Destructive persistent weak layers should still be on your radar if you're heading to the north of the region.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Expect 5-15cm of new snow to fall between Friday night and Saturday. An additional 5cm may fall on Sunday, while isolated flurries and sunny breaks are forecast for Monday. Winds on Saturday will be strong from the southwest, and then drop to light and northwesterly for Sunday and Monday. Freezing levels should hover between 500-700m for the entire forecast period.
Avalanche Summary
On Thursday, persistent and storm slab avalanches to size 3.5 were observed on a variety of aspects and elevations. They occurred in response to heavy storm loading from rain and snow. The mid-January surface hoar was the suspected culprit in many of these avalanches. Of note was a size 4 slab avalanche in Steep Creek in the Duffey Lake area. Thanks to the individual who submitted this great observation through the Mountain Information Network. Moving forward, cooling will likely put a cap on persistent slab avalanche activity, except on higher elevation slopes in the north of the region where reactivity is expected to persist. Forecast wind and snow will spark a new round of wind slab activity in high elevation lee terrain over the weekend.
Snowpack Summary
On Wednesday night and Thursday morning, heavy snowfall and strong winds continued to form deep and dense storm slabs in high elevation terrain and cause substantial cornice growth. The snow line fluctuated a great deal during the storm which dropped around 80mm of precipitation throughout the region. At treeline and below, heavy rain saturated the snowpack. With forecast cooling, slopes at these elevation bands may now be sporting a hard crust. Where it still exists, the mid-January surface hoar layer is between 60 and 130 cm below the surface. The combination of heavy storm loading and warm temperatures has likely flushed out this weak layer in many areas. I would still exercise caution on higher elevation slopes in the north of the region where continued reactivity at this destructive interface is likely to persist. New snow and wind on Friday night and Saturday will form new wind slabs in high elevation lee terrain.
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.
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.