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Observation: Little Cottonwood Canyon

Observation Date
12/23/2017
Observer Name
Greg Gagne with Patrice Duvernay
Region
Salt Lake » Little Cottonwood Canyon
Location Name or Route
Upper LCC Perimeter
Weather
Sky
Obscured
Precipitation
Moderate Snowfall
Wind Direction
Northwest
Wind Speed
Moderate
Weather Comments
Sustained winds out of the NW blowing and drifting snow. Periods of S2 snowfall in the very early afternoon. Outside of the wind, rather moderate temperatures (minus 6 C / 20 F).
Snow Characteristics
New Snow Depth
8"
New Snow Density
Medium
Snow Surface Conditions
Powder
Wind Crust
Snow Characteristics Comments

Difficult to determine storm snow totals with so much wind scouring and deposition, but in sheltered areas snowfall was about 20 cms (8") by early afternoon. Denser snow with rimed stellars.

Red Flags
Red Flags
Recent Avalanches
Heavy Snowfall
Wind Loading
Cracking
Collapsing
Poor Snowpack Structure
Red Flags Comments
Lots of red flags/bulls eyes today. Several human triggered avalanches, a couple of naturals, lots of cracking, some collapsing. Wind loading on a very weak snowpack. Interesting sympathetic/remote we triggered: Stomping on an East facing slope got a very small 15 cm (6") storm slab that didn't pull out very far before stopping in lower angled terrain, but a crack propagated to where the slope was steeper, and it pulled out a slab 30 to 45 cms (12 to 18") that was 150' wide and ran approximately 200' onto lower angled terrain. Unsure of the weak layer, but am guessing facets judging by the look of the bed surface.
Avalanche Problem #1
Problem
Wind Drifted Snow
Trend
Same
Problem #1 Comments

Wind loading seems the biggest concern right now. These wind drifts were failing at a density inversion within the storm snow or at the interface with the Wednesday storm snow. But they are also falling on a very weak pre-existing snowpack, and could easily trigger much larger - and more dangerous - avalanches. Drifts were 45 - 60 cms (18-24") in places, and were found down well off of ridge lines.

Avalanche Problem #2
Problem
Persistent Weak Layer
Problem #2 Comments

Small naturals from sheltered areas in Silver Fork today are evidence the weak, pre-existing snowpack won't take much of a load before failing.

Comments

[No photos or video as my phone battery once again died in the cooler temperatures ..... pretty soon I'll be using a teletype machine.]

Take home from today:

- Most hazardous day of the season so far. Touchy conditions with lots of red flags.

- Weakest snow was either (1) a density inversion within the storm snow, (2) the interface with the Wednesday storm snow (a thin layer of facets formed overnight Thursday into Friday), or (3) within the lower-density snow that fell on Wednesday.

These were the weakest layers today; as (1) and (3) gain strength, the persistent weaknesses deeper in the snowpack may become reactive.

- A few pits outside of wind-affected terrain showed 20-25 cms (8-10") of a denser storm snow slab. Full propagation extended column tests (ECTP 8 Q1) failing at the interface of - or within - the Wednesday night storm snow.

Less concerned about storm slab problem, and more about persistent slab.

Solid Considerable today, with an even higher spike in activity and conditions in the early afternoon.

Today's Observed Danger Rating
Considerable
Tomorrows Estimated Danger Rating
Considerable