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Avalanche Forecast

Archived

Feb 13th, 2018–Feb 14th, 2018

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

Wind slabs, cornices and persistent slabs make for a tricky situation out there. Ride in conservative terrain and avoid all overhead hazard to manage your risk.

Confidence

Moderate - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Wednesday will see an upslope event bringing modest amounts of low density snow, with higher amounts on eastern slopes. TUESDAY NIGHT: Flurries (5-10cm of low density snow). Winds moderate gusting strong from the north west. WEDNESDAY: Flurries (5 -15cm accumulation). Ridge wind moderate gusting strong from the north / west. Temperature -7. Freezing level valley bottom.THURSDAY: Sunny with cloudy periods. Ridge wind gusting moderate from the west. Temperature -9. Freezing level valley bottom.FRIDAY: Isolated flurries. Ridge wind becoming strong from the southwest. Temperature -5. Freezing level 1000m in the afternoon.

Avalanche Summary

On Tuesday we received reports of size 3 avalanches, likely from the weekend and likely triggered by cornice fall. 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

Wind has been the main weather story over the past few days, with gusts on Tuesday over 130 Km/ hr!The end result is 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.