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Confidence
Poor - Due to the number of field observations
Weather Forecast
A moist low pressure system will linger on Sunday bringing mostly patchy light snowfall, moderate northerly winds and freezing levels at valley bottom. By Monday a dry arctic ridge will set-up for the rest of the forecast period. With this ridge, we'll see mainly clear skies, light to moderate northeast winds and alpine temperatures around -20.
Avalanche Summary
Wind slabs to size 1.5 were observed in high elevation terrain on Friday. Strong winds on Saturday may still be blowing loose surface snow into fresh wind slabs, and reactivity may persist for a few days.
Snowpack Summary
On the surface, up to 30cm of low-density snow exists. In the alpine and at treeline, winds have been conducive to blowing this snow into wind slabs in exposed lee areas. 30 - 60 cm below the surface (more in wind affected areas) you may find a crust that may have surface hoar on top of it. The crust is reported to extend into the alpine to at least 2100 m, while the surface hoar was reported to be most reactive on shady aspects around treeline and just below, between around 1600 m and 1900 m. Where the crust exists, it may be bridging triggers from penetrating to deeper persistent weaknesses that formed earlier in the season. However, on high alpine slopes above where the rain crust formed, or in areas where rain didn't occur, facets, and/or buried surface hoar may still be susceptible to triggers.
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.