Avalanche Forecast
Regions: Cascades - West.
You may be able to trigger newly formed wind slab avalanches on steep slopes near and above treeline. Use visual clues such as fresh cornices, snow drifts, and uneven snow surfaces to identify and avoid wind loaded slopes. Very large triggers or small avalanches may step down into deeper older layers in the snowpack. You can avoid being involved in a deep persistent slab avalanche by staying out of large steep open slopes where these bigger avalanches may occur.
Detailed Forecast
Avalanche danger will continue to slowly decrease Saturday as newly formed wind slabs begin to gain strength. Even though the avalanche danger is decreasing, you may still trigger a small 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.
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.
Older weak snow still exists deep within the snowpack. Deep persistent slab avalanches have been the culprit in several accidents and fatalities over the last month. While we have not seen any recent avalanches on this layer, professionals in the field continue to find this layer 4-6 feet below the snow surface. This is a low likelihood high consequence scenario. While it may be difficult for a traveler to trigger a deep slab, smaller avalanches or other large triggers may step down into this deep layer creating large and destructive avalanches. You can avoid being involved with a deep slab avalanche by staying off large steep open slopes on W-N-E aspects near and above treeline.
Snowpack Discussion
Around 4-8†(10-20cm) of new snow fell near Stevens and Snoqualmie Passes Thursday and Friday. 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.Â
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.Â
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.Â
Avalanche 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: Possible
Expected Size: 1 - 1
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: Unlikely
Expected Size: 1 - 2