Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2
What is it? Forecasters have observed buried surface hoar in the upper 3' of the snowpack in the Northern and Southern Wallowas and Elkhorns, in the past week. A natural avalanche was observed on Sunday in the Southern Wallowas at Fish Lake, failing 1-3' in depth on a layer of surface hoar that was buried around January 4th. This avalanche was large enough to bury and kill a person.
Where is it? Various layers of surface hoar have been observed primarily near and below the treeline throughout our forecasting zones. The avalanche at Fish Lake was on a NE aspect at 7,000' on a steep slope >30 degrees. This avalanche likely reached the breaking point when the wind loaded the upper portion of the start zone. I found very reactive surface hoar buried around 1.5-2' deep this past week in both the Southern Wallowas and Western Wallowas below 7,000'.
How can you recognize it? The easiest Red Flag observation is a collapsing or whumpfing of the snowpack. Last weekend while snowmobiling between Schneider Meadows and Fish Lake, my group experienced many whumpfs. This tells me the surface hoar was present in those locations and was collapsing under the weight of my machine. If you take the time to dig into the snowpack, you may observe a thin grey-colored line, and observe clean shears where the buried surface hoar exists.
How can you manage it? Surface hoar is a sneaky weak layer. These avalanches may break wider than expected or higher on the slope above you. You may be able to trigger a slide after several riders have safely climbed or descended a given slope, and there's an outside chance you could remotely trigger an avalanche from gentler terrain above, below, or to the sides of steeper slopes. Avoid lower and middle elevation slopes on all aspects of the compass that are steeper than about 35* to significantly reduce the chances of finding trouble. Steer clear of large wind drifts or slabs in very steep terrain. Picture where even a small avalanche could carry you before committing to playing in large or consequential avalanche paths.