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Avalanche Forecast

Archived

Mar 29th, 2017–Mar 30th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
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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Very dangerous avalanche conditions are expected on Wednesday and travel in avalanche terrain is not recommended in this area. Wed AM Update: Added storm slabs to problem set due to delayed warming trend along the west slopes of the Cascades. 

Detailed Forecast

Wed AM Update: Added storm slabs to problem set due to delayed warming trend. 

A warm front is moving over the Northwest on Tuesday. A second frontal system will move over the Northwest Tuesday night and Wednesday. The second system will cause periods of moderate to heavy rain or snow with further gradually rising snow levels Tuesday night and Wednesday.

An avalanche cycle is expected by Tuesday night and should last into Wednesday. Snow that has accumulated since the mid-March avalanche cycle may be involved or entrained in avalanches.

SW alpine winds are likely to begin to build new wind slab on mainly N-SE slopes on Tuesday night and Wednesday. Alpine winds have been predominately from the SW-WSW the pasts few days with some SE winds seen at the top of Crystal. Hence NW to SE slopes will be indicated.

New and potentially deep storm slab is very likely at all elevations due to heavy precipitation rates and warming. Wet slab avalanches won't be listed but are possible at lower elevations later in the day.

Loose wet avalanches will be likely in the near treeline and below treeline where there is significant rain and warming.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Very dangerous avalanche conditions are expected on Wednesday and travel in avalanche terrain is not recommended in this area.

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 stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

A strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes on Friday, 3/17 and caused another wet snow avalanche cycle in the Mt. Baker area Friday night into early Saturday morning. 

This past week has also been active weather-wise, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 5 days ending Monday morning NWAC stations near and west of the Cascade crest have picked up 1.5-4 ft of snow with the most at Mt. Baker and above the Pass levels.

Recent Observations

North

The Mt Baker patrol reported heavy snow and increasing 6-14 in storm slab from explosive avalanche control on Tuesday morning.

Central

NWAC Observer Matt Schonwald was in the Alpental Valley Saturday and identified about 14-18 inches of storm snow over the past few days had a favorable density profile. There was evidence of shallow storm slabs being triggered earlier in the day, but gained significant strength through the day. On solar aspects daytime warming caused shallow, loose-wet slides, mostly D1, up to D1.5.  

South

On Friday, as the snow-line crept up during the day natural loose wet avalanches occurred along the Paradise Road above 5000'.  NPS rangers estimated the loose wet slides entraining about 8-10" of the recent storm snow. 

NWAC Observer Jeremy Allyn was in the Crystal BC Saturday, in Lakes Basin and near Norse Peak. Recent storm snow varied widely based upon elevation and aspect with up to 20 inches of storm snow or more near and above treeline on non-solar terrain, over the hard rain crust from last weekend. Many steep north aspects within the terrain had been skied both earlier and on Saturday with no triggered avalanches noted. Test profiles and snowpack tests at 6400 feet on a NW aspect below ridgeline, identified weak faceted snow,both above and below the rain crust buried about 20 inches in that location.  

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

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.