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Avalanche Forecast

Archived

Mar 24th, 2018–Mar 25th, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Snoqualmie Pass.

The threat of avalanches from new snow and wind is maintaining heightened avalanche conditions. You will be able to trigger a wind slab avalanche 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.

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 where you see 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 theses areas to avoid triggering a Wind Slab. Similarly, you can trigger Storm Slab avalanches in non-wind-affected, very steep terrain, on convex rollovers, and on unsupported slopes (ending in rocks or cliffs).

It has been more than two weeks since the last reported avalanche involving older weak layers. Many of these layers have rounded and they are now very difficult to trigger. With the current conditions Persistent Slab avalanches will remain unlikely. 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

About 1 foot (35cm) of new snow fell near Stevens and Snoqualmie Passes since Wednesday. A mid-storm instability has been the culprit weak layer for recent small avalanches. An observer reported remotely triggering some of these avalanches in the storm snow. Rain reached to near 5000’ during the early part of the storm before lowering snow levels brought snow to all elevations. This created a generally favorable bond between the old and new snow surfaces. Winds during the storm formed small wind slabs near ridges near and above treeline.

Above the rain line the new snow fell on a variety of snow surfaces. On sunny aspects a firm melt-freeze crust formed during the recent clear weather. On shaded slopes, settled powder and/or near surface facets have been reported earlier this week.

An old weak layer of sugary facets (2/13) can still be found in some locations just above a very firm and widespread crust (2/8). This pronounced crust has typically been found about 4-6 feet below the snow surface in the Stevens and Snoqualmie Pass areas. You are most likely to find this layer still intact on shaded aspects near and above treeline.

Observations

Stevens

On Thursday NWAC forecaster Josh Hirshberg traveled in the Stevens Pass backcountry. Josh reported 4-6 inches (10-15cm) of new snow in sheltered locations. Evidence of rain was observed up to around 5000 feet. Moderate winds at ridgeline were transporting the new snow forming small drifts up to 1 foot (30cm) deep. 

Snoqualmie

NWAC staff were in the Snoqualmie Pass backcountry Friday and Saturday. They found 12" (35cm) of low density new snow. Wind affected snow was only observed in isolated features near the ridgelines. Good visibility in the afternoon allowed them to see across the area. Numerous small slab avalanches were reported below treeline in steep terrain. 

NWAC staff were in the Alpental Valley Wednesday. They found a very firm and thick (10") melt-freeze crust on sunny aspects. On North aspects small near surface facets were observed. The 2/8 crust was found down 5 feet. 

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the 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. 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.