Another day of yellow! Though avalanche occurrences are becoming less frequent, the possibility of triggering persistent slabs still exists, and the consequences are high. A cautious approach to backcountry travel is recommended.
Weather Forecast
THURSDAY NIGHT - Cloudy with clear periods and isolated flurries / southwest winds 15-30 km/h / alpine low temperature near -14FRIDAY - Cloudy with scattered flurries, up to 10 cm / southwest winds 20-30 km/h / alpine high temperature near -12SATURDAY - Mainly cloudy with isolated flurries / northeast winds 10-15 km/h / alpine high temperature near -12SUNDAY - A mix of sun and cloud / northeast winds, 10-15 km/h / alpine high temperature near -14
Avalanche Summary
On Wednesday, there were numerous reports of human triggered avalanches up to size 2, primarily on east and southeast aspects at all elevations. Four of these were persistent slab avalanches that were triggered remotely (from a distance) by people. They were on east aspects below treeline and failed on a persistent weak layer that was buried mid January. This layer is discussed in more detail in the Snowpack Summary below.Over the past week there have been daily reports of avalanches being triggered by humans, explosives, and naturally. Many of these are wind slabs in the size 1-2 range, but some of them are also persistent slab avalanches in the size 2-3 range. Persistent slab avalanche activity on the mid January weak layer has slowed down to some degree but it has not stopped, and they continue to be triggered by humans on a somewhat regular basis. Avalanches on this layer are becoming fewer and further between, but they tend to be large, potentially high consequence events.
Snowpack Summary
Up to 10 cm of new snow is expected to fall on Friday with moderate southwest winds, which will likely form wind slabs in lee terrain. This new snow will fall on older wind slabs which may exist on all aspects due to previous variable wind directions. These older wind slabs will become more difficult to detect as they get buried.The new snow will bring recent snow amounts to 20-60 cm which overlies a weak layer of surface hoar (feathery crystals) and crust that was buried in early February. As this layer receives more snow on top of it, it may develop into a problem layer.Two additional weak layers of surface hoar have produced large avalanches in the region over the past month. A layer buried at the end of January is around 40 cm deep and a layer buried mid-January is between 50 and 90 cm deep. The mid-January layer may also be associated with a crust on southerly aspects. These layers are most prominent at treeline and below. The remainder of the snowpack is considered generally strong. However, there have been sporadic reports of very large avalanches that have released near the base of the snowpack. Most of these avalanches have been in the high alpine.
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.