Avalanche Forecast

New snow over the next 24 hours may form a slab that does not bond well to the existing snow surface. The new snow will also thicken existing storm slabs. Look for multiple density changes within the upper snowpack. Natural storm slab avalanches will be likely where these “strong over weak” layering configurations exist. Plenty of snow is available for transport, and winds may blow snow and create thicker and more reactive slabs. Avoid fresh drifts and steer around areas where the wind has stiffened the snow on leeward slopes at mid and upper elevations. Continue to watch for loose dry avalanches - they could knock you off your feet or pile up debris deeper than you expect. Use small test slopes to assess how the new snow is bonding to itself and older snow layers.

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Incremental snowfall is building a thick slab on top of a buried weak layer of sugary facets. Where this slab/weak layer combination exists, we may see large and surprising avalanches. The added load from incoming snow Thursday and Friday will stress this weak layer further - maybe to a breaking point. These avalanches could run naturally. Stay far out from under big avalanche paths and make sure not to get close to start zones. Several avalanches have already been triggered remotely - from a distance away on flat ridgelines, in gentler adjacent terrain, or from below. You do not need to be on a steep slope to trigger an avalanche.

This weak layer sits on a crust on solar aspects (SE-S-SW) - these areas have produced the most number of avalanches in the past few days. Avalanches have also occurred on northerly aspects. The slab/weak layer combination may not exist on all slopes, making it an unpredictable and challenging weak layer to track. Uncertainty exists regarding the reactivity and distribution of the problem. Look for recent avalanches and listen closely for collapses in the snowpack. Dig down about 3ft and look for a layer of sugary snow that, in some areas, may sit on a crust. Avoid slopes 30 degrees and steeper.

Release of a cohesive layer of soft to hard 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 Slabs.

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.