Northern San Juan
AvalancheChris Bilbrey · Mar 25, 2023 · Northern San Juan
Weather
Cool sunny day. Moderate westerly wind along ridgetops moving a decent amount of snow onto leeward slopes. A few passing clouds during the day, but the main cloud front stayed north of Lizard Head Pass.
Snowpack
We found 10 to 15 cm of F to 4F snow that fell overnight. Directly below the new snow is a micro-thin crust on slopes facing northeast through east to south. Small facets are beginning to develop below the crust. This shallow interface produced progressive collapse failures on both shady and sunny slopes with easy force in Compression Tests (CTs). On similar aspects, ski pressure yielded only minor, harmless sluffs on slopes pushing 35 degrees. Digging deeper, we found about 45 cm of storm snow from 3/20 to 3/23 around 10,000 feet increasing to about 80 cm on slopes at and above 11,000 feet. No obvious rain crust, from mid-storm temperature fluctuations, identified in snowpits or when probing along the skin track. In general, the storm slab felt less inverted than places further south and east where we traveled in the last week. However, we still observed planar failures on a northeast-facing slope with 22 taps where the slab transitions from P-/1F to 1F-/ 4F. This change in hardness ranged between 50 and 60 cm below the surface. On a southeast-facing slope, this density change was less obvious and the most notable failure was at the new/ old snow interface above a stout crust - resistant planar failures in CTs with 18 taps. The faceted interface buried between storms around March 15, 2023 produced sudden collapse failures with 25 taps. This was the only deeper PWL that failed in our northeast-facing snowpit. While excavating the snowpit on a southeast-facing slope, we observed a loud collapse when the shovel poked through the melt-freeze crust (3/20 interface) below. The slope settled a couple of inches, but a group around 100 meters away did not hear or feel the collapse. Below the crust is a 5 to 8 cm thick layer of rounding facets (1F-). We did not observe failures in facets below the crust. Settling of upper storm layers has improved travel conditions across aspects and elevations. Steady drifting along the ridgeline triggered a shallow, soft slab avalanche on an east-facing slope at treeline, but did not provoke a larger/ broader failure or entrain into deeper storm layers
Avalanche
Several size 2-2.5 natural avalanches near and above treeline that likely released around March 22, 2023. The crowns were mostly filled back in, but debris piles still evident below.