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Avalanche Forecast

Archived

Apr 19th, 2021–Apr 20th, 2021

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

Regions

Lizard-Flathead.

Anticipate a return to spring-time avalanche concerns with sunny weather and rising freezing levels. Stay alert to steep, sun-exposed slopes and cornices during peak daytime warming.

Confidence

Moderate - Uncertainty is due to limitations in the field data.

Weather Forecast

Monday night: Partly cloudy, light northeast winds, alpine temperature -6 C, freezing level dropping to valley bottom.

Tuesday: Sunny, light variable winds, alpine high temperature -2 C, freezing level 2200 m. 

Wednesday: Sunny, light southwest winds, alpine high temperature -2 C, freezing level 2600 m. 

Thursday: Cloudy, 10-20 cm of snow, moderate northeast wind, alpine high temperature -6 C, freezing level dropping throughout the day to valley bottom.

Avalanche Summary

On Friday and Saturday during peak warming and sunny weather, there were reports of natural wet loose avalanches ranging from size 1-2.5. 

Snowpack Summary

An upslope storm brought a trace to 5 cm of new snow to the region, with up to 15 cm accumulating in favored areas. Moderate northeast winds with strong ridgetop gusts have formed fresh wind slabs in exposed areas at upper elevations that may still be reactive to triggers. 

Sunny weather and rising freezing levels returns, which may initiate wet loose avalanches as the snow loses cohesion. Cornices are large and looming along ridgelines. Sun and warm temperatures will increase the chances of cornice failures, which could trigger avalanches on the slopes below. Check out the Forecaster's Blog for information on how to manage these spring conditions. 

The snowpack is overall strong and settled in most areas. However, steep and rocky alpine slopes with a shallow or thin to thick snowpack may still harbor deeply buried weak layers. Glide cracks releasing as full depth glide slab avalanches become more common in the spring and are extremely difficult to predict. Best practice is to avoid slopes with glide cracks.

The snow line is slowly creeping up the mountains, making some access areas snow-free. 

Terrain and Travel

  • Extra caution is needed around cornices under the current conditions.
  • Avoid steep slopes when air temperatures are warm, or solar radiation is strong.
  • As surface loses cohesion due to melting, loose wet avalanches become common in steeper terrain.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.

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.

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.