New snow and extreme winds will keep the avalanche danger elevated at higher elevations on Friday. The avalanche danger is expected to rise throughout the day.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Thursday
Weather Forecast
Friday: 10-20cm of new snow / Strong to extreme westerly winds / Freezing level at 1400mSaturday: Light flurries / Light and variable winds / Freezing level at 1200mSunday: 5-10cm of new snow / Light southwest winds / Freezing level at 2300mNote: Confidence is low for forecast wind values and precipitation amounts for Saturday and Sunday.
Avalanche Summary
Explosives control on Tuesday and Wednesday triggered lingering storm slabs at treeline and above - mostly in the size 1.5-2 range. The exceptions were a size 3 and a size 3.5 slab that were explosives-triggered on rocky, cross-loaded alpine slopes. The exact failure plane of these avalanches is unknown.Looking forward, continued snow and wind on Friday morning is expected to promote a new round of wind slab activity in higher elevation lee terrain.
Snowpack Summary
As of Thursday morning 10-15 cm of new snow had fallen and was redistributed by strong southerly winds into fresh wind slabs in exposed lee terrain. At lower treeline and below, generally light amounts of snow now overlie a melt-freeze crust which formed during last Monday's warm storm. The new snow adds to the 170cm of snow that fell during the last few weeks of January. These series of storms were accompanied by strong to extreme winds which resulted in extensive scouring and impressive cornice growth in upper elevation terrain. These accumulations sit on a crust from mid-January which generally shows signs of bonding to the overlying snow; however, it has the potential to "wake-up" with a large trigger such as a cornice fall.Below this, the snowpack is thought to be generally strong and well-settled.
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.