Professional Observation
Basic Information
Observation Details
Observation Date:
February 22, 2021 - February 24, 2021Submitted:
February 24, 2021Observer:
WAC - Caleb MerrillZone or Region:
Northern WallowasLocation:
Wing Ridge, Big Sheep, Little SheepSigns of Unstable Snow
Recent Avalanches?
YesCracking?
IsolatedCollapsing?
None ExperiencedSnow Stability
Stability Rating:
FairConfidence in Rating:
ModerateStability Trend:
SteadyBottom Line
As a warm front moved through the region on monday, the riding went from blower to slower. Warming temperatures and decreasing clouds created quite the greenhouse effect on the snow surface on the morning of 2/22. The afternoon produced rain- resulting in a rain crust that was noted to be up to 1/2" thick around 6200' on Tuesday. The rain crust was variable in thickness depending on elevation and aspect. The 2/22 rain crust will be a key interface to watch in the upper snowpack and should be monitored as more weight is added to it through new snow and wind blown snow.
The new storm snow that is under the 2/22 Rain crust is mostly right side up, and totals in the 2-3 foot range in the Northern Wallowas. A snowpit in the Big Sheep drainage (7k NW) produced propagation in an extended column test failing on buried surface hoar 65 cm from the surface. 2 other ECT's produced no propagation on this layer in similar pits. See photos below. Another pit in the Little Sheep drainage (7900' East) produced a compression test with a sudden planar result 65 cm down on this same interface, but no surface hoar was found at this location
Media
Advanced Information
Weather Summary
Wind:
Moderate , NW
Monday 2/22: Warm temperatures. Greenhouse effect followed by light rain up to ridgetops
Tuesday 2/23: 2" of new snow Monday night- Light snow throughout the day with partly cloudy skies. Snow increased in the late afternoon. NW winds overnight moving new snow into windslabs.
Wednesday 2/24: 8-10" of new snow fell Tuesday into Wednesday morning. NW winds loading leeward aspects.
snow subsided around 9:00. Clouds diminished around noon. Temperatures stayed cold.
Avalanche Observations
| # | Date | Location | Size | Type | Bed Sfc | Depth | Trigger | Comments | Photo |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Past 24 hours |
South Bowl E 8400' |
D2 | HS | I-New/Old Interface | 18" | N-Natural | Natural Wind Slab avalanche 50' x 12-18" x 500 feet |
|
Snowpack Observations
In the Big Sheep Drainage on 2/23, 2-3" of new snow fell over the variable thickness 2/22 rain crust. The riding was quite good above 7200' despite the crust. Below this elevation, riding quality decreased. The storm interface found 60-70 cm down produced mixed results in tests, with the most significant result failing on buried surface hoar. I do not believe this is widespread in the northern Wallowas.
On 2/24, along wing ridge, the NW winds had laid in sensitive wind slabs along leeward terrain features. Windward terrain features left little to be desired by the wind affect. See below.
Avalanche Problems
| Problem | Location | Distribution | Sensitivity | Size | Comments |
|---|---|---|---|---|---|
Wind Slab
|
|
Layer Depth/Date: 2/23/21 up to 18" deep Above Tree Line/SE aspects |
Terrain Use
We avoided wind loaded terrain features as well as terrain features that we thought had less snow in the start zones. A shallower snowpack holds more trigger points to impact deeper weak layers in the snowpack. We found the best riding on wind sheltered slopes where 8-10" of light density snow sat above the variable rain crust.
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