Lingering wind slabs may be difficult to detect under a dusting of new snow. A persistent slab problem still exists at lower elevations and has been responsible for sporadic human-triggered avalanches.
Weather Forecast
Convective snow showers on Thursday could mean that some areas receive higher than forecast snowfall amounts.WEDNESDAY NIGHT - Mainly cloudy with flurries, 2 cm / southeast winds 10-30 km/h / alpine low temperature near -10THURSDAY - Mainly cloudy with flurries, 3-5 cm. / southwest winds 15-30 km/h / alpine high temperature near -9FRIDAY - Mainly cloudy with sunny periods and isolated flurries / southwest winds 10-30 km/h / alpine high temperature near -12SATURDAY - A mix of sun and cloud / northwest winds, 10-20 km/h / alpine high temperature near -10
Avalanche Summary
On Tuesday, there were two reports of human triggered persistent slab avalanches that failed on the early February persistent weak layer (described in more detail in the Snowpack Summary). These were size 1 and 1.5, on west aspects.In recent days, there have been reports of size 1-1.5 wind slab avalanches on a variety of aspects in the alpine and at treeline, as well as size 1-1.5 loose snow avalanches, primarily on sun exposed slopes.Reports of persistent slab avalanches are becoming less frequent, suggesting our January/February weak layers have evolved into a lower likelihood/high consequence avalanche problem that still requires a measure of discipline to manage effectively.
Snowpack Summary
Around 5 cm of snow is expected to fall on Thursday. Some convective snow showers could mean that some areas receive closer to 10 cm. The new snow will sit on wind slabs and facets (sugary snow) in most areas, and a crust on sun exposed slopes. The wind slabs may continue to be reactive as they are also sitting on facets. There are up to three layers of surface hoar (feathery crystals) that were buried in mid and late January, and early February. These layers are around 30 to 80 cm deep and are most prominent at lower elevations - especially below treeline. The surface hoar may sit on a crust on south facing slopes.The base of the snowpack is composed of weak and sugary faceted grains that sit on a crust. This weak layer has produced large and destructive avalanches that are sporadic in nature and difficult to predict. See the Forecasters' Blog here for more information on this problem. Continued cold temperatures have been weakening the lower snowpack.
Problems
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.
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.