The full day of sun following cold stormy conditions is expected to destabilize the recent storm snow. Loose wet sluffing, cornice failures, and solar triggered slab avalanches are all expected. Use extra caution during the heat of the afternoon.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
On Wednesday, a ridge of high pressure should bring a mix of sun and cloud with moderate alpine winds from the SW to W. Freezing levels are expected to be around 700m in the morning and 1700m in the afternoon. On Thursday, 4-8mm of precipitation is expected with moderate-strong alpine winds from the SW.Freezing levels are forecast to be around 1200m in the morning and climb to around 1800m in the afternoon. A mix of sun and cloud is expected for Friday with light alpine winds and freezing levels reaching as high as 2500m. A weak storm system is currently forecast to reach the region Friday night.
Avalanche Summary
On Monday, a cornice triggered a size 1.5 avalanche in the Valemont area. This was 40cm deep and released on the mid-April weak layer. Around Blue River, a cornice was intentionally released but the cornice failure did not trigger a slab on the slope below. In this area where the storm snow amounts are less, wind slabs in specific areas are the biggest concern whereas deeper areas can expect a more widespread storm slab problem. In the North Columbia region, more widespread activity has been reported including natural and skier-triggered avalanches up to size 2.5. Remotely triggered avalanches have been reported from up to 200m away which suggests that the weak layer below the storm snow is very reactive in this region. On Wednesday, lingering storm slabs or wind slabs are expected to be reactive to human-triggering, especially in steep alpine terrain and wind-loaded terrain features. The sun is expected to trigger some natural activity on steep solar aspects including both loose wet avalanches and slab avalanches. Cornices will become weak in the afternoon and natural cornice falls have the potential to trigger large slabs.
Snowpack Summary
Recent observations have been limited and some of this discussion is extrapolated from the North Columbia region where conditions are expected to be similar. Around 30-40cm of recent snowfall overlies a weak layer that was buried on Friday. 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, strong SW winds have redistributed the recent storm snow forming thicker 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
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.
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.