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Avalanche Forecast

Archived

Mar 14th, 2017–Mar 15th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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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With high snow levels forecast through mid-day Wednesday, wet snow avalanches will become likely for all elevation bands along the east slopes of the Cascades. Fresh wind slab will likely build higher in the alpine. Dangerous avalanche conditions are expected Wednesday with increasing danger with elevation. 

Detailed Forecast

With high snow levels forecast through mid-day Wednesday, wet snow avalanches will become likely for all elevation bands along the east slopes of the Cascades. Fresh wind slab will likely build higher in the alpine. Dangerous avalanche conditions are expected Wednesday with increasing danger with elevation. 

Loose wet avalanches are likely on steeper slopes for areas and elevations that see rain. While most loose wet avalanches will be shallow, a few may gouge down to deeper layers and become more powerful than you expect. 

Cornices will be weakened and prone to failure due to rain and mild temperatures. Very large cornices have been reported in many areas and have been involved in recent accidents and close calls. A cornice failure could trigger a large and destructive wet slab avalanche. 

Wet slab avalanches are highly unpredictable and not necessarily tied to peak warming or rainfall. Besides their huge destructive potential, wet slab avalanches often surprise backcountry travelers when they run further than expected. 

Expect new or previous wind slab mainly on NW-SE aspects due to recent SW-W winds. Continue to watch for firmer wind transported snow on all aspects especially in areas of complex terrain.

The persistent slab problem previously listed in the NE zone has taken a temporary hiatus from the avalanche problem set until further snowpack investigation post-storm cycle. However be aware that avalanches stepping down to the Valentine's Day or 2/17 crust would be large and very dangerous.

Snowpack Discussion

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. 

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday 3/9. On Thursday night rain may have pushed up to about 4000 feet in the northeast Cascades and about 5000 feet in the southeast Cascades. By Friday morning 3/10 NWAC stations along the Cascade east slopes had about 0.35- 0.90 inches of WE (water equivalent) with only 0-5 inches of snow.

Moist W-SW flow and a stalled frontal boundary have produced periods of light to moderate rain and snow along the east slopes of the Cascades Monday and Tuesday. There was a strong downslope drying effect with much less precipitation east of the crest during this time. Precipitation totals through 4 PM PST (or 5 PM PDT).  Snow levels pushed above 6500 feet Tuesday afternoon with the summit stations at Mission Ridge, Washington Pass and Dirty Face Peak all above freezing.  

Recent Observations

North

On Saturday 3/11 a report for Delancy Ridge via the NWAC Observations page indicated many natural large loose wet avalanches on solar slopes.

The NCH on Saturday 3/11 reported significant recent warming and settlement. Recent natural size D1.5 - D2 wind and storm slab were seen in the Silver Star area. Pits in the upper Willow drainage had generally right side up layers with the Valentine's Day crust seen at 100-128 cm.

The NCH were in the Varden and Silver Star areas on Sunday 3/12 and reported many natural size D1-2 storm slabs on steep north slopes in the 5200-8400 ft range. Some cracking and propagation was observed. Shallow storm slabs were reactive about 10 cm down. 

Central

No recent observations. 

South

No recent observations. 

Problems

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.

Wet Slabs

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

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.