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Avalanche Forecast

Archived

Feb 16th, 2018–Feb 17th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
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 unlikely, human triggered possible.

Regions

South Rockies.

Periods of intense loading from snow and wind are expected to begin late Saturday afternoon into the evening. If the storm arrives earlier than forecasted, avalanche danger will increase sooner.

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Saturday

Weather Forecast

Snowfall is expected to begin Saturday afternoon, increase during the evening then ease overnight.TONIGHT: Light flurries. Accumulation up to 5cm. Ridge wind light, variable. Temperature -14. Freezing level valley bottom.SATURDAY: Snow. Accumulation 20-30 cm. Ridge wind moderate to strong, northeast. Temperature -14. Freezing level valley bottom.SUNDAY: Mainly cloudy, light flurries. Accumulation trace. Ridge wind moderate to strong, northeast. Temperature -22. Freezing level valley bottom.MONDAY: Mix of sun and cloud. Ridge wind light, northwest. Temperature -20. Freezing level valley bottom.

Avalanche Summary

Friday, ski cuts and explosive control produced loose storm snow avalanches up to size 1.On Thursday both natural and skier triggered wind slabs up to size 1.5 were reported as well as loose, storm snow releases from steep rocky terrain.Tuesday we received reports of size 3 avalanches that were likely triggered by cornice fall over the weekend and ran full path to valley bottom. See here for the MIN report. A widespread natural avalanche cycle occurred late last week with several storm slabs and deep persistent slabs to size 4 reported. See this MIN post for more information. These avalanches were failing on weak layers deep in the snowpack and running to valley bottoms. See this video for more details.

Snowpack Summary

About 20-40 cm on new snow and winds have led to the creation of wind slabs on leeward features at upper elevations on a wide range of aspects. In sheltered areas near treeline and below, these slabs overly a layer of surface hoar buried mid-February that has produced very easy, sudden results in recent snowpack tests.The lower snowpack in this region is weak with two main concerns; a widespread weak layer from mid-December composed of facets, crusts, and surface hoar that is 100-150 cm deep, and second, a rain crust with sugary facets buried in late-November near the bottom of the snowpack. Although the snowpack structure is variable across the region, these persistent weak layers are generally widespread.The take home message is that weak layers are still active and deserve a lot of respect. The solution is to stick to conservative terrain and avoid overhead hazard.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.