Avalanche Forecast

Today is not the day to go into avalanche terrain. If you venture into the backcountry, you must be confident of your ability to avoid avalanche terrain. You can trigger very large and deadly persistent slab avalanches on most slopes steeper than 30 degrees. Commonly traveled terrain such as the Source Lake area or Snow Lake Divide are exactly where these avalanches could occur.

Persistent slabs can break widely across terrain features, come down on top of you, and can be triggered from a long distance away. Stay far out from under any avalanche paths and make sure not to get too close to start zones. Persistent slabs can break onto low angle slopes.

In the past 2 days, observers have reported a number of obvious signs of Persistent Slab avalanches; 1) Remotely triggered avalanches (see photo). 2) Widespread collapsing of the snow underfoot or a "whumphing" sound. 3) A layer of weak, sugary facets and surface hoar 3-4 feet below the snow surface. The facets are widespread on all but the lowest, sun-exposed SE-SW slopes. The surface hoar may be found in more isolated, pockets. These weak layers are resting on a stout crust.

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.

While there is some uncertainty about triggering persistent slab avalanches, you can trigger large storm slabs even more easily. This is another reason to avoid avalanche terrain. Any avalanche triggered in the new snow could result in a much larger and more deadly persistent slab avalanche. Avoid drifts and steer around areas where the wind has stiffened the snow on leeward slopes at upper elevations. Use small test slopes to check how well the snow is bonding.

1-2 feet of warmer, more cohesive snow will fall on softer storm snow from the past 3 days. This will build a classic "strong-over-weak" layering, making avalanches in the new snow very easy to trigger. Wind will form stiffen snow further and form deeper drifts near and above treeline. 

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.