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Avalanche Forecast

Archived

Apr 17th, 2021–Apr 18th, 2021

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

Regions

South Rockies.

An upslope storm is forecast to bring up to 30 cm of new snow to favored areas. Watch for storm slabs building throughout the day and dial back terrain choices where you find more than 20 cm of new snow.

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain.

Weather Forecast

An upslope storm will bring varying snow totals to the region, favoring the eastern slopes.

Saturday night: Mostly clear, light northwest wind, alpine temperature near 0 C, freezing level 2900 m.

Sunday: Cloudy, 10-25 cm of snow, winds becoming northeast and increasing to moderate, alpine high temperature +1 C, freezing level dropping through the day to 1000 m.

Monday: Mostly cloudy, light northerly winds, alpine high temperature -2 C, freezing level 1700 m.

Tuesday: Mainly sunny, light southerly winds, alpine high temperature -2 C, freezing level 2300 m. 

Avalanche Summary

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 is expected to bring moderate northeast winds and enhanced snowfall to the eastern slopes throughout the day on Sunday. Amounts will generally be near 10 cm with pockets of up to 30 cm in favored areas. A new storm slab problem will likely build throughout the day as snow accumulates. Precipitation will likely start as rain, further complicating the amount of snow forecast. Cornices are large and fragile. Cornice falls could trigger avalanches on the slopes below.

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

  • Watch for fresh storm slabs building throughout the day.
  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Watch for changing conditions today, storm slabs may become increasingly reactive.
  • Extra caution is needed around cornices under the current conditions.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.