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
Wind Slab avalanches are maintaining Moderate Avalanche Danger. You could trigger a Wind Slab avalanche at upper elevations 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
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
Around 1 foot (35cm) of new snow fell near Stevens and Snoqualmie Passes since Wednesday. At Snoqualmie a mid-storm instability has been the culprit weak layer for recent small avalanches. An observer reported no new avalanches near Steven's Pass in the recent 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 wide spread 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. They found 6-8" (15-20cm) 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. The only avalanche activity they observed were a few small loose dry avalanches in very steep rocky 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
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
