Avalanche Forecast
Issued: Mar 22nd, 2018 11:00AM
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
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 by staying out of large steep open slopes where these bigger avalanches may occur.
Summary
Detailed Forecast
Slightly lower snow fall amounts will minimize but not eliminate the development of avalanche hazards near Stevens and Snoqualmie Passes Friday.
New small wind slabs have formed on a variety of aspects near and above treeline. You can avoid these avalanches by staying off of steep wind loaded slopes. Use visual clues to assess and avoid wind loaded features. Snow drifts, fresh cornices, and uneven snow surfaces with cracking can all indicate wind slabs exist on nearby slopes
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 4-5 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.
The new snow is falling on a variety of snow surfaces including firm surface crust and near surface facets. 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. This is most likely to occur in areas where more than 6-8 inches of new snow has fallen.
Snowpack Discussion
Roughly 4-6 inches of new snow had fallen as of Thursday afternoon in the Stevens and Snoqualmie Pass areas with generally cooling temperatures. Rain was reported to at least 5000 feet in the Stevens area. At lower elevations rain turned to snow as freezing levels lowered Thursday afternoon. This likely created a favorable bond between the new and old snow surfaces and generally right-side-up storm snow.
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.
Moderate occurring with the bulk of the precipitation transported new snow forming small shallow wind slabs near and above treeline. Wind deposition of snow may be found near ridgelines and mid-slope cross-loaded terrain features.
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 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: All elevations.
Likelihood
Expected Size
Valid until: Mar 23rd, 2018 11:00AM
