Northwest Avalanche Center

Observation: Public

All Observations

Observation Details

Name:
Matt Primomo
Observation Date:
January 27, 2021
Submitted:
January 28, 2021
Zone or Region:
East Central
Activity:
Skiing/Snowboarding
Location:
Blewett Pass

Triggered Avalanches

Did you trigger any avalanches? 
Yes
Was it intentional? 
Yes
Avalanche Type:
Soft Slab
Size:
Size 1: Relatively harmless to people
Elevation:
5800ft
Aspect:
N
Comments:
Easily triggered this 4F hard wind slab just off a ridgeline on the north side of Windy Knob about 200ft below the top. The crown was from 20-35cm deep x 7m wide, and it ran about 50ft downhill before stopping with lower angled slopes below. Wind slabs were isolated to near ridgeline and upper elevation terrain, but substantial drifting had occurred despite the light to moderate winds. This slab ran on weaker storm snow just above faceted grains over a stout crust.
Photo:

Observed Avalanches

Did you observe any avalanches? 
Yes
Avalanche Type:
Size:
Elevation:
Aspect:
Comments:

Signs of Unstable Snow

Did you see shooting cracks? 
No
Did you experience collapsing or whumpfing? 
Yes, Isolated

Observations

I experienced isolated cracking around 5,000ft elevation on open slopes. A closer look revealed the collapsing was occurring on a crust/facet/crust sandwich (beneath a thin crust on a layer of larger facets, above a more stout crust. See profile for details. Cracking and minor collapsing was very isolated, as it felt like there wasn't quite enough snow on it yet to cause larger, concerning collapsing. I've observed this sandwich structure within the zone at various elevations in different places and may make for a lasting weak layer structure as we slowly add more snow over it.
I was able to get results with tilt tests within the new snow 15cm down, and also at the interface of the storm snow and small grained facets above the thin crust. Storm snow was very soft, and uncohesive.

Media

Profile at site of minor cracking and collapsing revealed a facet sandwiched between two crusts as a concern with more snow.
East aspect at 5,800ft, with crystal card highlighting larger grained facets above a stout melt freeze crust. The upper crust was stout here, no facet sandwich.
WordPress Lightbox