Avalanche Forecast

The snowpack has seen a rapid and major change. Give the snow some time to adjust. Continue to stick to slopes under 35 degrees and smaller terrain features while the recent storm layers bond.

You can easily trigger large storm slab avalanches in the snow that fell over the past couple days. Watch out for a strong-over-weak snow layering in the top 3 feet of the snowpack. Use test slopes and quick snowpack tests to check how well the recent snow is bonding. Avoid leeward slopes at upper elevations where the wind has blown the snow into thick pillows and drifts. Avalanches on wind loaded slopes could be up to 6 feet deep. Any avalanche triggered in the recent snow could result in a larger and more deadly persistent slab avalanche.

Snoqualmie Pass has received over 30 inches of snow since the morning of the 11th and over 58 inches since the 8th. The top 2-3 feet fell warmer, more cohesive, and was wind affected at upper elevations and along the highway 2 corridor. This built a classic "strong-over-weak" layering, making avalanches very easy to trigger in the new snow.

Loose dry avalanches may be a concern at lower elevations. These may be large by Tuesday. Watch for fan shaped avalanches, and avoid hanging out in places where snow normally sheds off steep slopes from above. 

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.

Today is not the day to travel in large or complex avalanche terrain. If you venture into the backcountry, you must be confident of your ability to put enough distance between you and slopes that could produce life-threatening avalanches. Stay far out from under big avalanche paths and make sure not to get too close to start zones. Persistent slabs can break widely across terrain features, come down on top of you, and can be triggered from a long distance away. These avalanches are difficult to predict and break in ways much wider and in more surprising ways than avalanches that we typically see in the Northwest. 

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 4-5 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.