Regions
Northwest Coastal.
A special warning has been issued for the substantial warming and sun this week. Deeply buried weaknesses within the snowpack are expected to wake-up and produce very large avalanches. It is best to avoid avalanche terrain during the heat of the day.
Weather Forecast
The ridge is expected to persist until Friday evening. Thursday should be sunny with light alpine wind and freezing levels reaching around 3500m in the afternoon. Increasing cloud cover is expected on Friday but mostly sunny conditions are expected for most of the day with freezing levels above 3000m. Alpine winds are expected to increase on Friday evening as the ridge breaks down. A mix of sun and cloud is currently forecast for Saturday with freezing levels dropping to around 1500m and moderate southwest winds in the alpine. Light scattered flurries are possible on Saturday.
Avalanche Summary
On Tuesday, a skier triggered a size 1.5 wet slab avalanche on a steep roll on an east aspect at 1400m elevation. The slab was 10-30cm thick and slide on a crust. A natural size 3.5 was reported north of Stewart on a north aspect at 2200m. This was 3-4m thick and failed on the ground on a steep glaciated feature. Numerous natural cornice failures and loose wet avalanches were reported up to size 2. No new avalanches were reported on Monday. On Thursday, continued warming , sun exposure, and limited overnight recovery means natural cornice releases, deep persistent slab releases, and loose sluffing are expected to continue. Very large avalanches will remain a serious concern for the next few days.
Snowpack Summary
Moist surface snow is being reported to ridgetop on all aspects with wet surface snow below 1500m. Warm overnight temperatures means there was a limited refreeze of the snow surface. A thin crust may have formed in some areas but is expected to quickly break down in the morning. A weak crust/surface hoar layer which was buried over a week ago is down 30-40cm in the north of the region. A widespread crust/facet layer buried in early February and a lingering surface hoar layer from January can now be found down over a meter. These old weak layers still have the potential to produce large avalanches, especially with a large trigger such as a cornice fall. Extra caution is required during this period of substantial warming and intense solar radiation, as this is when these layers have the biggest potential to wake up.
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.
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.