22-23

Town Hill Lawn Mower Incident

Date

Sat, 03/04/2023 - 14:30

Activity

Snowboarding

Yesterday, I was in the party of three that had one skier injured on the Lawn Mower, and witnessed the slide.

Our pit results before the descent were stable (ECT-X), we made sure to dig past the problematic layer noted in the snowpit observation at town hill on 2/19.

We began our descent leapfrogging one another to zones of safety. Myself and the third person in our party below the skier in a safe zone when he was was caught. The avalanche broke above him and came down on to him to sweep him down into the gully. Based on how he trigged it toward the far edge of the path, best guess is he found a shallower trigger point that remote triggered above.

The two of use remaining split up so one could search above where we last saw him, and one going down the path. Got a signal and shortly spotted him buried with his head and arm out and conscious. After excavating him it was clear his leg was broken at the femur.

Two other groups showed up on the scene some time after to help.

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Outside of the observation above, one of the groups gave their extra jackets and we do not have contact information to return them. If possible would you be able to share that with me?

Region

Out of Advisory Area

Location (from list)

Lawn Mower - Town Hill

Observer Name

August Siebs

Town Hill (Absaroka) Lawn Mower

Date

Sat, 03/04/2023 - 05:20

Activity

Skiing

We toured up to the top of Town Hill (S. Fork Deep Crk.) we approached on the north west shoulder and stayed on the same ridge line to gain the “summit”. We noticed extreme variability as we crossed aspects on our skin track. There were areas that were nearly un-edgeable and slick, and areas that were faceted and hollow all the way to the ground. We observed and spoke to a group who had skiied the “Lawn Mower” on our way up and observed several tracks. We dug a pit at the top of the “Lawn Mower” in the burn area (~8900ft) before the slope steepened. We observed a HS of 230cm and had a test result or ECTX. We gave our extended column a pull to try and identify any potential slabs and observed two distinct slabs in the upper two feet, but they didn’t pull out any wider than the width of our shovel. We took this as further indication of low quality and potential for propagation across the snowpack.

We chose to ski one at a time to the toe of the slope. I skiied down first along the east side of the lawn mower and waited for the remaining skiiers to follow. As my partners descended we realized an avalanche had ripped earlier in the day and we skiied down the side of the debris and met the two parties at the tail of the slide debris. The third skier that triggered the avalanche had been caught and partially buried with his head above ground and badly injured. Part of our party stayed to stabilize the victim and provide support while I and two others from the other party left to bring SAR in.

Region

Out of Advisory Area

Location (from list)

Town Hill

Observer Name

Colter Mumford

GNFAC Avalanche Forecast for Sun Mar 5, 2023

Snowpack and Avalanche Discussion

A person can trigger large avalanches that break at least 3-4 deep on buried weak layers. The mountains near Cooke City and West Yellowstone received steady snowfall for the last two weeks, and moderate to strong winds drifted recent snow into thick slabs which pushed buried weak layers to their breaking point. More snow today will keep the snowpack on edge and large avalanches are likely for a person to trigger.

Over the last week there were at least five human-triggered avalanches near Cooke City. Three of these slides resulted in partial burials (<a href="https://www.mtavalanche.com/node/28338"><span… Hill</a>, <a href="https://www.mtavalanche.com/node/28322"><span… Lake</a>, <a href="https://www.mtavalanche.com/node/28331"><span…. Abundance</a>), and a skier triggered slide near Daisy Pass buried the group’s parked snowmobile (<a href="https://www.mtavalanche.com/node/28413"><span… and details</a>). Yesterday Doug looked at the crown of the recent slide near Daisy Pass. He found it broke on a layer of surface hoar that was buried in January (<a href="https://www.youtube.com/watch?v=nZFM_QZxG3Q&list=PLXu5151nmAvTi1DBS…;), and the slope angle at top of the starting zone was as shallow as 31 degrees.

Unfortunately, this deep slab avalanche problem is difficult to assess because the weak layers are deep, often not reactive in stability tests, and might not be easily found in every snowpit. Recent avalanche activity is our sign to take a big step back and choose to ski or ride slopes less than 30 degrees, and be extra cautious crossing below steep slopes as well. Today, cautious route finding and conservative decision making are essential. Avalanche danger is CONSIDERABLE.

Yesterday in Hyalite, skiers triggered a very large, deep slab avalanche that caught and partially buried one of them (<a href="https://www.mtavalanche.com/node/28455"><span… and photos</a>). Outside our advisory area in Paradise Valley, a skier was caught and badly injured in a large avalanche (<a href="https://www.mtavalanche.com/node/28462"><span… and photos</a>). Also, skiers triggered a shallow fresh wind slab in Hyalite. They were not caught, but were in high consequence terrain where even a small slide would have been potentially deadly (<a href="https://www.mtavalanche.com/node/28451"><span… and details</a>).

The mountains near Bozeman and Big Sky received less snow last night and the last couple weeks than the mountains to the south, but recent avalanches are a clear sign that buried weak layers make large avalanches possible to trigger (<a href="https://www.mtavalanche.com/images/23/snowmobiler-triggered-slide-buck-… Ridge Rider triggered</a>, <a href="https://www.mtavalanche.com/images/23/sled-triggered-avalanche-cabin-cr… Creek rider triggered</a>, <a href="https://www.mtavalanche.com/images/23/natural-avalanche-sage-basin-3"><… in Sage Basin</a>). While the likelihood of a large avalanche is relatively low where less snow has fallen, the consequences remain high. Take a step back from seeking bigger lines, and before riding or crossing any steep slope carefully assess the snowpack for buried weak layers and wind-loading. The most likely place to trigger an avalanche is on wind-loaded slopes. Buried weak layers may be difficult to find and assess, so steer clear of steep terrain if you have any suspicions. Smaller, fresh wind slabs are also possible. Watch for cracking across the snow surface as a sign fresh drifts are unstable, and evaluate terrain for the consequences of being caught in any size slide.

Today, avalanches are possible to trigger and avalanche danger is MODERATE.

Please share avalanche, snowpack or weather observations via our<a href="https://www.mtavalanche.com/node/add/snow_obs">…; website</a>, email (mtavalanche@gmail.com), phone (406-587-6984), or Instagram (#gnfacobs).

Dangerous avalanche conditions exist in Island Park. Over the last couple weeks, this area received feet of heavy snow which drifted into thick slabs, and more snow is expected today and tomorrow (<a href="https://www.mtavalanche.com/node/28422"><span… observation</a>). This steady, continuous addition of weight has pushed the snowpack to its breaking point. Large avalanches are likely to be triggered and could break naturally. Avoid riding on slopes steeper than 30 degrees, especially if they are wind-loaded, and be extra cautious crossing below steep slopes.

Read more about GNFAC Avalanche Forecast for Sun Mar 5, 2023

Upcoming Avalanche Education and Events

Our education calendar is full of awareness lectures and field courses. Check it out: Events and Education Calendar.

cornice fall in maid of the mist

Maid of the Mist

Northern Gallatin

3/4/2023

Code

C-N

Elevation

10200

Aspect

Latitude

45.41640

Longitude

-110.97000

Notes

Chunk of cornice fell off the top of Arden Peak. Notably the same aspect/elevation as the 2/27 observed natural slide on E face of Mt Bole but the cornice fall did not step down beyond the surface snow.

Number of slides

Number caught

Number buried

Avalanche Type

Cornice fall

Trigger

Natural trigger

Problem Type

Wind-Drifted Snow

Read more about cornice fall in maid of the mist

Images

cornice fall in maid of the mist

Snow Observation Source

cornice fall in maid of the mist

Slab Thickness units

centimeters

Single / Multiple / Red Flag

Single Avalanche

Advisory Year

22-23

Skier caught and injured on Lawn Mower

Lawn Mower - Town Hill

Out of Advisory Area

3/4/2023

Code

SS-ASu-R2-D2-O

Elevation

8400

Aspect

Latitude

45.50570

Longitude

-110.49600

Notes

From obs: "On March 4th around 2:15 PM my partner and I observed a skier (of a separate party that was caught and carried) in a D2 R2 on the Lawnmower in the Absorkas. I believe the slide was released on a buried weak layer (We only observed the crown from a distance). The victim came to a stop approximately 1,200 ft below the start zone and was buried up to his neck. There were multiple tracks on the slope prior to the avalanche. My partner and I skied up to the separate party with the victim approximately 30 minutes after the event."

From another email "FYI big skier triggered avalanche on the lawn mower slide path on town hill today around 3pm. One skier caught and buried to neck and later airlifted out with leg injury. Our party arrived maybe 45 min after the slide and assisted others who were there."

Number of slides

Number caught

Number buried

Avalanche Type

Soft slab avalanche

Trigger

Skier

Trigger Modifier

u-An unintentional release

R size

D size

Bed Surface

O - Old snow

Problem Type

Persistent Weak Layer

Read more about Skier caught and injured on Lawn Mower

Slab Thickness

30.0 inches

Vertical Fall

1200ft

Slab Width

300.00ft

Images

Skier triggered and injured Lawn Mower path

Victim on the surface of the slide

Snow Observation Source

Avalanche on Lawn Mower

Slab Thickness units

inches

Single / Multiple / Red Flag

Single Avalanche

Advisory Year

22-23

Read more about Skier triggered and injured Lawn Mower path

"FYI big skier triggered avalanche on the lawn mower slide path on town hill today around 3pm. One skier caught and buried to neck and later airlifted out with leg injury. Our party arrived maybe 45 min after the slide and assisted others who were there." Photo: D. Harnick

Avalanche Details: Skier caught and injured on Lawn Mower
Forecast link: GNFAC Avalanche Forecast for Sun Mar 5, 2023 GNFAC Avalanche Forecast for Mon Mar 6, 2023

Out of Advisory Area, 2023-03-05

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Read more about Victim on the surface of the slide

"On March 4th around 2:15 PM my partner and I observed a skier (of a separate party that was caught and carried) in a D2 R2 on the Lawnmower in the Absorkas. I believe the slide was released on a buried weak layer (We only observed the crown from a distance). The victim came to a stop approximately 1,200 ft below the start zone and was buried up to his neck. There were multiple tracks on the slope prior to the avalanche. My partner and I skied up to the separate party with the victim approximately 30 minutes after the event." Photo: A. Whitelaw

Avalanche Details: Skier caught and injured on Lawn Mower

Out of Advisory Area, 2023-03-05

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Read more about Crown on the upper slopes of Lawnmower

Out of Advisory Area, 2023-03-05

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Town Hill Lawn Mower Avalanche

Date

Sat, 03/04/2023 - 15:10

Activity

Skiing

FYI big skier triggered avalanche on the lawn mower slide path on town hill today around 3pm. One skier caught and buried to neck and later airlifted out with leg injury. Our party arrived maybe 45 min after the slide and assisted others who were there.

Region

Out of Advisory Area

Observer Name

Dan Harnick

Obs from the Skillet

Date

Sat, 03/04/2023 - 16:10

Activity

Skiing

Hey GNFAC,

I dug on a SE facing aspect at the top of the Skillet in Bacon Rind today.

I found a HS of 163cm, and a 40cm slab from the last two weeks of on top of at least four notable weak layers.

I was initially optimistic about the stability, but had an easy shovel shear test, followed by an ECTP17 Q1 at 120cm up from the bottom.

Our group opted not to ski the Skillet, and found some great turns in lower angle trees nearby.

Another observation of note is a widespread layer of surface hoar that was deposited about 4 days ago, and is now covered by about 10-12 cm of snow.

Region

Southern Madison

Location (from list)

Bacon Rind

Observer Name

Matt Standal

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