Avalanche Forecast
Issued: Mar 24th, 2018 10:28PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
The threat of Wind Slab avalanches is maintaining heightened avalanche conditions. You will be able to trigger a wind slab avalanche on steep wind-loaded slopes with convex rollovers, unsupported features, or below fresh cornices. You can stay safe by steering around these pieces of terrain where you see snow drifts, cornices, and firm snow that produces cracking.
Summary
Detailed Forecast
Today you can trigger avalanches in the recently fallen snow. New snow and wind are maintaining elevated avalanche conditions. You can trigger Wind Slab avalanches on steep slopes near ridge tops or in cross-loaded areas including: convex rollovers, steep unsupported slopes (ending in rocks or cliffs), or below fresh cornices. Clues such as variable snow height, drifts, cornices, and stiff snow surfaces that produce cracking are signs that you should stay off of steep, unsupported slopes. Steering around these features is the best way to keep from triggering an avalanche today.
It has been more than two weeks since the last reported avalanche involving older weak layers. Some of these layers have not gone away, but they are now very difficult to trigger. With the current conditions, Persistent Slab avalanches will remain unlikely, though not impossible. This is a low likelihood high consequence scenario. Snowpack tests can provide useful information about buried weak layers, they are not a decision-making tool.
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
Almost 2 feet of snow fell in the Cascade West South region since Wednesday. Winds during the storm transported snow forming wind slabs near ridgelines and on cross-loaded features. Observers reported avalanches within the recent snow and at the interface of the recent snow and old snow layers.
Rain on Thursday reach around 5500’ before transitioning to snow. This formed a new rain crust and created a generally good bond between the old and new snow. Below this rain line a wet and refreezing snowpack can be found. 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 soft settled snow were observed earlier in the week. This may form a new buried weak layer within the snowpack.
An old weak layer of sugary facets (2/13) can still be found in some locations just above a very firm and wide spread crust (2/8). This pronounced crust has typically been found about 4-6 feet below the snow surface in the Crystal and Paradise areas You are most likely to find this layer still intact on shaded aspects near and above treeline.
Observations
South
On Saturday NWAC Forecaster Dallas Glass reported multiple natural and triggered Wind Slab avalanches on North, East, and Southwest aspects above 5,500 ft. The largest was almost 2 feet deep and involved all of the snow that fell since Wednesday. Dallas also reported sudden test results on the 2/13 facets over 6 feet below the snow surface.
On Wednesday NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry. Jeremy found a firm and supportable surface crust on S-facing slopes. On shaded slopes, he reported soft snow. The 2/8 crust was down about 4 feet and still reactive in snowpack tests.
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
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 25th, 2018 10:28PM
