The new storm in the forecast is expected to drive avalanche danger up to HIGH over the next few days.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Cold arctic air is expected to persist overnight bringing alpine temperatures down to about -20 in the North of the region. Flurries are expected overnight with only a couple of cm accumulation, and another 2-3 cm during the day on Sunday. Snow beginning Sunday evening as the Southwest winds increase to moderate and the new storm moves into the region. Expect 5-10 cm of dry new snow by Monday morning, and another 5-15 cm during the day as Westerly winds increase to strong. Alpine temperatures are expected to gradually increase with highs of about -8 by Tuesday. The storm should continue on Tuesday with another 5-15 cm. Snowfall amounts are difficult to forecast due to the moist air moving in from the Southwest colliding with the cold air from the Northeast.
Avalanche Summary
Natural and sled triggered avalanches were reported up to size 1.5 in pockets of cross-loaded windslab in the alpine on West thru Northwest aspects. The mid-December persistent weak layer continues to be a concern for human triggered avalanches.
Snowpack Summary
New snow and moderate Southwest winds developed isolated pockets of touchy windslab in the alpine and at treeline. Since previous winds were from the north or northwest, wind slabs should be suspected on a wide variety of aspects in exposed terrain. A thin layer of surface hoar with variable distribution 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.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.