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Avalanche Forecast

Archived

Feb 14th, 2018–Feb 15th, 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 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 and human triggered avalanches likely.

Regions

South Rockies.

Wind slabs, cornices and deeply buried weak layers are making for tricky conditions.  The best approach is to choose conservative terrain, minimize your exposure to overhead hazards and avoid wind-loaded or cross-loaded slopes.

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

TONIGHT: Cloudy, flurries. Accumulation trace. Ridge wind light to moderate northeast. Temperature -20. Freezing level valley bottom.THURSDAY: Mix of sun and cloud. Ridge wind strong west. Temperature -15. Freezing level valley bottom.FRIDAY: Mix of sun and cloud, light flurries. Accumulation trace. Ridge wind moderate, northwest. Temperature -12. Freezing level valley bottom.SATURDAY: Snow. Accumulation 10-15 cm. Ridge wind moderate to strong, northeast. Temperature -20. Freezing level valley bottom.

Avalanche Summary

On 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 Thursday afternoon and overnight into Friday. Several storm slabs and deep persistent slabs to size 4 were reported when visibility improved on Friday. See this MIN post for more information. These avalanches are failing on weak layers deep in the snowpack and running to valley bottoms. See this video for more details.

Snowpack Summary

About 10-15 cm on new snow and strong to extreme winds have resulted in extensive scouring on windward slopes (generally north and west facing slopes) and the creation of wind slabs on down wind (lee) features higher up, on a wide range of aspects. 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 is 100-150 cm deep. Second, a rain crust with sugary facets buried in late-November is 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 while avoiding all 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.

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.

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.