Touchy storm slabs will make for tricky conditions on Sunday. Conservative decision making is critical. Stick to low angle slopes and avoid avalanche terrain.Check out our blog on the current conditions: http://goo.gl/i7J4MQ
Confidence
Fair - Due to the number of field observations
Weather Forecast
A weak ridge of high pressure should bring a mix of sun and cloud for Sunday. Freezing levels are forecast to reach around 1500m in the afternoon and alpine winds are expected to be light. Good overnight recovery is expected during the forecast period with freezing levels falling to around valley bottom each night. On Monday and Tuesday, mostly cloudy conditions are expected with light convective flurries. Afternoon freezing levels should reach around 1500m each day and alpine winds should remain light.
Avalanche Summary
Avalanches observations have been limited due to the storm conditions but a widespread natural avalanche cycle is expected to be occurring at higher elevations. On Friday, several natural size 2 avalanches were observed. On Wednesday, natural and skier triggered slab avalanches were reported up to size 2. On Sunday, natural activity is expected to taper off as temperatures become cooler. However, if the sun is out in the afternoon, natural avalanches are possible from sun-exposed slopes. Human-triggering of storm slabs is expected to be a major concern for the next few days, especially at higher elevations where the snow is still dry.
Snowpack Summary
At higher elevations, new storm snow (30-50cm) sits over a variety of surfaces including moist snow, crusts, wind affected surfaces, and/or old wind slabs. Anywhere from 50-80 cm now sits over the mid-February persistent weak interface. This interface has recently been very reactive and may continue to produce large avalanches. In exposed terrain, strong SW winds have redistributed new snow into wind slabs on leeward features. The snow surface has been reported to be wet to around 1700m and moist to ridgetop. The snow surface is now expected to undergo melt-freeze cycles as freezing levels fluctuate overnight. The late-January crust/surface hoar layer (over 1m deep) and the mid-January surface hoar (around 1.5m deep) have been dormant for several weeks.
Problems
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.