South San Juan Backcountry Avalanche Forecast Sat, Dec 19, 2020 at 7:17 AM Issued by: Chris Bilbrey Danger Rating Saturday and Sunday Above Treeline Considerable (3) Dangerous avalanche conditions. Cautious route-finding and conservative decision-making essential. Near Treeline Considerable (3) Dangerous avalanche conditions. Cautious route-finding and conservative decision-making essential. Below Treeline Moderate (2) Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully. Summary Recent snowfall has thickened slabs above weaker snow inching the snowpack toward its tipping point. Slopes that have not failed naturally may be sitting there waiting for the extra push from a rider or machine. For today, the threat of triggering an avalanche remains elevated and you’re more likely to get into trouble on northwest through north to east-facing slopes. Areas harboring the thickest drifts are the most dangerous and where a small slide could trigger a larger, more deadly one. Strong northwest winds today may help thicken and expand the distribution of wind-drifted slabs below ridgeline and in cross-loaded terrain features. Treat any steep slope where you see evidence of recent wind-loading, textured snow surfaces or smooth rounded pillows of snow, as suspect. Although the size of avalanche that you can trigger will be smaller below treeline, these avalanche can be just as dangerous if you are pushed into trees or buried in a terrain trap. Pay attention to surface cracking, audible collapses and stick to slopes less than 35 degrees to help reduce your avalanche risk. Avalanche Problem Persistent Slab Aspect/Elevation: N,E, NW,NE; All elevations Likelihood: Likely Size: Large - Small What You Need to Know About These Avalanches Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty. Forecast Discussion Consistent snowfall since December 11th has added small, incremental loads onto a very weak snowpack structure. Each new load adds a little more weight to the overlying slab and is inching the snowpack closer to its tipping point – and some slopes have already reached it. Our observations page is full of reports of rider-triggered slides and natural slides. Avalanches are the number 1 piece of bulls-eye data that points to a dangerous and reactive snowpack. If northwest winds increase today as forecast, the low-density snow that fell Friday will be easily transported onto leeward slopes helping build thicker drifts and may contribute to an uptick in natural activity. Pay attention to the direction a slope faces and don’t linger below steeper slopes that are being actively loaded. The vast majority of slides triggered by riders or those that failed naturally have remained on the smaller side, but volume and propagation continue to increase with each new load. These slides are breaking below newly formed slabs in a thick layer of near-surface facets above a harder mid-pack layer, in a layer of depth hoar at the ground, or around a faceted crust found on east and southeast aspects. The multitude of persistent weak layers is somewhat consistent across aspects and elevation in the San Juan Mountains but the biggest variable is how thick the overlying slab is. Right now, the most reactive and concerning layer to contend with are near-surface facets that developed during the prolonged dry spell from Late-November to Early-December. This layer ranges in thickness and you will find it buried anywhere from a foot deep to three feet or more in wind-loaded areas. Alpine slopes blasted by strong northwest winds on December 15th, the layer of near-surface facets is now capped by a hard, wind-slab. In these areas, it may be more difficult for a rider or machine to impact weak snow below the hard slab but if you do find the shallow part a slope, this harder layer may help slides propagate wider and potentially deeper than you might expect. Shallow parts of slopes are commonly found around rocky outcrops, near the margins of a slab, or on the backside of steep rollovers. Near treeline slopes still harbor the weakest overall snow structure and where observers continue to report loud, rumbling collapses. The most avalanche activity has been reported across the North San Juan, but similar signs of instability and snowpack structure exist in the South San Juan. This is probably a combination of fewer backcountry users, generally tougher access into avalanche terrain, and less new snow volume since December 11th as compared to the North San Juan. Regardless of where you are recreating in the Southern Mountains, if you find a cohesive-enough slab, which we have, and a steep enough slope, there is a good chance you will trigger an avalanche. The big question is how big will it be. Fresh snow, sunny days, and recreating with friends will undoubtedly drive the stoke this weekend. By no means is this the time to let your guard down. Avoid the temptation of getting lured out onto bigger, steeper slopes even if you don't observe signs of instability. Managing the uncertainty around a persistent avalanche problem is the biggest challenge and avalanches can potentially break above you once you have already committed to the slope.