Observation: NWAC Forecaster

All Observations

Basic Information

Observation Details

Observation Date:
December 20, 2021
Submitted:
December 20, 2021
Observer:
NWAC Forecaster - Andrew Kiefer
Zone or Region:
Mt Hood
Location:
Mitchell Creek + Hwy 35 (4200-6200ft, E-S-W aspects)

Signs of Unstable Snow

Recent Avalanches? 
Yes
Cracking? 
Isolated
Collapsing? 
None Experienced

Media/Attachments

An avalanche within storm snow on a steep slope above Hwy 35 (4300ft, E aspect). The avalanche broke 6-8in deep and impressively wide. Unknown trigger.
A small skier triggered avalanche (D1) that broke 4-6in deep within new snow near Vista Ridge (6200ft, E aspect)
A profile showing the 30in of new snow that fell since 12/18. Several inter-storm layers within the top 6-12in produced sudden planar fractures in tests but no propagation. The new snow was well bonded to the 12/18 crust at this location.
Rollerballs along Hwy 35 on a SE aspect near 4500ft.

Advanced Information

Weather Summary

Cloud Cover:
Obscured
Temperature:
30-35°F
Wind:
Light , SW
New/Recent Snowfall:
30in new snow since 12/18

Heavy, wet snow with snowfall rates of 1in per hour in the AM. Gradual warming throughout the day with precipitation transitioning to a rain/snow mix in PM up to ~5500ft. Light westerly winds with little blowing snow and very minor wind effect near and below treeline.

Avalanche Observations

 #  Date Location Size Type Bed Sfc Depth Trigger Photos Details
1 Dec 20, 2021
() 		Hwy 35
E 4400ft 		D1.5 SS-Soft Slab 0.5ft U-Unknown
An avalanche within storm snow on a steep slope above Hwy 35 (4300ft, E aspect). The avalanche broke 6-8in deep and impressively wide. Unknown trigger.
Report
Hwy 35

Triggered a small slab avalanche (D1) 4-6in deep within new snow on a steep convex roll (6200ft, E aspect). Observed another slab avalanche (unknown trigger) that broke 6-8in+ deep and impressively wide on a steep slope above Hwy 35 (4300ft, E aspect). See photos.

Snowpack Observations

Observed around 30in of new snow above the prominent rain crust buried 12/18. The top 6in+ of new snow were heavy and dense, and sat above less dense storm snow - an obvious upside down layering configuration (4F over F). Inter-storm layers ↓6in and ↓12in produced sudden planar fractures in snowpack tests but no propagation. These moist storm snow layers near the surface were challenging to accurately assess with snowpack tests but recent avalanche activity was bullseye data the snowpack was unstable. Below the top 6in of surface snow, the rest of the storm snow had a favorable hardness profile (F-4F) and was well bonded to the 12/18 crust where I looked in a test pit at 5500ft (see photo). Elevated danger and mediocre skiing aside, this heavy load of dense snow is great base building material and is really helping to fill in rough terrain and creek beds.

Avalanche Problems

Problem Location Distribution Sensitivity Size Comments
Storm Slab
Isolated
Specific
Widespread
Unreactive
Stubborn
Reactive
Touchy
D1
D1.5
D2
D2.5
D3
D3.5
D4
D4.5
D5
 Layer Depth/Date: up to 30in deep - storm snow since 12/18

Storm slab with wet characteristics

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