Professional Observation

Observation Details

Signs of Unstable Snow

Key Points

We covered many aspects and elevation bands in the June area looking at surfaces before the upcoming storm. We peeked into the snowpack to look for the persistent slab structure.

• Solar aspects below 10,000 feet began the melt-freeze cycle yesterday. We found melt-freeze crusts up to that elevation and saw rollerballs and small wet loose avalanches below 9,500 feet. Most of these slid yesterday, but we saw some new activity this afternoon under the intense March sunshine when temps climbed above freezing to at least 10,000 feet. We even found a thin, zipper melt-freeze crust on northerly slopes at lower elevations.
• We saw lots of evidence of extreme winds. Near treeline we found hard wind slabs that cracked underfoot, but no shooting cracks or other signs of instability. Above treeline we found hard sastrugi and wind board, primarily from north winds. At ridgelines, we found hard, unreactive wind slabs on both north and south aspects. We stomped around large, flat, windward slopes (fetches) and found our boots only sank about 2 inches into the hard surfaces. That means that increasing wind will likely need new snowfall for ammunition before new wind slabs form.
• We dug down to the persistent weak layer, which was buried 140 cm below the recent storm snow near treeline. The overlying slab increased from Fist-hard to Pencil-hard with depth. In this area, where the snowpack was over 4 m deep, the weak grains were smaller (0.5 to 1 mm), stronger (4F+), and bonding better (sintering FCxr) than we’ve seen in other parts of the forecast area. We got sudden planar results with hard force in our deep tap test on this layer.
• We saw several large crowns from the recent storm cycle. Some of these looked like wind slab avalanches, but some were deep, jagged, further down the slope that you’d expect wind slabs to form, and broke from rocky areas. In other words, persistent slabs.