Northwest Avalanche Center

Observation: Public

All Observations

Observation Details

Name:
Josh Hirshberg
Observation Date:
December 19, 2020
Submitted:
December 19, 2020
Zone or Region:
Stevens Pass
Activity:
Skiing/Snowboarding
Location:
Lanham/Henry creek

Observed Avalanches

Did you observe any avalanches? 
Yes
Avalanche Type:
Size:
Size 2: Could bury, injure, or kill a person
Elevation:
Near treeline
Aspect:
NE
Comments:
Arrowhead, NE bowl. Possibly D2.5. Ran at least 500ft vertical w debris stopping ~4.7k. Suspect this was a slab, running on Dec 12th PWL. Poor viz and couldn't confirm crown.

Signs of Unstable Snow

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

Observations

It appears there was a cycle of persistent slab avalanches overnight on Dec 18th on polar aspect. Mostly D2, some D2.5.
We stayed mostly below treeline. Found reactive fresh drifts on the ridge at 5.6k. Surface crust to 4.8k in the trees, maybe only to 4-4.2k in open terrain.
Profile at 5k, NNE:
Height of Snow (HS) 125cm
↓47cm Dec 12th facets (1.0mm), well-rounded
Test results did not indicate propagation at this location

Media

Profile Lanham creek, 5,000ft, NNE. Card marks the Dec 12th rounding facets 47cm below the surface
Cracking in a fresh drift at 5,600ft
Rain rills on the surface at 3,600ft
Profile graph from Lanham ck, 5,000ft, NNE

Advanced Observations

Observed Avalanche Problem #1: 
Storm Slab
Comments: 
Near and below TL. Possible. Up to large. Would have taken unsupported terrain, and slopes >40deg to
Observed Avalanche Problem #2: 
Persistent Slab
Comments: 
Did not directly observe. Suspect near and above TL, mostly on polar aspects. Very likely, Large to Very Large (D2-3).
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