Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain on Friday
Weather Forecast
Overnight Wednesday: Snow amounts 10-15cms. Moderate ridgetop winds from the SW. Thursday: A ridge of high pressure will likely build. This will bring dryer conditions, low cloud throughout the west Columbias, with possible sunny breaks to the east. Light snow amounts near 5cms. Ridgetop winds from the SW 70-90km/hr. Freezing levels around 1000m in the am, then rising to 1500m. Friday: Expect another frontal system spreading up to 20cms. Winds will be moderate from the South. Saturday: Mainly dry conditions.
Avalanche Summary
Natural activity up to size 2.5 has ramped up in the South Columbias. There are also continued reports of rider triggered avalanches up to size 2, and natural cornice failures triggering slopes below up to size 2.5. The focus is on elevations between 1300-2100m, and mainly on North-South aspects. These new avalanches suggest that the tipping point for activity on the mid-December surface hoar is happening, and the problem will likely get worse with more forecast snow, wind and rising freezing levels. Check out the Forecasters Blog to see an updated avi picture. It clearly indicates how touchy the surface hoar layer is on convexities and low angle terrain.
Snowpack Summary
The region has received storm snow amounts up to 40cms since December 26th. Wind slabs continue to form on North-South East aspects in the alpine and exposed treeline locations. New storm snow blankets all elevations except in valley bottom which sees more rain then the fluffy white stuff. In the upper meter of the snowpack two different weak layers exist. From the top down approx. 20-35cms lies the December 24th surface hoar layer. Field tests show easy shears on this layer and Rutchblock 3 (Whole Block) scores. This layer can be found in sheltered areas at treeline and below. Going down 70-90cms is the mid-December surface hoar layer. This layer has naturally reached its threshold in the southern part of the region. In the Northern locations this layer is very sensitive and just waiting for a trigger (new loading from snow, wind, rain, and/or human trigger). This surface hoar layer is still producing sudden planar results in stability tests. These are the layers to watch. Below this sits a well consolidated, strong midpack. Check out the forecaster blog for more info/ideas, and a photo showing our current problem.
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.
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.