Dangerous snowpack in Northern Whitefish Range

Location Name: Forecaster Observation - Mt. Barnaby, Northern Whitefish Range Observation date: Thursday, January 11, 2018 - 13:45	Is this an Avalanche Observation: Yes

Observation made by: Forecaster

Location

Tabs

Quick Observation

As much as 20" of new snow since 1/9 rain event doubled the depth of the snowpack here, making for reactive storm slab and persistent slab problems.
There were numerous soft slab avalanches, up to D2.5, that ran sometime in the past 24 hours. The crowns were 10" to 20" deep, and they failed on low density snow above our recent rain crust. These all failed above 6000', but one ran down to 4900' and snapped several tree limbs with an impressive amount of debris. This instability produced minor cracking under our feet, some localized collapses, and presented itself in pits with sudden planar, propagating results under moderate force.
Below 5,000 feet the snowpack is very shallow, purged by rain, and not enough new snow above the rain crust for avalanche concerns yet, apart from overhead hazards.
At 5,400 feet, we got a huge, rumbling collapse as I was stepping out of my skis to dig a pit. The structure is significantly shallower and weaker than other parts of our zone. 3 foot slab over large, weak facets on the ground (2mm, F hard). Tests on this layer produced sudden collapse results on isolation or under moderate force.
The 1/9 rain crust was present all the way up to at least 6700'. It was actually thicker at higher elevations (up to 2" thick), and thinner or non existent in the middle elevation band.

Snowpack, Avalanche, Weather Images:

Snowpack, Avalanche, Weather Videos:

January 11, 2018. Dangerous snowpack in Northern Whitefish Range

Travel Details

Region:

Whitefish Range - Northern (north of Coal Creek)

Route Description:

Mt. Barnaby to 6700 ft

Activity:

Skiing

Snowmobiling

Snowpack Details

Snowpack and Weather Details:

Terrain

Elevation of observation:

3500-5000 ft

5000-6500 ft

Above 6500 ft

Aspect(s) of observation:

Red Flags:

Collapsing / whumpfing noises

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

Persistent Weak Layers:

Buried

Total Snow Depth:

90 cm

More comments about the snowpack and weather:

See photo

Avalanche Details

Avalanche Details:

Date and Time of Avalanche:

Wednesday, January 10, 2018 - 14:45

Avalanche Type:

Soft Slab

Failure Plane/Weak Layer:

New/old snow interface

More information or comments about the avalanche:

One D2.5 sympathetically or spontaneously pulled in slabs from multiple parts of the start zone. Several other similar crowns that were R1/D1, smaller pockets.

Trigger

Trigger:

natural

Terrain

Aspect:

Southeast

Starting Elevation:

above-treeline

Size

Destructive Size:

D2.5

Relative Size:

R3 Medium

Crown Height:

1 ft

Avalanche Length (Vertical Run):

1900ft.

Avalanche Width (Average width):

350ft.

Avalanche Location:

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Dangerous snowpack in Northern Whitefish Range

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