CAIC: Colorado Avalanche Information Center

2019/12/01 - Colorado - Big Chute, Anthracite Range

Published 2019/12/05 by Ben Pritchett - Forecaster, CAIC

Avalanche Details

  • Location: Big Chute, Anthracite Range
  • State: Colorado
  • Date: 2019/12/01
  • Time: 1:00 PM (Estimated)
  • Summary Description: Solo backcountry tourer caught, partially buried, and self rescued
  • Primary Activity: Backcountry Tourer
  • Primary Travel Mode: Ski
  • Location Setting: Backcountry

Number

  • Caught: 1
  • Partially Buried, Non-Critical: 1
  • Partially Buried, Critical: 0
  • Fully Buried: 0
  • Injured: 1
  • Killed: 0

Avalanche

  • Type: SS
  • Trigger: AS - Skier
  • Trigger (subcode): u - An unintentional release
  • Size - Relative to Path: R1
  • Size - Destructive Force: D1.5
  • Sliding Surface: O - Within Old Snow

Site

  • Slope Aspect: N
  • Site Elevation: 11100 ft
  • Slope Angle: 42 °
  • Slope Characteristic: Planar Slope,Sparse Trees

Avalanche Comments

Skier 1 triggered two avalanches. He remotely triggered the first, and larger, avalanche from uphill of the starting zone. The avalanche was a soft slab that broke two to four feet deep and 40 feet wide across a shallow gully. It was small relative to the path and large enough to bury, injure, or kill a person. It broke in old snow near the ground (SS-ASr-R2D2-O). The avalanche split into two lobes of debris. The lobe on the left, looking upslope, stopped on a bench in the terrain about 350 feet below the crown. The right lobe, looking upslope, ran about 600 vertical feet before stopping on another terrain bench. Debris in the right lobe was 5 to 7 feet deep.

Skier 1 triggered and was caught in the second avalanche downslope of the first. It was a soft slab avalanche that broke about 1.5 feet deep and 30 feet wide. It was very small relative to the path and relatively harmless to a person. It broke on a layer of surface hoar above a thin melt-freeze crust. The avalanche gouged down into weak depth hoar once in motion (SS-ASu-R1D1.5-O). Most of the debris was three to four feet deep, but was up to six feet deep where Skier 1 was partially buried. Small blocks composed the avalanche debris.

Backcountry Avalanche Forecast

The Colorado Avalanche Information Center’s (CAIC) backcountry avalanche forecast for the Gunnison zone on December 2, 2019 rated the avalanche danger at Considerable near and above treeline, and Moderate below treeline. Persistent Slab avalanches were listed as the only avalanche problem type, highlighted on north, northeast, and east aspects near and above treeline. The likelihood of triggering was Likely and the potential size was small to large. The summary from the backcountry avalanche forecast:

Dangerous avalanche conditions exist. You can trigger large dangerous avalanches on north through northeast to east-facing slopes near and above treeline. Pay close attention to steep slopes in the near treeline elevation band especially if the slope ends in a terrain trap such as rocks or trees. Strong winds on Saturday deposited a lot of new snow in lower elevation areas. These areas were relatively safe before recent storms. Look for and avoid all areas where you see evidence of previous wind drifting such as smooth or hard pillows of snow. Steer clear of steep slopes below ridgelines if you see evidence of fresh cornices. You can trigger avalanches from below slopes or from a distance, so give north through northeast to east-facing slopes a wide berth.

You can find safer riding options on slopes with a slope angle less than about 30 degrees with no steep terrain above you, and in below treeline areas.

Weather Summary

A series of storms began November 20. Between November 20 and December 1, the Schofield SNOTEL site (about 15 mile north) gained 2.8 inches of snow water equivalent (SWE). Ski tourers found 12 to 14 inches of fresh snow in the Anthracite Range on the morning of December 1. Most of the snow had accumulated since November 29.

On November 29, wind speeds averaged 40 to 50 mph, with a peak gust of 83 mph at the Irwin Guides Ridgetop weather station (about 5 miles north). Winds shifted to the west overnight on November 30. Winds were light from the north on the morning of December 1 as the storm cleared.

Snowpack Summary

The bottom half of the snowpack was comprised of large (2 to 3 mm) and very weak (Fist on the Hand Hardness Index (HHI)) depth hoar grains. A thin melt-freeze crust topped the layer of depth hoar. Above the crust was a thin layer of surface hoar gains.

The overlying slab was formed with storms starting on November 20, was about two feet thick, and had a HHI of 4 Fingers. With the calm winds at the end of the storms, fresh snow blanketed unevenly distributed layers of dense, wind-drifted snow.

Investigators measured 3.2 inches of SWE in the slab at the avalanche crown. That was more SWE than accumulated at nearby weather stations, indicating winds had drifted additional snow onto the slope. Snowpack tests conducted on-site indicated the avalanche released on the surface hoar layer, and once in motion, broke through the thin crust into the depth hoar.

Events Leading to the Avalanche

Three backcountry tourers met for a day of skiing in the Anthracite Range on December 1. They read the avalanche forecast and discussed the avalanche danger, agreeing it was a day for conservative decisions. After skiing together for the morning, Skier 1 chose to make a ski run alone. He felt more comfortable descending alone in familiar terrain than skiing where the other two intended to ski.

Accident Summary

Around 1 PM, Skier 1 entered the top of an area locally known as Big Chute, a north-facing avalanche path. He was above the upper starting zone of Big Chute, skiing over scalloped wind-rolls to test conditions, when he remotely triggered the first avalanche on the slope below.  

Skier 1 descended towards the flank of the avalanche. He triggered additional avalanches in the  snow above the initial fracture line. He skied along the flank and rocky bed surface of the avalanche then traversed below the looker’s left debris pile.

Skier 1 traversed above a rock slab about 45 degrees in steepness. He turned and headed down towards an open slope below the rock slab. The avalanche fractured above him and knocked him off his feet. 

Skier 1 quickly curled into a ball and brought his hands toward his face to protect his head and airway. The avalanche carried him for about 200 vertical feet through small trees. As the avalanche began to slow, Skier 1 felt his skis sink and pull him down. Skier 1 was buried to his neck in a mostly standing position.

Rescue Summary

Only Skier 1’s head was above the avalanche debris. His hands were near the surface and he was able to push his arms free. Skier 1 used his hands to dig his backpack free. From there on he used his shovel to excavate his lower body and skis. It took about 20 minutes. He completed his self-rescue and returned to the trailhead.

Comments

Several other parties were ski touring in the area. One skier had veered into the lower portion of Big Chute approximately 20 minutes prior to the avalanches Skier 1 triggered. The first avalanche overran that skier’s tracks. The other parties were unaware of Skier 1’s self-rescue taking place nearby.

Eric Murrow of the Crested Butte Avalanche Center contributed to this report.

Media

Images

Snowpits

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Figure 6: A snow profile observed adjacent to the avalanche. Snow in the slab was mostly 4 Finger HHI. It released on a thin layer of surface hoar, above a crust. Once in motion, the avalanche gouged down into the depth hoar layer below the crust.