It's time for a mentality shift. Warm conditions are slowly destabilizing the upper snowpack and conditions are no longer "bomber" like they have been for the last few weeks. Check out the latest blog post on warming conditions: http://goo.gl/nS6uhF
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A weak storm system will bring light precipitation to the region on Wednesday night and Thursday. Models are currently showing 5-10mm for the north of region and less to the south. Unfortunately freezing levels are expected to stay around 2000m so much of this precipitation will fall as rain. Alpine winds on Thursday are forecast to be moderate-to-strong from the SW to W. On Friday, a ridge of high pressure should bring mainly sunny conditions. Freezing levels are forecast to reach as high as 3000m on Friday and alpine winds light from the SW. Unsettled conditions are expected on Saturday as a storm system moves into the region. Light precipitation is possible and winds are expected to become strong from the SW.
Avalanche Summary
Reports from Tuesday of loose wet avalanches and cornice failures from steep sun exposed slopes. No slab avalanches have been reported recently in the Purcells but there were some concerning avalanches in the Columbia regions. An natural avalanche failed near the ground in the South Columbia region and multiple skier-triggered avalanches were reported in the North Columbia region. On Thursday, light rain at lower elevations may further destabilize the warm upper snowpack and loose wet avalanches should be expected. Up high where the new precipitation falls as snow, wind slab formation is expected to continue. Finally, with the combination of unseasonably warm temperatures with little overnight recovery, recent strong solar inputs, and forecast rain, there is a concern for dormant persistent weak layers to wake-up which could result in large slab avalanches. This is a low probability, high consequence problem but deserves attention as we can expect several more days of warm conditions.
Snowpack Summary
A moist snow surface is being reported to around 2500m on solar aspects and 1800m on north aspects. Wind slabs are being reported in the alpine and may be overlying the early-March crust/facet layer. The late-Jan crust/surface hoar layer can be found about a metre below the surface in deeper snowpack areas. The mid-January surface hoar can be found below that. These layers have gained significant strength and have been dormant for several weeks but have the potential to wake-up with the current warm temperatures.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.