Avalanche Forecast

The recent skier triggered accidental wind slab in the Stevens Pass zone is bullseye information. Continue reading if you'd like advice to help avoid getting into a similar situation... New snow will easily be transported into sensitive slabs on Thursday. Older, but still recent wind slabs may be resting on a layer of near surface facets, or facets over a melt freeze crust and they may be disguised by the new snow. It may be easier to find wind slabs than to avoid them at the moment. Expect slabs on open slopes above, near, and even below treeline. Areas exposed to, and that funnel easterly winds are likely to have thicker and more dangerous slabs. Watch for uneven snow surfaces, hard drum-like sounding snow, and lens shaped drifts. Don't get lured out onto a wind slab on a slope steeper than 35 degrees, as they make break above you. Avalanches within upper snowpack layers have the potential to step down and create much wider persistent slab avalanche. Seek out sheltered terrain, thats where the quality powder is, anyway. 

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.

By and large, the best way to deal with this persistent slab problem is to avoid large, open slopes that are steep enough to avalanche. In the past 2 weeks the only steep slopes I've been on in the area are ones that I know have previously slid, and those that are completely faceted out with no slab on top. Terrain management is key. Choose well supported terrain, and minimize your exposure to avalanche terrain. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative uphill routes and descent options to minimize your risk.

Plenty of recent snowfall with cold temperatures have inhibited settlement, and have stressed old faceted weak layers. The most likely place to get one of these to release would be on shallow, unsupported features and large upper elevation start zones that have been recently wind loaded. Facets buried on 2/8 may be found closer to the surface on some previously stripped, rocky slopes. In these isolated areas where recently drifted wind slab overlies facets over a crust, a very large avalanche may still be triggered. This is especially true in the Wenatchee mountains, east of Hwy 97, and locations with less than 4 feet of total snow on the ground. The February 8th facet/crust layer can be found from 1-3 feet beneath the surface and may be most preserved on northerly aspects. Tests continue to indicate the potential for avalanches to occur on it. Shallow snowpack zones nearer the Columbia River and low elevation north facing slopes all over the zone have a weak snowpack structure. 

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.