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Avalanche Forecast

Archived

Apr 16th, 2021–Apr 17th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Inland.

No overnight freeze combined with 3000m freezing levels means less window for safe travel, even early in the morning. Steer clear of steep slopes when they are moist or wet and give cornices a wide birth. Check out the Forecaster's Blog for information on spring conditions.

Confidence

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

Weather Forecast

FRIDAY NIGHT: Clear / light, southwest ridgetop wind / alpine high temperature +3 / Freezing level 3000 m.

SATURDAY: Sunny / light, northwest ridgetop wind / alpine high temperature +8 / Freezing level 3000 m.

SUNDAY: Partly cloudy, clearing in the afternoon / strong east ridgetop wind / alpine high temperature 0 / freezing level 1700 m.

MONDAY: Sunny / moderate easing to light southeast wind / alpine high +7 / freezing level 2400 m.

Avalanche Summary

We don't have a lot of information coming in for this region, but there have been reports of 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. 

The lower snowpack has been strong and settled over the past month, however, the dramatic warming this week could potentially wake up weak facets at the bottom of the snowpack. This is particularly a concern on steep, rocky slopes with a shallow or thin to thick snowpack. Cornice failures or a rider hitting a thin spot in the snowpack are the most likely ways to trigger these weak layers.

Snowpack Summary

Lingering wind slabs may still be reactive in isolated locations on steep northerly aspects in the alpine that still have dry snow. All other terrain has been undergoing a melt-freeze cycle with successive days of warm sunny weather. Things will be "more melt, less freeze" on Saturday as we do not expect a good overnight recovery. 

Strong solar radiation and warming will trigger more wet loose avalanches on Saturday as the snow loses cohesion. Cornices are large and fragile. Cornice falls could trigger large avalanches on the slopes below.

The snowpack is overall strong and settled in most areas, however, sustained warming can increase the likelihood of large avalanches failing on deeply buried weak layers.This is most likely to occur on steep, rocky alpine slopes with a shallow or thin to thick snowpack.

Terrain and Travel

  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • Extra caution is needed around cornices under the current conditions.
  • As surface loses cohesion due to melting, loose wet avalanches become common in steeper terrain.
  • Back off slopes as the surface becomes moist or wet with rising temperatures.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.

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.