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Avalanche Forecast

Archived

Mar 9th, 2014–Mar 10th, 2014

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

Regions

Stevens Pass.

Less avalanche danger is expected on Monday.

Detailed Forecast

Mostly light rain or snow showers possibly heaviest in the central Cascades should be seen on on Monday and end Monday night at slightly lower snow levels. But amounts should be light in the Washington Cascades and not cause a significant or extensive new avalanche danger.

Further consolidation and stabilizing should be the main trend and avalanche danger will be lower on Monday than the past few days.

The equinox is not too far off and the sun is gaining power. Further triggered wet loose avalanches will be possible on Monday mainly below treeline. Watch for pinwheels and surface wet snow deeper than a few inches.

Old areas of wind slab should still be seen at the highest elevations. Watch for signs of firmer wind transported snow in the highest areas above treeline.

Deep persistent slab is deeply buried and should be hard to trigger but would be mostly to be triggered in areas with a shallower snow pack. This would be a low probability but high consequence event. It is probably still best to avoid shallow snow near large steep open avalanche terrain.

Snowpack Discussion

The past month has had 2 major storm cycles and periods of avalanches in the Cascades.

The first two week storm cycle ended about 25 February that produced about 7-12 feet of generally wet heavy snowfall and avalanches near and west of the crest.

The second storm cycle is coming to an end on Sunday morning. This storm cycle turned out to be basically as wet as the last one! Water equivalents and snowfall for the past week are about 9-13 inches and 1.5-6 feet respectively for NWAC stations near and west of the crest. So this has been another period of rain and wet heavy snowfall. This produced many avalanches the past week including reports from Holden Village, Stevens Pass, Tumwater Canyon, Alpental and Snoqualmie with pass closures at times last week.

Wet loose avalanche on 5 March on Spiral Butte at White Pass. Photo John Stimberis/WSDOT.

The last front of the second cycle moved across the area on Sunday morning and a few more avalanches were seen in some areas. The Mt Baker ski patrol reported 2 explosively triggered large wet loose avalanches that destroyed trees but nothing human triggered. The Alpental ski patrol Snoqualmie DOT reported saturated surface snow layers but limited effects from all the rain Saturday night. A snow cat triggered a large wet loose avalanche at White Pass on Sunday morning at 5000 feet.

Looking down on a snow cat triggered large avalanche at White Pass on Sunday morning 9 March. Photo by Chris Talbot/White Pass Ski Area.

The NWAC received a call this afternoon from the Crystal Mountain ski patrol where explosive avalanche control is producing consistent results to deep layers from early this winter. This is generally on east to northeast slopes at about 6000 feet with crowns up to 6 feet and some trees destroyed. So persistent deep slab layers will still be listed as a concern near and west of the crest. These avalanches may still be releasing at the January crust and faceted layers from early February. We will continue to try to track this layer as we move into the spring. Here is a link to more information about deep persistent slab which is unusual in our area.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the 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. 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.