Monday's danger ratings assume that a major storm will arrive in the morning. The timing of this system is uncertain and danger ratings may be a bit too high if the storm arrives later.
Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain
Weather Forecast
Batten down the hatches!Monday: A deep Pacific low spreads moderate to heavy snow to valley floor (25-30cm). Severe winds and thunderstorms are possible. The timing and intensity of this system are uncertain.Tuesday: Light convective snow. Moderate winds, gusty at times. Freezing level around 600m.Wednesday: The next low pressure system arrives with moderate to heavy snow, strong south-westerly winds and freezing level rising slightly.
Avalanche Summary
It's been an active week for avalanche activity. Saturday's storm snow was triggered naturally, by skiers and by explosives to size 2 on a variety of slopes. Many occurred on wind-loaded north-east to north-west aspects. Two skier involvements were reported on north/north-west aspects at 2100m. Earlier in the week, snowmobilers remotely triggered avalanches from 200m below the fracture line. There was an avalanche fatality at Grizzly Lake on Powder Mountain on Tuesday involving a snowmobiler. The avalanche occurred on an east aspect near 2000m. The slab was 80cm-150cm thick and 525m wide and failed on the mid-February persistent weakness. An unnervingly large natural avalanche cycle occurred last weekend, with slabs up to size 4.5 propagating 2-3km along ridgelines and stepping down to the mid-February layer. Similar avalanches are possible as storm loading continues this week.
Snowpack Summary
Stormy conditions are creating ever-deeper wind slabs and storm slabs. These overlie various old surfaces including crusts, moist snow, old wind slabs and old storm slabs. Reports from the field suggest variable reactivity in the recent storm snow. A key concern is the potential for a storm- or wind-slab to step down and trigger deeper weaknesses, formed in mid-February, which are still producing clean and fast shears in snowpack tests. Very large avalanches are possible, which could be remote-triggered, triggered mid-slope, and/or propagate into low-angled terrain.
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.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.