Confidence
Fair - Intensity of incoming weather is uncertain
Weather Forecast
Unstable conditions expected through the weekend. Two cold fronts back to back, will bring light to moderate snow amounts, accompanied by a strong upper SW flow. The Western ranges may see higher snow amounts. The second frontal system looks to be a bit punchier. Friday: Dry in the am. Snow amounts 5-15 cm. Snow continued overnight. Ridgetop winds moderate from the SW. Treeline temperatures near -6. Freezing levels 1200-1400 m. Saturday: Snow amounts 10-20 cm. Ridgetop winds moderate-strong from the SW. Freezing levels 1000-1400 m. Sunday: Light-moderate snow amounts as the front moves through. Freezing levels should drop to 1000 m by Sunday night into Monday.
Avalanche Summary
On Wednesday operators did full explosive testing and reported numerous large avalanches. This may indicate the triggering potential that lies ahead. The snowpack is susceptible to new storm slab, wind slabs, cornice fall, sled and skier triggers. The scary fact is the avalanche size. Even a small avalanche could step down and trigger a persistent weak layer, which in turn will create a large, destructive avalanche. Remote triggered avalanches (from afar) are still being reported up to size 2.5. Many skier triggered size 1-2 avalanches have also been reported. Monday/Tuesday the region saw a widespread natural avalanche cycle in response to snow, wind and warming. With more snow and wind in the forecast the avalanche danger will elevate.
Snowpack Summary
The weekend past, up to 80 cm of storm snow fell. Strong winds from the SW and fluctuating temperatures were the perfect recipe for storm slab, and wind slab instabilities. A 80-170cm thick slab sits on variable old surfaces (surface hoar, facets, and crusts) that formed mid-February. These persistent weak layers have remained touchy all week and are a key concern at all elevations. Easy-Hard test results, with sudden planar shears have been reported on the weak layer. If triggered, the avalanches will be larger than expected. There may even be step-down potential on deeply buried weak layers. Large cornices have formed and loom as potential triggers on the slopes below.
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.
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.