The storm is over and the new storm slab may take several days to settle and bond to the old surface. The mid-December buried surface hoar crust combination is expected to continue to be a tricky problem to solve.
Weather Forecast
Warm air that has been trapped in the alpine should move out and let the alpine cool down to about -8 by Thursday morning. The freezing level should also drop down to valley bottoms overnight, and stay there throughout the forecast period. Moderate Northwest winds overnight should become light Northerly by Thursday morning. Light winds, cooling temperatures, and clearing skies are expected during the day Thursday. Mostly overcast with light Southerly winds on Friday. Flurries beginning Friday evening and becoming light snow by Saturday morning. Saturday should be overcast with flurries or light snow combined with moderate Southerly winds.
Avalanche Summary
Natural cornice fall resulted in a size 2.5 avalanche and did not trigger the persistent weak layer. Natural storm slab avalanche activity probably ended on Wednesday, with human triggering the storm slab or the persistent weak layer still very likely.
Snowpack Summary
After the storm we have been left with a new storm slab that is 40-60 cm thick and it is sitting above a mix of old surfaces including wind slabs, old storm slabs, and pockets of surface hoar. Warm air in the alpine and at treeline has settled the new storm slab into a cohesive slab that is easier to trigger and may allow for longer fracture propagations. Some areas have experienced strong Westerly winds that may have developed new windslabs by transporting the new snow. Below the storm slab there is a weak layer of surface hoar and crust that was buried in mid-December that continues to be a concern for human triggering. The mid-December weak layer may be buried up to 100 cm deep and continues to give easy or moderate sudden planar results in snow profile tests. Avalanches releasing on this layer may be large and destructive.
Problems
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.
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.