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Avalanche Forecast

Archived

Apr 13th, 2021–Apr 14th, 2021

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 possible, human triggered probable.

Regions

Northwest Inland.

Danger will elevate throughout the day as slope warm up. Plan on finishing early and minimizing your exposure to avalanche terrain once it heats up. Read more in this Forecasters' Blog.

Confidence

Moderate - Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

A high pressure ridge brings clear skies and progressively warmer temperatures.

TUESDAY NIGHT: Clearing skies, light west wind, freezing level climbs from 1700 to 2000 m by the morning with treeline temperatures climbing to +2 C.

WEDNESDAY: Mostly sunny, light south wind, freezing level climbs from 2000 to 2700 m throughout the day, treeline temperatures climb to +5 C.

THURSDAY: Sunny, light south wind, freezing level steady around 2800 m, treeline temperatures around +8 C.

FRIDAY: Mostly sunny, light south wind, freezing level steady around 3000 m, treeline temperatures around +8 C.

Avalanche Summary

Warm sunny weather this week will cause wet loose avalanches and cornice falls. Natural slab avalanches will become an increasing concern after several days of warm weather.

On Sunday and Monday there were many small (size 1) wet loose avalanches on south-facing slopes at all elevations and a few size 2-2.5 cornice failures on north and east facing slopes. On Saturday there were a few naturally triggered wind slab avalanches (up to size 2) on northeasterly aspects in the alpine.

Snowpack Summary

Sunny skies and rising freezing levels will result in melting surfaces everywhere except north-facing alpine terrain. In the alpine 5-20 cm of snow is quickly settling and strengthening, although some wind slabs could linger on steep slopes. The recent snow has covered a variety of snow surfaces, including wind affected snow, crusts on solar aspects and at lower elevations, and soft snow on sheltered slopes at upper elevations.

The lower snowpack has been strong and settled over the past month, however the dramatic warming this week could potentially wake up weak basal facets. This is particularly a concern on steep, rocky slopes with a shallow or thin to thick snowpack.

Cornices are large, looming, and capable of triggering large avalanches when they fail.

Terrain and Travel

  • Avoid exposure to sunny slopes, especially in the afternoon.
  • Cornices become weak with daytime heating, travel early on exposed slopes.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.
  • Watch for areas of hard wind slab on alpine features.

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.

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.