Avalanche Forecast
Issued: Mar 21st, 2018 11:10AM
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
Avalanche conditions are changing Thursday as new and reactive avalanche problems develop and grow larger. Use visual clues such as blowing snow, plumes, and fresh cornices to identify and avoid slopes where new wind slabs are forming. Older weak layers of snow still exist with in the snowpack. Stay off of large open slopes where new snow loads may make triggering these deeper layers easier.
Summary
Detailed Forecast
Expect winds to transport new snow throughout the day Thursday forming new and reactive wind slabs especially at higher elevations. You can use simple observations to identify where these new wind slabs are forming. Blowing snow, fresh cornices, and plumes all indicate winds slabs are forming on slopes below. Stay off of wind loaded features where you will be able to trigger new wind slab avalanches.
Older weak snow still exists in the snowpack. Persistent slab avalanches have been the culprit in several accidents and fatalities over the last several weeks. New precipitation may change how these layers react making them easier to trigger. 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.
Snowpack Discussion
New snow will exist in most locations by early Thursday morning. This new snow will be falling 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 over the past several days. Rain turning to snow at lower elevations will help create a more favorable bond between the new and old snow. However, above the rain line a new buried weak layer may develop.
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 3 avalanche fatalities occurred on persistent weak layers 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 8-14" 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.
Observations
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.
Central
NWAC professional observer Matt Primomo traveled in the North Fork of the Teanaway River on March 15th. (just north of Cle Elum). The 3/8 buried surface hoar layer was found about 12 inches below the snow surface. It was reactive in some snowpack tests. The 2/5 layer was also found in this area about 3 feet 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 22nd, 2018 11:10AM
