Public Observation #2022-912
Submitted:December 19, 2022 1:00 PM
Observation Date:December 18, 2022
Zone or Region:Central Cascades
Did you trigger any avalanches?Yes
Was it intentional?Yes
Avalanche Type:Hard Slab
Size:D1: Relatively harmless to people
Comments:Intentionally trigger test slope. Wind slab over surface hoar.
Signs of Unstable Snow
Did you see shooting cracks?Yes
Did you experience collapsing or whumpfing?Yes
ObservationsI accessed Black Crater on foot from the end of the plowed old Mackenzie Hwy. I stayed to low angle slopes on that long approach and compared observations to the conditions witnessed on the exact same approach on 12-16. There had been significant surface hoar on 12-16; that morning it had been -16C at the parking area. This was mostly gone on 12-18 and there were indications of a fair amount of wind transported snow in our old skin track and elsewhere (this likely blew away much of the surface hoar in exposed areas).
Wind slabs had formed on N facing slopes, and I suspected that the surface hoar had been sheltered there long enough to be buried. Two separate test slopes near our ascent track proved extremely reactive. one at 1,540m facing 25 degrees and the second at 1,898m facing 15 degrees. The first was able to be triggered with a mid-slope stomp and partially slid with a 20cm crown. The second triggered with a light step, remotely, from over 15m away horizontally in a lower angle, area. The second had a crown of over 40cm.
We continued past these test slopes on low angle terrain and skied lower angle terrain with a more westerly aspect safely down.
My take away is that wind loaded slopes might be extremely unstable where the surface hoar was preserved before wind deposition. The test slopes were 5-8 m in height, and without slopes below or above or terrain traps. A larger slope with this problem would have been extremely dangerous.