Avalanche Forecast
Issued: Mar 21st, 2018 11:10AM
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
Avalanche conditions will change Thursday as new shallow wind slabs develop and grow throughout the day. You are most likely to trigger a wind slab on steep slopes where active wind deposition is occurring. A low likelihood-high consequence deep persistent slab still exists in the snowpack. You can avoid this difficult to assess problem by staying off of large steep open slopes on shaded aspects.
Summary
Detailed Forecast
Small shallow but reactive wind slabs will form during the day Thursday. You can use simple observations to identify where new wind slabs are forming. Blowing snow, fresh cornices, and plumes all indicate winds slabs are forming on slopes below. Stay off of steep wind loaded features where you may trigger new wind slab avalanches.
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. This is a low likelihood high consequence scenario. While it may be difficult for a traveler to trigger a deep slab, smaller avalanches 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.
Snowpack Discussion
A few inches of new snow will fall around the area by Thursday morning. At lower elevations rain will change over to snow during the morning. The transition from rain to snow may allow for a more favorable bond between the new and old snow. Above the rain line new snow will be falling in a variety of snow surfaces. On sun exposed aspects very firm and thick surface crust have formed during the recent sunny weather. In shaded areas near surface facets and soft settle snow have been observed within the past week.
The mid-snowpack generally consists of several well bonded layers of melt-freeze and strong rounded grains. In isolated locations, thin buried persistent weak layers have been reported. You are most likely to find a buried persistent weak layer in shaded locations (see a recent public observation), at higher elevations, and/or in terrain slightly east of the Cascade Crest.
An old weak layer of sugary facets can still be found around the 2/8 crust. Very firm and thick surface crust on sun exposed slopes will greatly reduce the ability for a deep slab avalanche to occur on SE-SW aspects. However, where this protective layer does not exist, smaller avalanches may step down to this deep layer. This very distinct crust can generally be found 4-6 feet below the snow surface. While snowpack tests are good for demonstrating the presence of a weak layer, they are not a decision-making tool to determining whether or not a slope is safe.
Observations
Stevens
NWAC forecaster Josh Hirshberg reported melt form snow grains in the top 2 feet of the snowpack. The 2/13 layer (about 3 feet below the surface) had received liquid water and transitioned to melt forms, as well.
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.
On Saturday, observers reported a large wet natural avalanche that stepped into older layers on a SW aspect at 4500 ft near Red Mountain and several large loose wet avalanches on E-facing aspects in Gold Creek with numerous small Loose Wet natural avalanches on all aspects.
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 22nd, 2018 11:10AM
