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Avalanche Forecast

Archived

Apr 14th, 2017–Apr 15th, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
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

Stevens Pass.

Afternoon sunshine or sunbreaks should increase the loose wet potential on steeper solar aspects Saturday. Initially shallow loose wet avalanches could entrain deeper layers and become large in specific locations. Watch for lingering wind slab on lee slopes in steeper terrain primarily above treeline. Give cornices a wide safety margin.

Detailed Forecast

The cool upper level trough that's been plaguing the Pacific Northwest for several days should finally begin to loosen it's grip on the area Saturday. Partial clearing is expected in the southwest and central-west Cascades Saturday afternoon while the northwest Cascades should retain more cloud cover. Snow levels will remain low and winds should be light. The Mt. Baker area will maintain the Considerable rating above treeline due to recent wind transport, continued cold temperatures and the potential for larger wind slab in this zone/elevation band.  

After a cool night Friday, snow levels will remain on the cool side Saturday. However, afternoon sunshine or sunbreaks should increase the loose wet potential on steeper solar aspects Saturday. Initially shallow loose wet avalanches could entrain deeper layers and become large in specific locations. This time of year even brief sunbreaks combined with subtle daytime warming can quickly activate loose wet avalanches. Be especially wary of the loose wet avalanche potential near terrain traps. 

Older wind slab may still be sensitive and linger on lee slopes in steeper terrain, primarily on NW-SE slopes and above treeline on Saturday.  

Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. There have been numerous recent cornice failures with some being very large in the Washington Cascades. Five people were tragically killed by a cornice release in British Columbia on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

Heavy rain received in mid March has left behind a well consolidated snowpack with one or more strong melt freeze crusts in the upper portion of the snowpack in the near and below treeline elevation bands. 

A series of strong spring storms occurred during the first week of April with high water amounts seen particularly on the volcano stations (Baker and Paradise) and with lower amounts in the Cascade Passes.

Mostly sunny weather Tuesday helped storm snow begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow.

A weather system passing through the area Wednesday night produced half an inch to 1 inch of water across the west slopes of the Cascades through Thursday morning. This translated to a few inches of new snow around 4000 feet with 4-6 inches at the higher NWAC stations at Alpental, Paradise, Chinook Pass and Green Valley at Crystal. Showers increased during the day on Thursday with light additional snow accumulations mixing with graupel during more intense showers. By Friday morning an additional 2-4 inches of snow had accumulated above 4000 feet. Friday was also cool, cloudy and showery. 

Recent Observations

North

NWAC's Lee Lazzara and Jeremy Allyn were in the Mt Baker backcountry Tuesday, 4/11. Cornices were once again observed to be very large with at least 5 large cornice failures noted, likely releasing in the past day or two. These cornices pulled out D2 sized slabs on the slopes below. 

Mt. Baker pro-patrol reported one natural wind slab off of Mt. Herman on Thursday. In area the new snow was not reactive to ski cuts at any elevation or aspect. Despite a steady W-SW wind atop Pan Dome Thursday, little loose surface snow was available for transport near and below 5000'. Fresh wind transported snow was observed above treeline outside the ski area. 

Central

Stevens Pass pro-patrol saw little in the way of results with explosives Thursday. 

Alpental and Crystal resorts had minimal avalanche control results on Friday. 8-10" of recent storm snow was at the top of Alpental above wet grains, but was stubborn to trigger and did not entrain deeper layers.  

South

WSDOT Chinook Pass crews checked in Wednesday and Thursday.  On Wednesday, shallow loose wet avalanches were sensitive to ski cuts below 5000' feet and running well. On Thursday, cooler temperatures helped limit the loose wet potential to below 4800' on solar aspects. Isolated pockets of wind slab could be found on lee slopes below ridges. About 14-18" on storm snow sat above the most recent crust in non-sun/wind affected terrain. 

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.

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.

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.