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Avalanche Forecast

Archived

Mar 22nd, 2021–Mar 23rd, 2021

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

Regions

South Columbia.

Carefully assess your line for wind slabs before committing. Steep, convex slopes below ridges are the most likely places to trigger these slabs.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

MONDAY NIGHT: Patchy clouds. Moderate northwest wind, alpine low -12C, and freezing level valley bottom.

TUESDAY: Patchy clouds, sunny breaks, and isolated flurries. Light to moderate west wind, alpine high -1C, and rising to 1500 m.

WEDNESDAY: Snow, 5-15 cm. Moderate southwest wind, alpine high 0C, and rising above 1500 m.

THURSDAY: Flurries, 5 cm. Light west wind, alpine high -1C, and freezing level beginning about 1600 m.

Avalanche Summary

On Monday, explosives triggered several size 1-1.5 avalanches on NW-N-NE aspects above 1800 m. At upper elevations, skies triggered small dry-loose avalanches in steep terrain. Below tree line, loose-wet and wet-slab avalanches gained reactivity to skiers in the afternoon. And several natural storm slab avalanches size 2-2.5 were reported in Glacier NP, occurring on north-northwesterly aspects above 1950m.

On Sunday, explosives and skier traffic triggered numerous size 1 avalanches, new and touchy cornices were observed. A few small dry loose avalanches and sluffing in steep terrain was also reported.

On Saturday, explosives triggered small (size 1-1.5) storm slabs from north-northeast aspects above 2000 m. A skier also triggered a small (size 1) wind slab on a steep convex roll on an easterly aspect at treeline. Sluffing was reported in steep terrain. In Glacier NP, a handful of storm slabs size 1.5-2.5 released naturally from steep north-northwest aspects above 2000m.

On Friday, several glide slab avalanches (size 2-3) were observed on south and east aspects around the TCH highway corridor. Loose wet avalanches up to size 2.5 were reported around the region; a natural loose-wet cycle was reported around Rogers Pass initiating with evening rainfall.

Snowpack Summary

Southwesterly winds have developed slabs in lee features. 20-35 cm storm snow (up to 60 cm in areas) covers a handful of surfaces: dry settled snow and surface hoar (up to 10 mm) on northerly aspects above 1800m, and crusts on solar aspects and lower elevations. Sun and warm temperatures have encouraged settlement. Large cornices loom over alpine ridgetops. Below 1500 m, the snowpack turns moist.

Persistent weak layers of surface hoar, crusts, and/or facets 80-120 cm down have recently been unreactive and no recent avalanches have been reported on these layers. 

Terrain and Travel

  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Carefully evaluate steep lines for wind slabs.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.