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Avalanche Forecast

Archived

Dec 27th, 2011–Dec 28th, 2011

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

South Columbia.

Confidence

Fair - Timing of incoming weather is uncertain

Weather Forecast

Overnight Tuesday: Strong SW ridgetop winds. Snow amounts 15-25cms. Freezing levels could rise to 1000m. Wednesday: Ridgetop winds 60-70km/hr from the SW. Snow amounts 5-15cms. Freezing levels could rise to 1600m. Thursday: Ridgetop winds 55-90km/hr from the W. Snow amounts 5-10cms. Freezing levels dropping back down to valley bottom. Friday: Snow amounts 10-20cms. Freezing levels at valley bottom starting to rise in the evening.Check out the new Forecasters Blog Post to learn more about weather patterns, and timing in regards to how it affects our avalanche bulletins.

Avalanche Summary

Natural activity up to size 2 has started to ramp 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. The focus is on elevations between 1500-2100m, 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

Up to 35cm of new snow has fallen over the past three days forming wind slabs and soft slabs in exposed open areas at treeline and above. The new snow also overlies soft slabs on North through East aspects and some spotty, small buried surface hoar/surface facets down 10-20cms in sheltered locations at treeline and below. Field tests show easy shears on this layer. The new snow is creating an additional load to a buried surface hoar layer that is found down 60-80cms in the snowpack. This layer we are referring to as the mid-December surface hoar layer. This layer has naturally reached its threshold in the southern part of the region. In the northern part it sits waiting for a trigger. This surface hoar layer is still producing sudden planar results in stability tests and a whole block rutchblock 4 was also observed. This is certainly the layer to watch; especially if it reaches its threshold by additional snow loading or rider triggers. Check out the forecaster blog for more info/ideas on this current problem. The midpack is well consolidated and strong.

Problems

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.

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.