Avalanche Forecast

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 can break wide and in surprising ways by wrapping around terrain features. 

If you hear large whumpfs, experience sudden collapses in the snowpack, see shooting cracks in relatively flat terrain or remotely trigger an avalanche, nature is trying to tell you deadly persistent slab avalanches are on the menu. Be especially suspicious on north through east aspects near treeline where weak snow over a firm crust was last identified. 

Ask yourself, if a very large avalanche released from the slope above, would I be in the runout zone? If the answer is yes or if you can’t answer the question, then choose much simpler terrain that’s much lower angled and not connected to anything capable of producing a very large avalanche. 

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.

SE winds likely formed fresh wind slabs Thursday. You will need to approach steep unsupported slopes with wind-drifted snow cautiously, feeling for firm or hollow sounding snow as a sign that wind slabs may be present. You can stay safe by traveling on ridges, wind-scoured areas and slopes less than 30 degrees.

In wind sheltered areas, storm slabs that formed over the last few days should be trending toward stubborn and unreactive. Any avalanche triggered in the recent snow could result in a larger and more deadly persistent slab avalanche. 

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.

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.