 ft wide and ran a couple hundred feet. They were on south east and northeast facing slopes at around 9000 ft.

Photo: GNFAC

Small avalanche in 2nd Yellowmule. Likely broke during storm on 12/24. Less than a foot deep but ~150 ft wide.

Ice climbers had a very hard wind slab collapse, crack and "whumph" while approaching a climb in Flander's drainage. Winds were blowing strong and drifting snow. They noted their tracks were filled in when they left, and fresh drifts cracked when isolated.

From IG message: "5' thick wind slab just behind little bear remote triggered riding along the ridge." @skidooin_it

Riders near Fairy Lake reported this slide on Saturday 12/21. Photo: @turbo_dieshall

Skiers with a level 2 class near Cooke City reported: "...toured into East Hayden today with the Level 2 course. We noticed some recent natural activity on the S end of the Climax path (see photo). 

We dug on a N aspect... around 9500'. HS 100 cm, CT22 SP down 60 cm on FC above the Thanksgiving crust. ECTP21 on the basal facet layer. 

The students dug on E and W aspects... around 9800'. On both aspects HS was around 110-120 cm, and both the midpack FC/crust and basal facet layers were less reactive. Between six snowpits we didn't have a single ECTP, and only a handful of CT results on either layer. 

All aspects were heavily wind affected."

1. From an email: The wind sure moved snow around up Hyalite last night, and our party observed several small to medium-sized releases that occured on a heavily-loaded east aspect overnight. None stepped down in the snowpack, but they certainly could have taken a person for a ride. Overalll, stability seemed to be good, with the exception of isolated wind-loaded areas.

2. From IG: Couple of small natural wind slabs around 10,200’ NE in Hyalite. Not sure exactly how recent. Played it conservative and chose to stay off a similar slope.

Avalanche crown(s) seen Wednesday (12/18), probably ran Monday or early Tuesday. Broke 200-300' across between the peaks and south central. Debris ran 1,500' vertical.  Photo: GNFAC

Additional info from a skier on 12/19/19: 

"Hard to tell how deep it was. Looked like 15” max maybe. Tapered off quick.  Looked like probably that south wind load. 

One crown was north between the peaks about 60 ft from the ridge top. Then it seemed to take the legs out of the stuff in South Central since the crown was 150 ft below ridge top. Then a little dribble made its way into Argentina bowl on the south aspect. Debris terminated right at the bottom of each chute. 

Since the crown was so low I never got eyes on it. Plus it has blown in a little. I stayed high near the ridge line to the pinnacles. But it looked and felt like a bomber crust is what it ran on. Hard slab with pretty small chunks in the debris. 

The one picture didn’t come out well but it looked like goat tracks went right into that higher north crown. Maybe a dead goat...  they are usually pretty smart though."

Riders reported three recent avalanches that happened on or prior to 12/15. A couple looked natural, one had a lot of tracks nearby and may have been snowmobile triggered.

Went up for a closer look at a recent natural avalanche in the bowl of Mt. Republic today, just lookers left of the typical Fin ski zone.  An easterly aspect. Photo: B Fredlund

While teaching up at Second Yellowmule we received a secondhand report of riders in the Slat Creek area triggering 4 avalanches. It sounds like no one was caught, injured, or buried. 

From e-mail: "... This picture is from what I call the Sirens. Looks pretty recent. Steep terrain and repeat offender every season. We got some isolated collapses while skinning today. West face snowpit at 9600' revealed no issues with Thanksgiving later, but did get it to go 90cm deep on a Ect31, big wallop on basal facets."

While on a tour in Hyalite yesterday, we were able to find stable snow on a south aspect at 9500 feet. Although we found no propagation in our ECT, we observed a defined crust layer about 2.5 feet down which was well bonded to snow above. On some steeper pitches near rocks bands, we observed top layer slough that moved naturally. We also observed a small crown from a day or two ago that broke between two buried rocks and only ran about 50 feet down slope.

From email: "Saw 3 different slides today, all had sled tracks near or into them, and were breaking from rocks... The other slides were smaller, had broken from rocks at the top of ridges. Also saw some cracking mid slope, on this same aspect, a ridge over down left of this photo."

Photo is of avalanche on SE aspect of Crown Butte.
 

Photo of recent avalanche up on Amphitheater Mtn., as seen 12/15 from Silver Gate.  NE aspect around 9800'

From IG post: "Pictures don’t do this monster avalanche justice. We helped the kid recover his sled, which was buried between 4and 10 feet. luckily it wasn’t him and no one was hurt." @mackleymtvet

E-mail from uninvolved group: "Incident occurred on an East aspect of the far south shoulder of Abundance.  Did not witness incident or know the party involved, but arrived shortly after it occurred. Involved party was still on scene. Rider was approaching blind roll over from the top when slide was triggered at the visible crown. Rider attempted to bail, but both the individual and machine were carried over and through the rocks and strainers below. Rider and machine were deposited in the debris pile as seen in pictures. Rider was unharmed. Machine was pretty well trashed. Information based on first person observation and rider testimony."

From e-mail: "Remote triggered this one today. Knew it was up there and touchy. Took the safe line along the trees.... certainly a sign of what is possible...."

From email: "Significant collapsing throughout the day. After the first collapse, we backed off and kept slope angles well below 30°. A couple of collapses were widespread with significant settling."

Forecaster obs: We rode into Taylor Fork on 12/13 and saw recent crown lines along the long ridge in Carrot basin. Appeared to be natural and broke on weak snow at the base of the snowpack. They were buried by recent snow so at least one day old, but timing is estimated.

From an email, "Signs of instability were impossible to miss in Bacon Rind today. Lower elevations with thick trees produced localized collapsing under our skis. At higher elevations, and in open meadows, collapsing and whumpfing communicated across whole slopes, shook snow off trees, and sent birds flying. 

Riders reported multiple natural slab avalanches on N facing slopes.

Skiers at Targhee Pass near West Yellowstone reported: "We got a few good collapses while skinning up through east-facing low angle meadows, and got some serious cracking and collapsing while stomping on fresh wind pillows just below ridgeline on a northeast-facing slope at about 8200'. We dug down at one of the cracks and found a 35cm thick F+ slab that had failed on a layer of 4-6mm depth hoar. Cracks propagated close to 100'..." 

Bill Whittle of Cooke City SAR reported that one sledder triggered a slide in a terrain trap to the northwest of Lulu Pass (45.06703, -109.97180). It caught him, but he was not buried. It only buried the tail of his sled. He hit his "SPOT" rescue beacon because he was stuck.

Separately, a little while later, a rider on a rental sled was unable to ascend Daisy Pass to return to town. As he was walking up the pass and others were retrieving his sled, a 2 foot deep (est.) natural slide released on the northeast aspect of Crown Butte that missed them.

Since the storm began 26 hours ago, 1.4" of snow water equivalent fell which is estimated to be 2 feet of snow.

Winds were W-SW averaging 10-20 mph with gusts reaching 44 mph (Lulu Pass weather station)

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."

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.

From an email:

"Chris and I toured up the East Ridge of Mineral today and dug a pit on a NE aspect around 9000'. HS 75 cm, mostly facets with the exception of the new snow. ECTX. We found more of a windslab on some more exposed features although it wasn't reactive for us and we saw no natural avalanches with the exception of a small slide in the couloir skier's left of the summit of Meridian. We were still a ways away but it appeared to have run on the old/new snow interface."

From Obs.: "New snow from 24 hours ago blew around the ridge tops from 11pm last night until 7am this morning ~30mph. This soft 4 finger slab was about 6”-7” deep and broke on a convex roll... We were assessing the terrain carefully and expected this result."

Observations from Hyalite today:

1. When approaching the base of Mt. Blackmore this morning at roughly 8:45am we observed strong swirling winds at higher elevations (summit of Blackmore and the Elephant/Blackmore Saddle). The winds we observed were primarily loading snow onto E and SE facing aspects. Furthermore, we witnessed four naturally triggered avalanches over a twenty minute span. All slides appeared to be D1/D2 on E and SE aspects and, seemingly, restricted to the newly loaded snow.

2. My partner and I climbed Zach Attack today. We intentionally triggered numerous size 1 wind slabs in the gully approaching the climb, which has slopes up to 35 degrees. These slabs were between 5 and 15 cm thick, up to 5m wide, 4F in hardness, touchy, and failed on lower density new snow. Although none of them ran more than 20m in this terrain, it would be a different story on steeper slopes, and if you were in an exposed position they had enough mass to push you around. Strong down and cross-slope winds formed these slabs at and below treeline, while the more alpine terrain of the climb itself was scoured.

3. Skied up in the main drainage of Hyalite today. Triggered some small wind slabs and propagating cracks. We backed off our main objective (a north facing couloir) because of these red flags and multiple crown lines on the same aspect and elevation.

We were able to trigger dry loose avalanche in the 3-5" of new snow that ran on firm underlying crusts.

Skiers saw natural soft slabs that broke below the cornice along the ridge near Slushman's lift.

Another group reported: "I spotted a buddy skiing the lower nose and he started a couple of slow moving sluffs, well in his wake... We did see a recent point release in that zone, from a ski track, that ran over 200 feet in steep terrain."

From obs.: "8500 feet Northeast aspect. Extended column test ECTP 9 Failed near the ground on prestorm facets. 60 cm total snow depth. Widespread collapsing in open terrain."

From an email:

"At about 9,000ft on the northern aspect of Overlook mtn we dug a pit to the ground (90cm) and got ECTX and very stable results. We found the basal faucets to be quite rounded except for some large ones growing off rocks. About 500ft higher we found a pervasive wind slab that stretched across the face. We got some large shooting cracks while skinning and ended up turning around. Picture is attached. Before the wind picked up we found surface hoar almost 3/4 of an inch deep!

Sometime last week (11/20-11/22) these large avalanches occurred on heavily wind loaded northeast facing slopes in the Absaroka. The snowpack there is thin and faceted, similar to the southern ranges and mountains near West Yellowstone. Photo: B. VandenBos

Thursday, November 21:

1. "I observed 2 natural avalanches on the slopes between the Apron and Ramp. No ski tracks in those areas. I thought I took a picture but didn’t. Looks like they both started in the rock bands below the ridge. One was small and the other was considerably larger. Hard to know if they ran today or yesterday. Looked like it was just new snow. Pretty much east aspect."

2. "Toured up to the ridge at Bridger today and traversed into Bridger Gully. At around 8,000' in the trees above the most prominent snowfield, my partner and I observed very small dry sluffs confined to the new snow. Bellow Bridger Gully, my partner was able to release a slide while traversing through rocks above his intended line. This was on an east-southeast aspect at 7,500'. Snow just a few degrees more to the east was dryer. The snowpack nearby thinned greatly on the east-southeast aspect.

Wednesday, November 20:

1. " Noted some activity (D1-D2) isolated to the new snow around rock features and ridgetops that were wind-affected.  Our pit revealed a weakening and thin snowpack due to the drier weather, however, the facet growth at the ground has remained fairly slow and has maintained moisture."

From Doug Chabot:

I went ice climbing in Flanders and got hit with a graupel storm (0830) which avalanched off the climbs on east-facing slopes for over an hour. It was pretty spectacular and we found knee deep graupel (small, ball-bearing like grains) 200 vertical feet from the bottom of the cliffs. The climbs that we saw this on were Big Sleep, Bobo Like and Killer Pillar. The snow appears to be sliding on a razor thin ice crust under the 3" of new snow that fell last night, which was dense (estimated 15%).

I dug a hand pit to the ground and found 2' of mostly solid snow (no obvious facets) with a couple ice crusts in the upper third. A CT cut with a ski pole yielded no fractures. Elevation 8,000', east aspect.

From an email: "I observed a wet loose avalanche that most likely occurred yesterday afternoon. Occurred at an elevation of 8,900' on a WSW aspect, along the runout of a chute sometimes referred to as "the gem". It wasn't very large, but it ripped out to the ground in sections and looked like it could do some damage if caught off guard. I assessed the snowpack next to the small slide, and found a snowpack of only 30 cm composed of facets from 0-14 cm above the ground (F hardness), a crust of large bonded grains from 14-16 cm above the ground, small facets / rounding facets from 16-23 cm above the ground, a rain crust from 23-27 cm that blended into wet snow on the surface becoming slabby in the afternoon sun."

Outside advisory area, but seemed relevant to report a climber triggered avalanche today (12/11/2019) at around 9000 ft, on a 35 degree slope on a NW aspect. Solid size 2, triggered roughly 15m below the crown (depth 20-40 cm, width 15m) Failed on a density change below a fresh windslab, ran around 100m and deposited 1m of debris towards the bottom of the runout. HS-AF-D2/R3-I. No injuries.

Two parties reported shooting crack at Bridger Bowl today (11/11/2019).

"While skinning up to the top of Powder Park lift at Bridger, we had roughly 15' cracks propagate on an E/SE aspect. A snowpit showed roughly 8" of wind slab above a few inches of facets, on top of last week's rain crust."

Natural wind slab avalanche on the N face of Mount Blackmore on 11/10/2019.

From email: "Noted recent activity on the N face which likely happened yesterday during the storm; upper half of the avalanche and crown had been scoured by wind, but the toe of the debris was still visible and had traveled quite far"

Several small wet loose avalanches were observed at Bridger Bowl on 11/5. 

"I postholed up to Zach Attack today and dug a pit in the gully a few hundred feet below the route. We found about a meter of snow on the ground, the upper 65 cm being generally unconsolidated (fist-4F hardness) snow largely transported by the wind. Below that was a 5-10 cm MFcr on top of small (1-2 mm) basal facets. Higher up in the gully we found a few isolated winds slabs, and on the way my back down one of my partners felt a wumph as we crossed above a leeward convexity."

From e-mail: "Skied in the the beehive area today. Triggered some small wind slabs that were sliding on a crust. 

E aspect 9,100': Triggered from a stomping a small cornice, which landed on a steep slope below and popped a small wind slab.

NE aspect 10,000': Triggered while skiing through a choke point. This one was thin but ran down slope quite a ways because the terrain was steeper. It was easy to avoid by skiing off to the right, since the gully was just cross loaded a bit on the left.

SE aspect 9,800': The third was just some propagating cracks. The slope angle wasn't quite steep enough to slide here.

These instabilities appeared to be confined to areas of dense wind drifted snow on top of a crust layer and did not appear to be wide spread."

From obs., "Activity I observed was restricted to the new snow. A fair bit of loose snow....  Skiers on the E bowl of Blackmore triggered some D1 dry sluffage, running on a crust beneath the new snow.... No slab activity observed until the wind started pushing more snow around this afternoon. I watched a small (D1) wind slab build and fail as the wind picked up near the Blackmore/Elephant saddle (happened just before 2pm). Slab couldn't have been much thicker than 15cm, broke 30-40m wide. These slabs were building rapidly..."

Went for a midday skin up Thunder Road during the maelstrom today. It was blowing gale force, mostly from the south but seemingly from all directions at times...here’s a pic from a north facing slope below Thunder Road, about midway up PK. We saw a few more shooting cracks in the flats below the top of PK, and the surface wind slab where this photo was taken was ~10 cm.
 

Dave Zinn triggered a small wind slab off the ridge on Mt. Blackmore.

Several reports of small skier triggered slides at Bridger Bowl (10/29).

From instagram:

"I kicked off a small 6 inch slab on a rollover skiing down east of PK."

"Skied ptarmigan this morning, kicked off a decent sized slab rolling over deer park road cross from avy gulch."

"I got a 2 foot wind slab to crack on a test slope. There was lots of evidence of heavy wind affect. Saw a 5 inch by 20 foot slide in sluice box gully (skier triggered)."

From an email: "Skinned part way up Slushmans yesterday, saw that a chunk of cornice had fallen over the south side of the football field and took a medium-sized slab with it. The debris didn’t make it over the cliffs, but I did see debris further to the south."

A skier in Hyalite wrote, "Natural activity that I observed was failing at new/old interface and (generally) occurred in predictable, wind loaded locations near ridge lines. The one exception is the avalanche with the freshest debris; looks like it failed right at the end of the storm in a somewhat sheltered, mid-slope location." Photo: B. VandenBos

Gusty North wind and moderate snowfall created sensitive drifts. From obs. "Some moved while I was skinning up, some slid naturally (hidden/[Apron area]) mostly recent wind deposition.

Small storm slabs triggered at Bridger Bowl on 10/26. From email: "slab formation appeared isolated to leeward sides of midslope terrain features and rollovers as well as at ridgetops". 

From email: "Found this large slide just north or Imp today [10/25]. Right around 10k, N-NE aspect, crowns spanned estimated 800-1,000' and ranged from ~2-10' thick. Appeared to activate by both a weak layer near the ground and a mid-pack interface. Lower interface was pencil hard, variably icy melt freeze crust of some sort, with 10-20 cms of 2-3mm facets underneath. Didn't get a good handle on the mid-pack interface, mostly assumed its presence based on shallower crowns.

Also saw several other crowns, at similar and slightly lower elevations on similar aspects to the SW of above described slide. Other crowns appeared thinner and less wide."

A skier reported lots of wind transport and triggering several thin wind slabs on the east face of Mt. Blackmore, in the northern Gallatin Range (10/25).

Skiers noted recent activity on Saddle isolated to fresh cornice fall and loose snow sluffing. The snow still available for transport was isolated to upper elevations, as mid/lower elevations the snowpack was wet and warm.

Ice climbers triggered a small avalanche while approaching the Twin Falls ice climb in Hyalite Canyon on Sunday (10/20).

From e-mail on Saturday 10/5, climbers on the Sphinx found these conditions: "We found a lot of snow on the Sphinx today following heavy precip since Sam’s report two days ago [see photo]. 3-5 cm in the parking lot, 20 cm at the Sphinx-Helmet col. Moderate SW winds had created drifts almost a meter deep in places on the traverse to the ice routes on the north face. I backed off after a whumph on a particularly exposed section of the traverse. We tried different ledge systems above and below the normal approach route but found similar snow conditions throughout."

Photo: From 10/3 prior to 8-10" new snow, "No snow hazard of much concern on the approach. However, a quick hasty pit in an isolated wind pocket revealed a CT1 on this layer.  ice axe is to the ground."

From e-mail: "The only places one could ski were wind pockets blown into the limestone fin features that in places were well over a foot. These spots were also where the snow was weirdest/posed any sort of avalanche hazard as you pretty much had to find a wind slab in order to ski anything! One group reported triggering a slab in one of these spots, but vis was too poor to see."

From e-mail: "A couple wet slides buried the trail up to 4’deep on a sw facing slope approaching 9000’ in elevation. Probably set off yesterday with clearing skies. Snow depth was about 14” at Jewel Lake."