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Avalanche Forecast

Archived

Dec 26th, 2019–Dec 27th, 2019

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

Regions

South Coast Inland.

Light new snow amounts and increasing winds are expected to form new wind slabs at higher elevations. In the north of the region, this adds a layer of complexity to a serious persistent slab problem. Danger in the south of the region is MODERATE and limited to wind slab concerns.

Confidence

High -

Weather Forecast

Thursday night: Cloudy with scattered flurries bringing up to 5 cm of new snow by morning. Moderate to strong southwest winds.

Friday: Mainly cloudy with lingering isolated flurries and a trace of new snow. Light to moderate west or northwest winds. Alpine high temperatures around -7.

Saturday: Cloudy with scattered flurries developing later in the day and bringing up to 5 cm of new snow by morning. Light to moderate south or southwest winds. Alpine high temperatures around -6 in the north of the region, closer to -3 in the south. 

Sunday: A mix of sun and cloud. Light south winds. Alpine high temperatures around -4.

Avalanche Summary

Avalanche observations have been on a declining trend since the widespread avalanche cycle observed across the region at the end of last week. During that cycle, avalanches were reported to be running to valley-bottom in the north of the region, failing on the weak layer described in the snowpack summary below.

The possibility for large human-triggered persistent slab avalanches remains a very serious concern at higher elevations in the northern half of the region (e.g., Duffey, Hurley, etc.). See this MIN report of an avalanche involvement on Monday for an example of the issue at hand.

Given our current snowpack structure, we recommend very conservative route-finding and decision making in higher elevation avalanche terrain in the north of the region.

Snowpack Summary

Surface conditions over the region have likely evolved into a mix between newly wind-affected surfaces at exposed higher elevations and large new surface hoar in more sheltered areas. 

Below the surface, the storm at the end of last week brought over 120 cm of snow to the south of the region and about 40 cm to the north of the region. This snow is gaining strength as the days pass but snowpack conditions found beneath this layer are variable over the region and are in some areas highly problematic.

In the north of the region, the above-mentioned storm snow brought a critical load to a now 30-70 cm-deep weak layer from late November composed of sugary faceted grains and hard melt-freeze crust. This structure is a recipe for large and destructive avalanches and a problem that may persist for weeks to months. This problematic layer is largely absent in the south of the region.

Terrain and Travel

  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Persistent slabs have potential to pull back to lower angle terrain.
  • Avoid steep convexities or areas with a thin, rocky, or variable snowpack.
  • Carefully evaluate steep lines for wind slabs.

Problems

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.

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.