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Avalanche Forecast

Archived

Mar 27th, 2017–Mar 28th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Fresh wind slabs and cornices will be the main problem Monday. Loose-wet avalanches will become likely on steeper sun exposed slopes during extended sun breaks Monday. Keep terrain selection simple and conservative. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches. 

Detailed Forecast

Rain and snow late Sunday should change to showers with cooling overnight. Showers and moderate ridgetop winds should persist early Monday before tapering late Monday. This should maintain unstable wind slabs on lee slopes below ridges, in exposed terrain near and especially above treeline. 

Recent winds have been mostly SW-S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline. However, due to terrain effects, wind slabs may have formed on a variety of aspects 

Recent cornices are very large in many areas. Make sure to avoid areas on ridges where there may be an overhanging cornice as well as travel on steep slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Snowpack Discussion

Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please see NWAC's blog post issued Sunday, March 26. 

Weather and Snowpack

The first week or so of March was very cool and snowy. NWAC and NRCS stations indicate about 2 feet of snow in the northeast Cascades with less elsewhere along the Cascade east slopes. 

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered more rain than snow to lower/mid elevations along the east slopes of the Cascades. At least two regional avalanche cycles occurred during the stretch. The most recent cycle from mid-March had many massive slides that covered Hwy 20 near Washington Pass up to 40' deep in places! Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. Far less precipitation was seen further east of the crest during this period for areas like Blewett/Mission Ridge. 

A strong low pressure system brought 6-12 inches of snow in the northeast and central-east slopes of the Cascades Friday, 3/17 and mostly rain for the southeast Cascades. This was followed by snow levels rising to 6000-6500 feet in the northeast and central-east Cascades and likely 7000 ft in the southeast Cascades by Saturday morning 3/18. 

During this past week, weaker fronts crossed the Northwest from Tuesday through Saturday with light amounts of new snow along the east slopes of the Cascades, mainly near and above treeline and near the Cascade crest. The Barron Yurt near Hart's Pass reported about 20 cm (8 inches) of recent storm snow as of Saturday afternoon. About 6 inches of storm snow had accumulated in the Mission Ridge area.   

Recent Observations

North

Guides near Washington Pass Tuesday, 3/21 reported hearing many natural avalanches near midday following a prolonged sunbreak and subsequent brief warm up. Numerous avalanches were likely small, however, some avalanches sounded much larger, likely involving deeper layers below the relatively shallow recent storm snow.  

NCH reported no significant new avalanche activity observed outside of one small natural storm slab on a north aspect above treeline Thursday.

Reports from the Barron Yurt on Friday and again Saturday indicate the recent wind transport was mainly shallow but evident near ridges. Recent SW winds were building shallow wind slabs as of Saturday and were avoided. There were a few small natural avalanches that occurred on very steep shaded terrain, mainly above 40 degree's. These were D1-D1.5 in size and unknown whether they loose-dry or shallow wind slabs. No triggered slides were observed Saturday. 

Central

On Friday, Mission Ridge pro-patrol reported very sensitive storm and/or wind slab up to 6-8 inches deep on lee aspects near and above treeline. NW aspects were especially sensitive.

NWAC pro-observer Tom Curtis was on the lower slopes of Mt. Cashmere Friday below treeline. About 5 cm (2 in.) of new snow had accumulated in this area. On steep test slopes, Tom easily triggered loose wet avalanches entraining moist underlying snow with the potential to become large. Although not directly observed, Tom heard several large natural avalanches release up the Trout Cr drainage along ridge-crest in the morning. It is possible these avalanches were cornice triggered.  

South

No recent observations. 

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Loose Wet

Loose Wet avalanches are the 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. 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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.