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Avalanche Forecast

Archived

Jan 4th, 2021–Jan 5th, 2021

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

Regions

Glacier.

Natural activity has tapered off, but skier triggering of recently developed slabs remains likely.

If wind and snowfall pick up earlier than forecast this afternoon, overhead hazard will once again become a concern.

Weather Forecast

A spike in wind, temps, and snowfall will accompany a front passing our area late this afternoon.

Today: Flurries (7 cm). Alpine High -3 C. Ridge wind light (increasing to strong late in the day) SW. Freezing level (FzL) 1600 m.

Tonight: Flurries (7 cm). Low -11 C. Moderate SW wind.

Tuesday: Mainly cloudy. High -9 C. Light S wind.

Weds: Snow (15cm)

Snowpack Summary

60cm of snowfall over the weekend was accompanied by strong to extreme winds from the SW. A surface hoar layer, buried Dec 26, may be preserved in sheltered areas at and below treeline, down 70+cm. The Dec 13 surface hoar can still be found down 120cm and the Dec 7 crust/facet/surface hoar (aspect/elevation dependent) layer is down 135+cm.

Avalanche Summary

There was widespread large natural avalanche cycle Saturday through early Sunday morning.

Artillery control Saturday night produce numerous avalanches up to size 4, notably MacDonald West shoulder and Crossover resulted in deposits on the highway.

Control just West of the park on Sunday morning also produced avalanches to size 4 from a North aspect.

Confidence

Due to the number of field observations

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.