Storm conditions will quickly elevate the hazard on Saturday. We are uncertain how the snowpack will react to this new loading and are recommending very conservative terrain choices if you head out during the storm.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A warm storm hits the region on Saturday. Models are currently showing 15-30mm of precipitation but freezing levels are forecast to stay around 1800m during the day so much of it will fall as rain. Freezing levels are forecast to drop down to below 1000m by Sunday morning so the rain may switch to snow at the end of the storm. Alpine winds are forecast to be strong during the storm and ease by Sunday morning. On Sunday, a mix of sun and cloud is expected with freezing levels around 1000-1500m and light-to-moderate alpine winds. On Monday, unsettled conditions are expected as a weak storm system passes to the south of the region.
Avalanche Summary
On Thursday, natural, skier-triggered, and explosive-triggered avalanches were reported up to size 2.5. Much of this activity is releasing on the mid-February layer down 30-60cm. On Wednesday, several natural size 2 avalanches were reported but no details were provided. A skier remote avalanche was also reported to have been triggered from 20m away. Finally, ski cuts released several small pockets of wind slab which were overlying the crust/facet layer. Saturday's storm is expected to quickly build touchy new wind slabs where the precipitation falls as snow. At lower elevations where the precipitation falls as rain, loose wet avalanches can be expected. The major question and my biggest concern is whether deeply buried persistent weaknesses will wake-up with the new loading. These persistent weak layers could have the potential to result in some very large persistent slab avalanches. However, this is probably a low probability, high consequence type of problem.
Snowpack Summary
A moist snow surface has been reported to around 2500m on solar aspects and around 2000m on north aspects. Strong winds will continue to build wind slabs in leeward terrain features in the alpine. Up to 60cm of snow sits over a weak facet/crust layer that was buried in mid-February. The late-Jan crust/surface hoar layer (over 1m deep) and the mid-January surface hoar (around 1.5m deep) have gained significant strength and have been dormant for several weeks but now have the potential to wake-up with the current warm temperatures and new storm loading.
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.