Regions
Northwest Inland.
Substantial warming and sun exposure this week are expected to wake-up deeply buried weaknesses within the snowpack. Conservative terrain selection is critical and it is best to avoid avalanche terrain during the heat of the day.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
A blocking ridge of high pressure is the dominant feature for the week. Dry and clear conditions are expected for Tuesday through Thursday. Freezing levels are expected to climb to around 2700m on Tuesday afternoon and should be above 3000m on both Wednesday and Thursday afternoon, with cooling during the overnight periods. Strong northwest winds are expected on Tuesday in exposed terrain. Light to moderate northwesterly winds are forecast for Wednesday and light northwesterly winds for Thursday.
Avalanche Summary
On Sunday, a natural icefall entrained surface snow and was reported as size 2. Isolated natural wind slabs were reported to size 1 on east aspects at treeline. In the far north, natural cornice releases were reported to have triggered slabs up to size 3.5 which were stepping down to ground. Point releases were reported up to size 2 and glide cracks up to size 2.5. On Tuesday, substantial warming and sun are expected to result in natural cornice releases and may also result in deep persistent slab releases. Lingering wind slabs also remain a concern.
Snowpack Summary
At treeline and above small wind slabs overly a variety of surfaces include 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. Surface hoar from March 23rd is still being mentioned in the northern half of the region, and now may be covered by up to 20cm of recent storm snow. Professional operators are tracking two deep weak layers from early-January and 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 some areas and have been shown to be reactive in the north of the region. Rapid warming may wake up deeply buried weak layers.
Problems
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.
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.
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.