Avalanche Forecast
Issued: Mar 23rd, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Wind slabs formed over the past few days will slowly gain strength Saturday. You are most likely to trigger a wind slab avalanche steep wind loaded slopes with convex rollovers, unsupported features, or below fresh cornices. Use visual clues such as snow drifts, cornices, and firm snow with cracking to identify and avoid this wind loaded terrain. Older deeper persistent slabs can still be found. You can avoid triggering a persistent slab by staying off of steep large open slopes where these bigger avalanches may occur.
Summary
Detailed Forecast
Avalanche danger will slowly decrease Saturday as new wind slabs begin to gain strength. Even though the hazard is decreasing, you may still trigger a wind slab on steep slopes near ridgeline. You are more likely to trigger a wind slab on convex rollovers, steep unsupported slopes, or below fresh cornices. You can use visual clues to identify and avoid wind loaded slopes greater than 35 degrees. Snow drifts, cornices, and firm snow surfaces with cracking all indicate that nearby slopes received wind deposited snow.
Observations continue to show older weak snow in the snowpack. It can be difficult to identify exactly which slopes have persistent weak layers and which ones do not. Persistent slab avalanches have been the culprit in several accidents and fatalities over the month. New precipitation may change how these layers react. While snowpack tests can provide useful information about buried weak layers, they are not a decision making tool. You can avoid persistent slab avalanches by staying away from large steep open terrain where these large avalanches may occur.
The strong March sun can quickly change conditions on slopes receiving direct sunshine. If the sun comes out, expect small loose wet avalanches, rollerballs, and pinwheels. Be ready to move off of and avoid steep sunny slopes during periods of intense sunshine.
Snowpack Discussion
Approximately 8-10” of snow has fallen in the Washington Pass area since Wednesday evening. Winds during the storm transported snow forming new wind slabs on lees slopes and cross-loaded features near and above treeline. 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.
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.
Below the most recent snow and crust, several older persistent weak layers continue to be found. While there have not been any new avalanches reported on these layers, snowpack tests continue to demonstrate that avalanches can occur. Persistent weak layers were responsible for 3 avalanche fatalities in the past month. The exact persistent weak layer depends on your location.
Commonly seen persistent weak layers are:
- Surface hoar and facets were buried on 3/8. This layer is typically found 18-24” (45-60cm) below the snow surface on shaded aspects near and below treeline.
- 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 3-5 feet (90-150cm) 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
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
Expected Size
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
Expected Size
Valid until: Mar 24th, 2018 11:00AM
