A crust/surface hoar layer from mid-December remains the primary concern and requires careful terrain selection. New snow may conceal wind slabs in exposed terrain.
Confidence
Fair - Intensity of incoming weather systems is uncertain on Monday
Weather Forecast
Saturday: mostly cloudy with flurries. Treeline temperatures around -13C and winds light or moderate from the west. Sunday: Dry during the day with snow starting late in the afternoon. Treeline temperatures around -16C, winds light and variable. Monday: 10-20 cm new snow expected by the end of the day Monday. Freezing levels are expected to remain below valley bottom, with treeline temperatures around -10C. Winds anticipated to rise to around 50 km/h from the southwest.
Avalanche Summary
Explosives control on Thursday produced several avalanches generally in the 1.5 to 2.5 size range with some of these failing on persistent weak layers, while others only involved surface snow. One very large avalanche (size 3.5) was explosively triggered that released on an early season weak layer with a deep, 3 m crown and very wide propagation. A small (size 1) skier-triggered avalanche was reported from near Golden on a steep convex roll in forested terrain at 2200 m on a northeast aspect.
Snowpack Summary
Most of this region only saw light snow accumulations during the storm that ended Friday. However, winds were sufficiently strong for fresh wind slabs to be of concern. Since previous winds were from the north and more recent winds anywhere from southwest to northwest, wind slabs should be suspected on a wide variety of aspects in exposed terrain. A spotty thin layer of surface hoar was buried on December 27th by about 5-10 cm of light dry snow. As more snow accumulates, this interface could become one to watch. Deeper, at about 40-60 cm below the surface, a persistent crust/surface hoar layer from mid-December can be found. Recent snowpack tests show moderate "pops"-type results on this layer, indicating the potential for avalanches to propagate if triggered on this layer. This persistent slab problem is reasonably widespread across the region and is not expected to improve quickly. A deeper crust/facet layer which formed early in the season may still be triggerable from thin or rocky snowpack areas.
Problems
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.