This Will be a Week of Dubyas: Windy, Wet and Warm! Expect increasing hazard and widespread natural avalanche activity by Tuesday.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Monday: Periods of rain or snow – 20-30mm or cm. The freezing level is around 1800 m and winds are 30-60 km/h from the S-SW. Tuesday: Rain, heavy at times. The freezing level could climb as high as 2500 m and winds could be gusting close to 100 km/h from the south. Wednesday: Continued heavy rain or snow. The freezing level remains above 2000 m and southerly winds should keep cranking.
Avalanche Summary
There have been no recent reports of avalanches in the past several days; however, I suspect it may be possible to trigger new wind slabs in open leeward terrain, particularly in the alpine.
Snowpack Summary
Recent snowfall amounts vary but in some areas above 1800 m there could be 20-40 cm of moist storm snow, which now covers the previous snow surface of surface hoar or facetted snow in sheltered areas, and pockets of old wind slab or a crust (from last weeks rain) in open wind-exposed terrain. Fresh new wind slabs are likely below ridges and terrain features on exposed north and east facing slopes. This new snow may also be covering old dense or hard wind slabs from the outflow winds we experienced during the recent dry spell. A solid rain crust is buried 20-50 cm deep up to around 2000 m. Another crust that was buried in mid-November is down 40-60 cm. The deeper crust may be associated with a layer of facets above or below. The snow pack depth drops significantly below treeline with essentially no snow below 1600 m.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.