Avalanche Forecast
Regions: Cascades - North West.
Wind slabs formed over the past few days will slowly gain strength Saturday. You will be able to trigger a wind slab avalanche steep wind loaded slopes with convex rollovers, unsupported features, or below fresh cornices. Use visual clues such as snow drifts, cornices, and firm snow with cracking to identify and avoid this wind loaded terrain. Older deep persistent slabs can still be found. You can avoid triggering a deep persistent slab by staying off of steep large open shaded slopes where these bigger avalanches may occur.
Detailed Forecast
Avalanche danger will slowly decrease Saturday as new wind slabs begin to gain strength. Even though the hazard is decreasing, you can still trigger a wind slab on steep slopes near ridgeline and on cross-loaded mid-slope features. 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 loose wet avalanches, rollerballs, and pinwheels. Be ready to move off of and avoid steep sunny slopes during periods of intense sunshine. With higher snowfall totals near Mt Baker, these loose avalanches may become large.
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 5-6 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.
Snowpack Discussion
Around a foot of snow has fallen in the Mt Baker are as of Friday afternoon. Winds during the storm transported snow forming wind slabs near ridgelines and on cross-loaded features. Several small wind slabs were triggered by backcountry travelers in the Mt Baker area Friday.
Rain on Thursday reach around 5000’ before transitioning to snow. This formed a 1†(2-3cm) thick rain crust and created a generally good bond between the old and new snow. Below this rain line a wet and refreezing snowpack has been observed. 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.
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 5-6 feet below the snow surface in the Baker area. You are most likely to find this layer still intact on shaded aspects near and above treeline.
Observations
NWAC professional observer Lee Lazzara traveled in the Mt Baker backcountry Friday. Lee reported 1’ (30cm) of new snow over a growing melt freeze crust. He observed several small skier triggered wind slab avalanche near ridges and on cross-loaded terrain.
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: Likely
Expected Size: 1 - 1
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
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