A warm, wet and windy storm will bring the Avalanche Danger to High
Weather Forecast
Up to 30mm of precipitation is forecast for Wednesday night with another 20cm of snow expected on Thursday. Ridgetop winds will remain extreme from the southwest while freezing levels will drop throughout the day from 2000m to about 1500m. On Friday and Saturday the region will see generally light flurries, significantly decreased winds and freezing levels around 1000m.
Avalanche Summary
In recent days, backcountry avalanche observations have been extremely limited, mostly due to stormy weather. Looking forward, continued warm, wet and windy weather is expected to spark ongoing storm slab activity in high elevation lee terrain, but warm temperatures and loading will also increase the likelihood of waking up any lingering persistent weaknesses that were not triggered by last week's storm.
Snowpack Summary
On Wednesday night and Thursday morning, moderate to heavy snow and strong winds will continue to form deep and dense storm slabs in high elevation terrain and cause substantial cornice growth. At treeline and below, rain is forecast to further saturate the snowpack. In many areas, the early and mid-January surface hoar layers have been wiped-out by last week's storm and subsequent avalanche cycle; however, "sudden" test results and isolated persistent slab avalanches prove these interfaces should remain on your radar. These layers would be down between 80 and 150 cm deep, and still have the capacity to surprise with nasty consequences. The mid and lower snowpack is generally strong, with the exception of shallower snowpack areas that may be more faceted.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.