Regions
Northwest Inland.
Benign weather on Monday should lower the hazard throughout most of the region.The hazard may be higher than posted in the far north of the region where buried weak layers may still be reactive to human-triggering.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Tuesday
Weather Forecast
A ridge of high pressure is expected to keep the region mostly dry on Monday. Freezing levels are expected to stay below 900m with light alpine winds. A storm system is expected to reach the region on Monday evening or overnight and should continue through Tuesday. 5-15cm is expected between Monday evening and Tuesday afternoon. Freezing levels are forecast to reach around 1000m and alpine winds are forecast to be moderate from the southwest. Wednesday is expected to be mainly dry with freezing levels below 700m and light alpine winds.
Avalanche Summary
On Saturday, a natural size 1.5 wind slab avalanche was reported on a west aspect at 2200m elevation which was 20cm thick. No new avalanches were reported on Friday. On Thursday in the far north of the region, a skier accidently triggered a size 2 persistent slab avalanche on a northwest aspect at 1650m and explosives triggered a size 2.5 persistent slab on a northeast aspect at 1400m. These both failed on the early January surface hoar layer down 100-150cm. Wind slabs are expected to remain reactive to human-triggering on Monday in steep, unsupported leeward features and around loaded convexities. In the far north, deeply buried persistent weak layers may still be reactive to large triggers.
Snowpack Summary
Recent snowfall and mostly southwestern winds have formed wind slabs in the leeward features in the alpine and at treeline. Storm snow from last week has settled due to recent warm temperatures. There may be a buried layer of surface hoar in the top 30 cm of the snowpack from early February, this layer was found at Hankin, but has not been widely reported. Crusts may be developing at mid-elevations as the freezing level slowly descends. The early January persistent weak layer of buried surface hoar is down 50-80 cm in most of the region, but may be down a metre or more in the deeper snowpack areas in the southwest of the region. Shallow snowpack areas in the east and north of the region may have a weak base layer of facetted snow on or just above the ground.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.