Observation: Cottonwood Canyons

Observation Date
02/1/2018
Observer Name
Greg Gagne

Region:

Location Name or Route
LCC > BCC (Silver/Days Forks)
Weather
Sky
Overcast
Precipitation
Light Snowfall
Wind Direction
Northwest
Wind Speed
Light
Weather Comments
Gusty northwest winds. A trace of snow in the early afternoon.
Snow Characteristics
Snow Surface Conditions
Wind Crust
Melt-Freeze Crust
Rain-Rime Crust
Damp
Snow Characteristics Comments

The Sunday Jan 28 rime crust has almost entirely broken down, and where it is present it disintegrates very easily.

Unreactive pockets of wind drifts at the mid and upper elevations.

Some northerly aspects > 9000' have 1 - 1.5 meter snowpack depths.

Red Flags
Red Flags
Poor Snowpack Structure
Avalanche Problem #1
Type
Persistent Slab
Trend
Same
Problem #1 Comments

Poor structure still in place with a stronger slab on top of weaker snow down near the ground. 

All pits today were ECTX (no fracture) or ECTN (no propagation - mostly requiring 20 or more taps)

What I did find interesting was at least three locations where there was a stronger slab on top of weaker faceted snow, the slab was over 1 meter in thickness. Performed a propagation saw test (PST) where the slab sheared during isolation, down 120 cms. The strong slab was on a steep slope and was connected to a much thinner area of the snowpack (where an avalanche could be triggered from.) Am wondering if we have isolated areas in the upper Cottonwoods where there is a deep, persistent slab problem.

Comments

Photos of two different pits: First highlights strong  over weak. Second shows the PST results where the column failed  (collapsed and sheared) during isolation, down 120 cms.

Low or Moderate? Our avalanche hazard toolbox struggles with this because our current persistent slab problem can easily be defined as Low probability with signficant consequences. and this does not neatly fit into a single, specific hazard rating.

The North American Danger Scale distinguishes avalanches problems based upon (1) likelihood, and (2) avalanche size and distribution. Our current setup is probably Low with regard to likelihood (human-triggered avalanches are unlikely), but Moderate with regard to size and distribution (large avalanches in isolated areas). Given the significant consequences of getting caught in a persistent slab avalanche which will break down 2-4', keeping the current hazard rating at Moderate seems appropriate.

Today's Observed Danger Rating
Moderate
Tomorrows Estimated Danger Rating
Moderate
Snow Profile Coordinates