Regions
Northwest Inland.
Give recently wind-loaded slopes a wide berth.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Expect around 5 cm snow on Saturday with moderate to strong SW winds and the freezing level near 800 m. A second system is forecast to affect the area on Sunday, petering out slowly on Monday. A further 5-10cm snow is expected, with strong S-SW winds and freezing level around 600 m.
Avalanche Summary
Skiers triggered size 1 slabs on wind-loaded features at treeline on Friday. Further west there have been reports of large natural avalanches on N and NW facing slopes around 1800m within the last couple of days.
Snowpack Summary
Strong southerly winds are building wind slabs, generally depositing snow on N and NE aspects. A rain crust is still out there up to 1600m and a melt freeze crust to 1850m. Keep in mind a surface hoar layer reported to be 15 to 20cm below the surface in areas protected from the wind. We're watching the December crust, but we haven't seen any activity on it recently, most likely because it hasn't received a significant load on it. Near the base of the snowpack is a November crust-facet combination that could remain problematic for some time.
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.