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Avalanche Forecast

Archived

Jan 12th, 2019–Jan 13th, 2019

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 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.

Regions

Lizard-Flathead.

A big rise in freezing levels may mean that a deep persistent slab wakes up. A lot of uncertainty exists as to what this layer will do as temperatures climb. High levels of uncertainty are best managed by choosing conservative terrain.

Confidence

Low - Freezing levels are uncertain

Weather Forecast

A classic temperature inversion sets up Saturday night which will likely stick with us until Tuesday.  No significant precipitation is expected until possibly Friday.SATURDAY NIGHT: Temperature inversion with cold (below freezing) air in the valleys and above freezing temperatures between 1900 and 3000 m, light to moderate west/southwest wind, no significant precipitation expected.SUNDAY: A few clouds, temperature inversion with cold (below freezing) air in the valleys and above freezing temperatures between 1900 and 3000 m, light southwest wind at most elevations with moderate west/northwest wind at ridgetop, no significant precipitation expected.MONDAY: Clear skies, temperature inversion with cold (below freezing) air in the valleys and above freezing temperatures between 1500 and 3000 m, light variable wind, no significant precipitation expected.TUESDAY: A few clouds, freezing level at valley bottom, light variable wind, no significant precipitation expected. 

Avalanche Summary

On Friday natural avalanches size 1.5 to 2 were reported from northeast through northwest facing terrain at and above treeline.  Control work produced storm slab avalanches to size 2.5 on similar aspects.  Control work also caused cornices to fail on southeast facing terrain which subsequently produced storm slab avalanches to size 1.5.Numerous storm and wind slab avalanches were observed in the region on Thursday. Most of them were small (size 1 to 1.5), between 1600 m and 2300 m, and on all aspects. They were triggered naturally, by skiers, and explosives.For the weekend, the likelihood of triggering storm slabs may increase due to the substantial amount of warming expected around upper below treeline, treeline, and alpine elevations. The likelihood of triggering deeper weak layers may also increase too. Watch out in thin snowpack areas, where the likelihood of triggering deeper layers is the highest. See here for an example.

Snowpack Summary

Warm upper-level air temperatures may be impacting the snow surface. You may find dry snow, moist snow, or possibly a frozen melt-freeze crust, depending on how warm the air temperatures are. The warm air may produce touchy storm slabs at all elevations. At the highest of elevations, wind slabs may still linger in lee and cross-loaded terrain features.The middle and lower portions of the snowpack are generally well-settled and strong. However, there still remain a few deeper weaknesses in the snowpack around treeline and alpine elevations. Professionals are still tracking a layer around 150 to 200 cm deep, composed of sugary faceted grains, feathery surface hoar, and a sun crust. The base of the snowpack may also still be composed of weak faceted grains. The likelihood of triggering these layers may increase with the warm air incoming this weekend. These layers would most likely be triggered by humans in areas where the snowpack is shallow.

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.

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.