Stepping back after a sneak storm

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

Observation made by: Forecaster

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Quick Observation

Shortly after we left the trailhead, it was apparent that this area picked up substantially more snow than weather forecasts and remote stations had indicated. 6-8" of dense snow at lower elevations and up to 18" of dense snow at 6,000'. We stepped back to a conservative mindset for the day.
Numerous slab avalanches ran naturally in the past 6 hours, ranging from D2 to D3 in size from upper elevations. We saw 2 or 3 slides that appeared to go about 5 feet deep on the Thanksgiving crust, D3 in size (estimated from a distance). We saw 3 storm slabs that looked to be 1 to 2 feet deep, D2 in size.
Below 6,000 feet, there were dozens of loose snow avalanches, mostly D1.5, that ran overnight or today. At lower elevations, these were smaller and wetter. We also skier triggered several similar slides.
Several quick pits between 5400' and 6000' showed 70 to 80 cm of recent storm snow over the 1/9 rain crust, with non propagating results in ECTs. The loose snow avalanches at this elevation band did not trigger slabs or propagate wider as they ran, and we saw only isolated cracking. Regardless, we remained wary of storm slab issues given the dense and rapid load and slab avalanche activity observed higher in the terrain.
The freezing level rose to ~5,000 feet by the afternoon. Snowfall above/ rain below continued for most of the day but tapered in intensity from heavy (S5) to light (S1). There were moderate to strong wind gusts, variable directions but mostly NW. Snow transport was relatively light where we traveled given the dense snow surface.

Snowpack, Avalanche, Weather Images:

Travel Details

Region:

Flathead Range - Middle Fork Corridor

Route Description:

Wahoo Creek to the 6,000 feet. We climbed to both ridges on either side of the creek.

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:

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

Rain on snow

Blowing snow

Rollerballs / pinwheels

New Snow in the past 24 hours:

15.00in.

Total Snow Depth:

260 cm

More comments about the snowpack and weather:

We dug about a foot below the 1/9 crust to look for 1/5 surface hoar but didn't see signs of it from multiple pits.

Avalanche Details

Avalanche Details:

Date and Time of Avalanche:

Saturday, January 13, 2018 - 06:00

Avalanche Type:

Hard Slab

Failure Plane/Weak Layer:

Old snow

More information or comments about the avalanche:

Mt. Nyack. Crowns viewed from a long distance away so sizes are rough estimates

Trigger

Trigger:

natural

Terrain

Aspect:

Northeast

Starting Elevation:

above-treeline

Size

Destructive Size:

D3 - Could destroy a car, a wood house, or snap trees

Relative Size:

unknown

Crown Height:

5 ft

Avalanche Width (Average width):

500ft.

People Involved

Number of people caught:

Avalanche Location:

Date and Time of Avalanche:

Saturday, January 13, 2018 - 08:30

Avalanche Type:

Soft Slab

Failure Plane/Weak Layer:

Within storm snow

Trigger

Trigger:

natural

Terrain

Aspect:

North

Starting Elevation:

above-treeline

Size

Destructive Size:

D2 Could bury, injure, or kill a person.

Relative Size:

R2 Small

Crown Height:

2 ft

Avalanche Location:

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