Avalanche and poor stability in JFS Canyon

Location Name: BNSF Avalanche Program-Snowslip Mtn-Glacier Park Observation date: Thursday, December 13, 2018 - 23:30	Is this an Avalanche Observation: Yes

Observation made by: Public

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Quick Observation

Travelled up the Shed 6/7 ridge for a welfare check on the Shed 7 weather station and to assess stability now that our weak, pre-storm snowpack was recently buried by a slab. In general, where a snowpack existed, which was mostly on easterly aspects above 5500 feet elevation, stability was poor and we intentionally triggered one avalanche at about 5600 feet elevation.
Mostly cloudy skies, S-1 snowfall, and moderate to strong westerly winds that were actively loading easterly aspects. Air temperatures in the 20s °F above 5500 feet.
Below 5500 feet there is still generally less than 25 cm of snow on the ground. Above that elevation snow depths quickly increased and ranged from 50-150 cm above 6000 feet elevation.
Above 5500 feet there were touchy wind slabs on the east-facing slopes. We observed many signs of instability such as audible collapses, shooting cracks, and avalanches.
In some exposed locations above 6000 feet the wind-loading has been heavy and we estimate there are locations with new wind slabs well over 1 meter thick.
Extended Column tests in a pit at 6200 feet on an east aspect resulted in propagation on the new storm snow/old snow interface (top of the facets) about 35 cm down and also on another layer of facets about 60 cm down from the surface.
Intentionally triggered a wind slab avalanche by walking near the flank. Failure was on the facets just below the base of the wind slab. Location was at 5600 feet on a SE aspect in a location known as Test Slope 2. Avalanche classified as HS-AF-D1.5-R4-I, crown ranged from 20-50 cm thick and about 25-30 m wide. Debris ran about 50-70 m down the slope.
No naturally releasing avalanche activity was observed save for some cornice fall at the top of Shed 7 West.

Snowpack, Avalanche, Weather Images:

Travel Details

Region:

Flathead Range/Glacier National Park - John F. Stevens Canyon

Activity:

Hiking

Snowpack Details

Snowpack and Weather Details:

Terrain

Elevation of observation:

3500-5000 ft

5000-6500 ft

Aspect(s) of observation:

Red Flags:

Shooting cracks

Collapsing / whumpfing noises

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

Blowing snow

Persistent Weak Layers:

Facets or Faceted Crust

Buried

Blowing Snow:

Intense

Wind Speed:

Strong (Whole trees in motion)

Wind Direction:

West

Air temperature:

Below Freezing

Sky Cover:

Overcast (OVC)

Avalanche Details

Avalanche Details:

Date and Time of Avalanche:

Thursday, December 13, 2018 - 23:30

Avalanche Type:

Hard Slab

Failure Plane/Weak Layer:

New/old snow interface

Trigger

Trigger:

on-foot

Trigger Modifier:

Intentionally Triggered

Terrain

Aspect:

Southeast

Starting Elevation:

near-treeline

Size

Destructive Size:

D1.5

Relative Size:

R4 Large

Crown Height:

1 ft

Avalanche Length (Vertical Run):

200ft.

Avalanche Width (Average width):

80ft.

Avalanche Location:

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