Regions
Northwest Inland.
The hazard may go higher than forecast if temperatures rise unexpectedly. Pay close attention to the danger from cornices and wind slabs.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain on Monday
Weather Forecast
The Northwest Inland will see light precipitation through the weekend. TODAY: Freezing level around 1200 m, trace of snow, light to moderate southwest winds. SATURDAY: Freezing level will drop overnight to around 700m, then rise to 1500 m during the day. 1 to 5 cm of snow in the forecast, light to moderate south/southwest winds. SUNDAY: Freezing level dropping to 700 m overnight, then rising to 1300 m during the day. 1 to 5 cm of snow, light winds from the west. MONDAY: Freezing level dropping to 600 m overnight, then beginning to rise to 1500 m by midnight. No precipitation in the forecast.
Avalanche Summary
Loose wet avalanches on solar aspects below treeline are in the news from yesterday. It is spring, after all ! Recently a cornice failure triggered a size 3 slab avalanche in the southwest corner of the region. This avalanche occurred on an east aspect in high alpine terrain and stepped down to persistent weak layers from January and December. A great example of the ability of a large load to trigger weaknesses that are buried deep in the snowpack..
Snowpack Summary
Recently, up to 20 cm of new snow has fallen in some parts of the region accompanied by moderate to strong southerly winds. At treeline and in the alpine small wind slabs are expected to have formed over a variety of surfaces that include a hard crust on solar aspects above 1300 m, moist or refrozen snow on all aspects below 1300 m, and settled wind slabs in exposed high elevation terrain. Of note is a surface hoar from early March that can be found down 50 to 70 cm but only seems to be a problem for the northern half of the region. Professional operators are still tracking two deep weak layers from early-January and early-February that are now down about 1 meter or more. These layers are generally dormant but might wake up with increased warming or a heavy trigger (like a cornice fall). Basal facets exist in most areas and have been shown to be reactive in the north of the region.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.