Confidence
Fair - Intensity of incoming weather is uncertain
Weather Forecast
Unstable conditions expected through the weekend. The first front will bring light to moderate snow amounts, accompanied by a strong upper SW flow. The Western ranges may see higher snow amounts, and the second frontal system looks to be a bit punchier. Friday: Snow amounts 15-30 cm. Ridgetop winds moderate-strong from the SW. Treeline temperatures near -8. Freezing levels 1200 m. Saturday: Moderate snow amounts. Ridgetop winds moderate from the SW. Freezing levels 1200-1400 m. Sunday: Snow amounts 5-10 cm by late afternoon, then heavier amounts may extend into the night. Freezing levels falling to 10000 m.
Avalanche Summary
On Wednesday a natural size 3.5 was reported including several size 2.5, that remotely (from afar) triggered low angle slopes. Operators did full explosive testing and reported numerous large avalanches. This could indicate the triggering potential that lies ahead. The persistent weakness is susceptible to new storm slab, wind slab, cornice fall, sled and skier triggers. The scary fact is the size of the avalanches. Even a small avalanche could step down and trigger a persistent weak layer, which in turn will create a large, destructive avalanche. Many rider 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, 60-100 cm of snow fell. Strong winds and fluctuating temperatures have created storm slabs and wind slabs. Buried below 100-180 cm sit the mid-February persistent weak layers (surface hoar, facets, crusts).These persistent weak layers have remained touchy all week and are a concern at all elevations. They could be triggered by storm slabs, wind slabs, or the additional load of a sled/skier. Avalanches may be unexpectedly large, and destructive. Large cornices have formed, and could act as a potential trigger on the slope 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.