Phase one of a two-phase storm system passed to our south and east overnight. Light snowfall and strong south to southwest winds built fresh, shallow surfaces slabs throughout the Southern Mountains. Generally, one to two inches of new snow is reported with up to four inches on Wolf Creek Pass. An additional three to five inches of snow is possible today before the storm quickly exists to the east. The next round of precipitation comes with a bump in wind speeds from a cold front this afternoon contributing to more blowing and drifting snow.
This new, light load should not overload and tip the scale on the generally strong snowpack in the San Juan Mountains. Smaller snowfall totals in the Sangre de Cristo range means no change to the avalanche danger for today, but an overall shallower, weak snowpack structure exists there. With each strong wind-loading event during January in the San Juan Mountains, we’ve seen natural avalanches popping in the alpine – be on the lookout for similar conditions today and tomorrow.
The most obvious and easier to manage avalanche problem today is newly formed wind-drifted slabs on leeward slopes. Thickest and potentially more reactive drifts are found in areas that received the most snowfall – the South San Juan zone favoring the eastern side. These fresh slabs formed above a variety of old snow surfaces such as wind-stiffened alpine surfaces, thin melt-freeze crust on sunny slopes, and decomposing fragments/ near-surface facets on shady slopes. New snow densities around 10% may promote good bonding with firm surfaces but the new snow may be easier to trigger where softer snow is found below. Denser snow increases slab strength and this may allow it to rest above weaker snow waiting for an extra push from a rider or machine before avalanching. On the flip side, a more cohesive slab has potential to break wide than expected, run farther, and entrain more snow leading to a larger, more violent avalanche.
Continue to use caution on slopes facing east, southeast, and south at higher elevations. Since January 1st, the largest proportion of natural avalanches have occurred on those aspects (49 out of 72) in the North and South San Juan zones. Although rider-triggering on those same aspects has been limited, a rider in the North San Juan zone on Tuesday triggered a large avalanche near treeline on a southeast-facing slope. This avalanche is bulls-eye information that our Persistent Slab avalanche problem may be coming back into play near treeline in more areas. A series of buried crust/ facet combinations as well as lingering depth hoar remain along with an overall shallower snowpack. Slopes with overlapping Wind Slab and Persistent Slab avalanche problems have potential for a shallow slide to step down into deeper weak layers producing a more dangerous slide that would be difficult to survive.
Northwest aspects still remain the most concerning of north-facing terrain due to more scouring and loading. This has contributed to highly variable snow depth and overall snow cover. Shallower areas with weak, faceted, cohesionless snow may have stiffer snow above making for a complex pattern. Managing your terrain selection is key to avoiding an avalanche encounter today and strive to avoid likely trigger locations such as the edge of the slab or near rocky outcrops.