Wind slabs sit above a weak mid-pack, creating the potential for small avalanches to step down to deeper layers.
Weather Forecast
SATURDAY: Flurries with accumulations of 5-10 cm, light southeast winds, alpine temperatures around -12C.SUNDAY: Dry and clearing, light northwest winds, alpine temperatures around -15C.MONDAY: Dry and clear, west winds increasing throughout the day reaching 50 km/h, alpine temperatures around -12C.
Avalanche Summary
On Thursday, two size 2 persistent slab avalanches were triggered with explosives in the southeast part of the region. The avalanches ran on weak faceted (sugary) grains between 50-100 cm deep. Earlier in the week, wind slabs were reactive to both explosive and human triggers (mostly size 2), while some stepped down produce larger persistent slab avalanches in the size 2.5-3 range. On Saturday, wind slabs are expected to be the primary concern resulting from new snow and strong to extreme southwest winds from earlier this week. There is a bit of uncertainty regarding how last week's snow will bond to old surfaces that formed during the cold snap. Due to these potentially persistent weak layers, wind slabs may remain sensitive to human triggering for some time. In the southeast corner of the region, there are concerns about a deep persistent problem. A weakness at the bottom of the snowpack has produced some large avalanches recently. Human triggering may be possible in thin areas or smaller avalanches have the potential to step down to this layer resulting in full depth avalanches.
Snowpack Summary
10-20 cm of new low density snow sits above 15-40 cm of settled storm snow. The storm snow has been redistributed into wind slabs in exposed terrain by strong to extreme winds. The new snow buries a variable surface that developed during the cold, dry, and windy conditions from earlier this month. This interface consists of scoured surfaces and wind slabs in wind exposed terrain, weak faceted (sugary) snow, and feathery surface hoar up to 20 mm in sheltered areas. In sheltered areas, there may be another layer of surface hoar in the upper snowpack which was buried around December 10. Observations have been limited, however, the reports we've received suggest the mid and lower snowpack are somewhat unconsolidated and faceting exists to varying degrees. Below treeline, the snowpack is very shallow and early season hazards such as stumps, rocks, and open creeks are still a major concern.
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.