Small storm slabs and one nasty persistent slab

Location Name: 
Forecaster Observation - Flathead Range
Observation date: 
Friday, February 5, 2021 - 17:45
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Tabs

Quick Observation

Today we went into the Flathead Range for a quick day to ride low angle terrain and get a feel for storm snow and buried week layers. Knowing what lurked beneath the storm snow kept us out from any terrain steeper than 30 degrees.

  • At 5000' there was roughly 18 inches of new and recent snow on the 1/13 crust.
  • Ascending a ridgeline, we got numerous shooting cracks in the new/old snow interface. This interface was roughly 6 to 10 inches below the surface. 
  • We remotely triggered small avalanches and cornice failures. All of which were small and failed in the storm snow. 
  • It wasn't until our high point of roughly 6200' where a cornice fall triggered a large, persistent slab avalanche (more info in avalanche tab). This was 2 to 3 feet thick and propagated roughly 600 feet wide. 

We had a lot of discussion on our ascent about recent avalanches and the lack of feedback we were getting prior to the large avalanche we triggered. Our theory is that the weak layer is just starting to get buried to a depth that is making it more difficult for the weight of the person to trigger, at least in this location. This makes a very dangerous situation. Feedback may become less "in your face" with a slab that is more dangerous. 

Snowpack, Avalanche, Weather Images: 
Snowpack, Avalanche, Weather Videos: 

Storm and Persistent Slab Avalanches

Travel Details
Region: 
Flathead Range - Middle Fork Corridor
Activity: 
Skiing
Snowmobiling
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
3500-5000 ft
5000-6500 ft
Aspect(s) of observation: 
N
NE
NW
Red Flags: 
Avalanches from the past 2 days
Shooting cracks
More than a foot of new snow or heavy snowfall rates (>1"/hr)
Blowing snow
Persistent Weak Layers: 
Surface Hoar
Facets or Faceted Crust
Buried
New Snow in the past 24 hours: 
8.00in.
More comments about the snowpack and weather: 
We dug down to the December facet/crusts to do a PST and see if we could initiate a crack. PST90/100 (end) on December 9 crust facet. Grains appeared to be rounding around the crust and locating the weak layer was challenging. The goal of this was to see if there is a potential for a significant loading event to step down to deeper weak layers from earlier in the year. A PST on the layer of surface hoar resulted in a PST40/100 (end).
Blowing Snow: 
Moderate
Wind Speed: 
Moderate (Small trees sway)
Wind Direction: 
West
Air temperature: 
Below Freezing
Sky Cover: 
Overcast (OVC)
Highest Precipitation Rate: 
Very Light Snowfall (S-1)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Friday, February 5, 2021 - 14:30
Number of avalanches: 
3
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
New/old snow interface
More information or comments about the avalanche: 

Numerous small avalanches were easily triggered on anything steep enough to slide (touchy). These were all harmless to a person due to the terrain we were in. 

Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Start Zone Slope Angle: 
35
Aspect: 
North
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Avalanche Location: 
Date and Time of Avalanche: 
Friday, February 5, 2021 - 14:45
Number of avalanches: 
1
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
Old snow
More information or comments about the avalanche: 

We triggered this avalanche at our high point. Other than wrapping up the ridge, I almost couldn't imagine it propagating any wider. Crown height averaged around 2 feet thick with some wind loaded areas being up to 3 feet. 

Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Start Zone Slope Angle: 
38
Aspect: 
Northeast
Starting Elevation: 
near-treeline
Hide Size
Destructive Size: 
D2.5
Relative Size: 
R4 Large
Crown Height: 
3 ft
Avalanche Length (Vertical Run): 
600ft.
Avalanche Width (Average width): 
600ft.
Avalanche Location: 
Date and Time of Avalanche: 
Friday, February 5, 2021 - 08:00
Number of avalanches: 
3
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
Unknown
More information or comments about the avalanche: 

Visibility made identifying any specifics challenging. Speculated time of occurrence. The only thing we could confirm was debris in the runouts of 2 paths. It seems fair that they were at least D2 in size.

Hide Trigger
Trigger: 
natural
Hide Terrain
Aspect: 
Southeast
Starting Elevation: 
above-treeline
Hide Size
Destructive Size: 
D2 Could bury, injure, or kill a person.
Relative Size: 
unknown
Avalanche Location: