Watch out for new wind slabs as well as thick, old wind slabs. They have been reactive to natural and human activity. Don't be complacent with the weak layer lurking in our snowpack, as it has produced large avalanches recently.
Weather Forecast
TUESDAY: Cloudy with light snowfall, accumulation 2 to 5 cm, light to moderate west winds, alpine temperature -7 C, freezing level 1200 m.WEDNESDAY: A mix of sun and cloud, light southeast winds, alpine temperature -6 C, freezing level 1300 m.THURSDAY: Morning sun and afternoon clouds, light southwest winds, alpine temperature -5 C, freezing level 1400 m.
Avalanche Summary
On Sunday, a large (size 3) persistent slab avalanche was remotely triggered on a north aspect in alpine terrain after substantial air temperature and solar warming over the day. The avalanche likely released on the weak layer described in the Snowpack Summary.Otherwise, loose wet avalanches were easy to initiate in the afternoon with solar warming. Evidence of large slab avalanches was observed, likely occurring early in the weekend.
Snowpack Summary
10 cm of new snow has been redistributed into lee terrain features from strong north to northeast winds. This overlies variable surfaces, including wind slabs at high elevations, a sun crust on south aspects, and a temperature crust on all aspects up to 1900 m. Previous wind slabs are up to 1 m deep from strong south winds. Ridgelines are heavily corniced and if they fail they could trigger slabs on slopes below.A weak layer consisting of surface hoar, facets, and/or a melt-freeze crust from late March is now buried about 60 to 140 cm. This layer is spotty in its distribution and has recently produced large and destructive avalanches. It is mostly likely to be problematic on west, north, and east aspects between 1900 m and 2250 m. Professionals are treating it seriously and avoiding terrain because of it.Below this, the snowpack is well-settled and strong.
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.