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Avalanche Forecast

Archived

Mar 24th, 2017–Mar 25th, 2017

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

Regions

North Columbia.

Tricky conditions are expected to persist at higher elevations with new snow and wind building slabs as well as the continuing possibility of avalanches releasing on deep weak layers. Conservative terrain selection remains highly recommended.

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Saturday

Weather Forecast

5-15 cm of snowfall is expected Friday overnight with moderate to strong southwest wind in the alpine and freezing levels around 1000 m. Another 3-6 cm is expected on Saturday with the possibility of sunny breaks in the afternoon. Alpine wind is forecast to be moderate from the southwest and freezing levels are expected to reach around 1500 m. On Sunday, a mix of sun and cloud is expected with light alpine wind and freezing levels reaching around 1700 m. Mainly cloudy conditions are forecast for Monday with light snow flurries.

Avalanche Summary

On Thursday, a natural cornice release occurred on a north aspect at 2100 m. A few natural wind slabs were reported as well as a few solar triggered loose avalanches. Skiers were triggering wind slabs and storm slabs up to size 1.5, mainly on northwest through east aspects in the alpine. Most of these slabs were 10-30 cm thick but one was 50-60 cm thick and release on the mid-March crust. Explosives triggered a few cornices and slabs 25-50 cm thick. One of the cornices triggered a deep slab which slid on the November crust. On Wednesday, a natural size 2 slab was observed on a south aspect at 1800 m which was 100 cm thick. Natural sluffing up to size 2 was reported as well as two cornice failures. One of these triggered a size 3 slab which released down 200 cm. On Saturday, storm slabs are expected to be reactive at higher elevations, especially in wind loaded terrain and on steep convexities. Cornices are large and may become weak with daytime warming or during stormy periods. We are in a low probability, high consequence scenario for persistent slab avalanches failing on buried weak layers. Click here for more details. Click here for photos of the recent large avalanche cycle.

Snowpack Summary

15-30 cm of new snow overlies a rain crust below around 2000 m or a sun crust on solar aspects at higher elevations. Alpine wind has recently been strong mainly from south through west directions and has loaded leeward slopes in exposed terrain at treeline and in the alpine. Large cornices are also reported on northerly aspects in the alpine. The rain crust which formed last week is now down 50-60 cm and generally seems to be well bonded to the surrounding snow. The February weak layers are down 120-150 cm and woke up during the recent storm cycle with many avalanches stepping down. The deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack and a few avalanches and cornice falls have stepped down to these layers recently resulting in very large avalanches.

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.

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.

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.