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
A ridge of high pressure dominates for the rest of the week. Sunny and dry conditions are expected for Wednesday through Friday. Freezing levels are expected to be above 3000m each afternoon, with cooling overnight. Light to moderate outflow winds are expected in the alpine for the forecast period.
Avalanche Summary
No new avalanches were reported on Monday. On Sunday, a skier triggered a size 1.5 persistent slab avalanche on a steep roll on a northeast aspect at 1400m elevation. The slab was typically 40cm thick and released on a layer of surface hoar that was buried on March 23. A ski cut at the top of a steep north facing chute at 1100m elevation triggered a size 2 persistent slab which failed on a crust. A helicopter remotely triggered a size 1.5 wind slab from 150m away on an east aspect. 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. Explosives also triggered a couple size 2 wind slabs on north aspects around 1500m. All of this activity was reported from north of Stewart. On Wednesday, continued warming and sun exposure may result in natural cornice releases, deep persistent slab releases, and loose sluffing.
Snowpack Summary
10-30cm of recent storm snow has accumulated in sheltered areas at higher elevations. In exposed terrain, southerly winds have formed soft slabs in leeward features. A weak crust/surface hoar layer which was buried last week 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 metre. These layers still have the potential to produce large avalanches, especially with a large trigger such as a cornice fall. Extra caution is required during periods of substantial warming and intense solar radiation, as this is when these layers have the biggest potential to wake up. At treeline and below, spring melt-freeze conditions are already being reported. These conditions are expected to extend well into the alpine this week with the forecast warming and sun.
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.