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Avalanche Forecast

Archived

Apr 20th, 2021–Apr 21st, 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 possible, human triggered probable.
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.

Regions

South Rockies.

One more day of warm sunny weather before the next snowfall arrives. Avoid sun-exposed slopes and cornices at peak midday warming on Wednesday. 

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Tuesday night: Clear, light winds becoming northwest, alpine temperature -4 C, freezing level dropping to valley bottom. 

Wednesday: Sunny, light west winds, alpine high temperature -1 C, freezing level 2600 m. 

Thursday: Cloudy, 10-20 cm of snow, light northeast wind with moderate ridgetop gusts, alpine high temperature -1 C and dropping through the day, freezing level dropping to valley bottom.

Friday: Cloudy, light southerly wind, alpine high temperature -3 C, freezing level rising to 1800 m. 

Avalanche Summary

Recent observations from this region are limited. 

On Saturday, there were reports of numerous wet loose avalanches releasing naturally on steep east-facing alpine slopes early in the day. Observers also reported two natural wet slab avalanches that occurred on northeast aspects above 2000 m.

Snowpack Summary

An upslope storm on Sunday brought 5-10 cm of new snow to the region, with 20+ 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 is forecast to stick around for Wednesday, which will likely initiate wet loose avalanches in the new snow as it 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.

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

Terrain and Travel

  • 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.
  • Extra caution is needed around cornices under the current conditions.

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.

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.

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.