Avalanche Forecast

Issued: Mar 22nd, 2018 11:00AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

You will be able to trigger wind slab avalanches on steep wind loaded slopes at higher elevations Friday. Use visual clues such as fresh cornices, snow drifts, and uneven snow surfaces to identify and avoid wind loaded terrain. Several older weak layers can still be found within the snowpack. You can avoid being involved with these low likelihood-high consequence avalanches by staying off of large steep open slopes until these layers have time to heal.

Summary

Detailed Forecast

New wind slabs have formed on a variety of aspects near and above treeline. You can avoid these avalanches by staying off of steep wind loaded slopes. Use visual clues to assess and avoid wind loaded features.  Snow drifts, fresh cornices, and uneven snow surfaces with cracking can all indicate wind slabs exist on nearby slopes.

Older weak snow still exists in the snowpack. Persistent slab avalanches have been the culprit in several accidents and fatalities over the month. New precipitation may change how these layers react. You can avoid persistent slab avalanches by staying away from large steep open terrain where these large avalanches may occur.

The new snow is falling on a variety of snow surfaces including firm surface crust, near surface facets, and potentially surface hoar. Be ready to dial back your terrain if you experience any signs of unstable snow such as observing new avalanches, hearing whumphs, or seeing shooting cracks. This is most likely to occur in areas receiving higher snow fall amounts such as near Washington Pass.

Snowpack Discussion

Four to six inches of new snow fell in the Washington Pass by Thursday afternoon. At lower elevations rain slowly turned to snow as freezing levels lowered throughout the day. This likely created a favorable bond between the new and old snow surfaces.

Above the rain line, the new snow fell on a variety of old snow surfaces. On sunny aspects, firm melt-freeze crusts have formed during the recent clear weather. In shaded locations, near surface facets and surface hoar were observed earlier in the week. This may form a new buried weak layer within the snowpack.

Moderate winds likely transported new snow forming soft wind slabs near and above treeline. These wind slabs will generally be shallow and isolated to exposed ridgelines and mid-slope terrain features.

Below the most recent snow and crust, several older persistent weak layers continue to be reported. Persistent Slabs in the upper snowpack are easier to trigger, but may be more isolated in their distribution. Persistent Slabs deeper in the snowpack (2/13 or 16 layer) are more widespread, but are harder to trigger. Observations from the Eastern areas of the Cascades have found these layers reactive and they were in involved 3 avalanche fatalities in early March. The exact persistent weak layer depends on your location.

Commonly seen persistent weak layers are:

  • Surface hoar and facets buried on 3/8. This layer is typically found 10-16’  below the snow surface near and below treeline. Professionals near Washington Pass have reported that the surface hoar is fairly widespread in that area. The distribution of this layer may be more variable in the Central and Southern zones. 
  • A more widespread persistent weak layer is found just above a firm crust layer buried on 2/5. This layer of weak sugary facets (2/13 or 16) can be found 2-4 feet below the snow surface. Professionals near Washington Pass and the north end of the East Cascades have reported that this layer most problematic below treeline.
  • Other shallow and very weak areas of snowpack may be found further east of the crest or in areas with less snow.

North

On Wednesday NCH found surface hoar and near surface facets present on shaded slopes. The 3/8 buried surface hoar was reported as 10-14 inches below the snow surface.

Problems

Wind Slabs

An icon showing Wind Slabs

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.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Persistent Slabs

An icon showing Persistent Slabs

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.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 23rd, 2018 11:00AM