Northern San Juan

Snowpack

Bill Nalli · Apr 10, 2020 · Northern San Juan

Weather

Warm with a slight breeze keeping conditions cooler in the alpine.

Snowpack

I went to look at the slide that happened on April 9 in lower Senator Beck and hunt out an elusive avalanche critter, the Corn Slab. This tricky slab and weak layer combination is basically a wet slab but the processes that form the slab are the same that allow it to fail and propagate. Most of us are familiar with how repetitive cycles of melt during the day and freeze at night successively turn snow near the surface into larger and larger rounded crystals. After a few days they resemble small kernels of corn. When it’s beginning to melt, and just before it becomes too melted and slushy, it is very easy to get your ski edge into and is an often sought after condition for good riding. Warm temperatures and short wave radiation from the sun conspire to melt the surface snow and cause individual crystals to round and increase in size. At the same time, excess liquid water moves through the snowpack looking for the easy path back to the ground. If it finds a weaker layer, it pools and follows the path of least resistance. As it cools again at night the upper layers of the snowpack move back toward the freeze side of the cycle and the rounded grains solidify back into a crust. Not good skiing in this frozen phase but the upper 20-40 centimeters is now a strong cohesive mass. Below this the residual heat from the last day’s warming keeps a layer of rounded crystals wet. So wet in fact that by the next day, liquid water is still present. The snow above starts out frozen and slowly softens. Right at about the same time this softening becomes good corn skiing, the strength of the frozen slab weakens just enough to fail but it is still cohesive enough to transfer energy and propagate into a slab. You can see in the crown profile photos taken at 4pm that the surface has started to refreeze while the layer below is still wet. Stability tests that isolate a column of snow effectively remove some of the strength of the slab. Compression tests failed at 8 taps from the wrist and extended column tests propagated on 6 taps, over 24 hours after the avalanche. It’s a reminder that when hunting out spring corn skiing it’s important to know the condition of the snow below what you see and feel at the surface. If there is a wet layer where water is pooling, reconsider riding on steep slopes.