If the sun comes out in the afternoon, loose wet sluffing, cornice failures, and solar triggered slab avalanches are all possible. A tricky weak layer is expected to be most reactive on high elevation north aspect slopes.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Unsettled conditions are expected for Friday as a weak frontal system reaches the region from the north coast. Mostly cloudy conditions and light precipitation are expected while the front passes sometime in the morning or early afternoon. A mix of sun and cloud is expected in the afternoon after the front passes. Alpine winds are forecast to be moderate-strong from the SW to NW directions and freezing levels are expected to reach 2000m or higher. On Saturday, mostly sunny conditions are expected with light NW winds in the alpine. Freezing levels are expected to be below 1500m in the morning and around 2000m in the afternoon. A mix of sun and cloud is expected for Sunday with light alpine winds and similar freezing levels.
Avalanche Summary
On Wednesday, loose wet avalanches to size 1.5 were reported below 2100m. Widespread wind slabs were reported in the alpine and treeline. On Tuesday, natural storm slabs up to size 2.5 were reported. These slabs had depths from 20-80cm. Ski cuts triggered a couple size 2 wind slabs with 20cm thickness. A remotely triggered avalanche was triggered from 10m away. Remote triggering suggests the weak layer below the storm snow has been very reactive in some areas. On Monday, a natural cornice release triggered a size 1.5 slab avalanche on the slope below. Solar triggered slab avalanches, cornice releases, and loose wet sluffing are all expected to have occurred on Thursday with the big warm-up. On Friday, the same types of solar related activity are expected if the sun comes out in the afternoon. If a supportive crust forms Thursday night, stability will begin to improve. Human-triggering remains a major concern at higher elevations, especially north facing terrain where the storm slab has been unaffected by the sun.
Snowpack Summary
Warm temperatures and strong solar are rapidly changing the upper snowpack. On Thursday, a moist snow surface was reported to 2300m elevation on north aspects and is expected to ridgetop on solar aspects. Overnight refreeze on Thursday night is expected to form a widespread surface crust which is expected to be supportive and capping at lower elevations. Down 30-40cm is a weak layer that was buried last Friday and has been reactive recently. This weak layer typically consists of surface hoar and facets overlying a melt-freeze crust that exists everywhere except high elevation north-facing terrain. In exposed alpine terrain, recent strong SW winds had formed wind slabs in leeward features. Large cornices exist in the alpine and may become weak with daytime warming. There are three dormant persistent weak layers that we are continuing to track. The late-March crust is down 50-70cm and was reactive last week during the warm period. The mid-March and mid-February layers are typically down between 70 and 100cm and have been dormant for several weeks. These layers have the potential to wake up with sustained warming, a significant rain event, and/or a big cornice fall.
Problems
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.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.