Avalanche Forecast

Archived

Apr 17th, 2021–Apr 18th, 2021

Alpine

Natural avalanches possible, human triggered probable.

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.

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.

New snow and northeast wind are expected to form wind slabs in lee features at upper elevations that will likely be reactive to triggering. Stay alert to changing conditions on Sunday.

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 west wind, alpine temperature near 0 C, freezing level 2900 m.

Sunday: Cloudy, 5-15 cm of snow, winds becoming northeast and increasing to moderate, alpine high temperature +1 C, freezing level dropping through the day to 1300 m.

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

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

Avalanche Summary

Natural wet loose avalanches, ranging from size 1-2.5, were reported over the past few days.

Snowpack Summary

An upslope storm is expected to bring enhanced snowfall to favored eastern slopes in the region throughout the day on Sunday. Amounts will generally range from 5-10 cm with pockets of up to 15 cm possible in favored areas. Precipitation will likely start as rain, further complicating the amount of snow forecast. Moderate northeast winds with strong ridgetop gusts will likely build a new reactive wind slab problem throughout the day. 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. 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

Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
Wind slabs are most reactive during their formation.
Extra caution is needed around cornices under the current conditions.

Problems

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.