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Avalanche Forecast

Archived

Feb 3rd, 2019–Feb 4th, 2019

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

Regions

Purcells.

Human triggered avalanches remain likely, especially in wind loaded areas and at lower elevations where a buried surface hoar layer has produced avalanches recently.

Confidence

Moderate -

Weather Forecast

SUNDAY NIGHT - Mainly cloudy with clear periods / light northeast winds / alpine low temperature near -22MONDAY - Mainly cloudy with sunny periods / northeast winds 10-15 km/h / alpine high temperature near -17TUESDAY - Sunny with cloudy periods / northeast winds 15-25 km/h / alpine high temperature near -14WEDNESDAY - A mix of sun and cloud / northwest winds, 15-35 km/h / alpine high temperature near -14

Avalanche Summary

Several explosives triggered avalanches to size 2.5 were reported on Sunday.Widespread avalanche activity continued on Saturday with reports of numerous natural avalanches to size 3.5, as well as explosives triggered avalanches to size 2.5, and human triggered avalanches to size 1.5. Many of these avalanches failed on a persistent weak layer that was buried in mid January.A widespread natural avalanche cycle to size 3 was reported on Friday. There were also several human triggered avalanches to size 1.5 reported in the region on Friday. Some of these reportedly failed on the persistent weak layer that was buried in mid January. This MIN report illustrates how reactive the new snow was on Friday.There is a great MIN report from January 22nd that shows a natural avalanche (size 3.5) in International Basin, on the deep persistent layer. Check it out here. While this is an old observation, it is relevant as this layer is still lurking, and may catch people off guard.

Snowpack Summary

Approximately 30-60 cm of new snow has fallen in the region since Thursday. This new snow sits on wind slab, surface hoar (feathery crystals), facets (sugary snow) and a crust on sun-exposed slopes. A persistent weak layer that was buried in mid January is now buried approximately 50-90 cm. This layer consists primarily of surface hoar, however there is also a crust associated with it on sun-exposed slopes. This layer has been most reactive at treeline and below.The base of the snowpack has a deep persistent weak layer near the ground. This layer consists of facets over a crust. This weak interface continues to produce large and destructive avalanches that are sporadic in nature, and very difficult to predict. This layer is most likely to be triggered from areas where the snowpack is shallow and weak. Rocky alpine bowls, ridge crests and rocky outcroppings are some examples of the kind of terrain to be wary of.

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.

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.

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.