Storm slabs in the southern WF Range

Location Name: Forecaster Observation - Canyon Creek, southern Whitefish Range Observation date: Saturday, January 13, 2018 - 15:00	Is this an Avalanche Observation: Yes

Observation made by: Forecaster

Location

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Quick Observation

After an enjoyable morning presenting avalanche education to the FVSEF race and freestyle teams, we took a short tour east of Flower Point to assess storm slab instabilities and persistent slab structure.

We sympathetically triggered 3 storm slabs, up to 1 foot in depth and 20' wide, from a distance of 20 or so feet. These were all on steep convex terrain on northeasterly aspects just below the ridgeline at upper elevations. We also noticed several natural storm slab avalanches of similar size.
The density change between the dense overnight snow and the low density underlying snow was the failure layer.
Shooting cracks and cracking beneath our skis was observed in several locations.
No instability was noted on our descent of a SE aspect.
Due to our time constraint, we did not observe the middle elevation snowpack.

Travel Details

Region:

Whitefish Range - Southern (south of Coal Creek)

Route Description:

Traveled the ridge east of Flower Point.

Activity:

Skiing

Snowpack Details

Snowpack and Weather Details:

Terrain

Elevation of observation:

Above 6500 ft

Aspect(s) of observation:

Red Flags:

Shooting cracks

More than a foot of new snow or heavy snowfall rates (>1"/hr)

Persistent Weak Layers:

Buried

New Snow in the past 24 hours:

7.00in.

Total Snow Depth:

104

More comments about the snowpack and weather:

Dug a pit on a northeast aspect at 6300'. Total snowdepth was 104" (240 cm). Surface instabilities were noted in the surface snow The 12/9 and Thanksgiving rain crusts were obvious and buried 50" and 58" below the surface respectively. Faceted snow surrounding these crusts has become noticeably stronger. The mid pack is made up of a 40" 1 F hard slab 1-2 mm depth hoar was found at the bottom of the pack partially due to the shrubbery that we had in the bottom of our pit.

Avalanche Details

Avalanche Details:

Date and Time of Avalanche:

Saturday, January 13, 2018 - 13:00

Avalanche Type:

Soft Slab

Failure Plane/Weak Layer:

New/old snow interface

More information or comments about the avalanche:

We were able to sympathetically trigger 3 slabs while approaching these slopes. We also noticed a few natural avalanches that failed during the storm cycle. There was an obvious difference between the overnight dense snow and the underlying low density snow from Thursday which resulted in these slides.

Trigger

Trigger:

skier

Trigger Modifier:

Intentionally Triggered

Terrain

Start Zone Slope Angle:

Aspect:

Northeast

Starting Elevation:

above-treeline

Size

Destructive Size:

D1 Relatively harmless to people.

Relative Size:

R1 Very Small

Crown Height:

Less than 1 ft

Avalanche Length (Vertical Run):

75ft.

Avalanche Width (Average width):

20ft.

Avalanche Location:

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