Avalanche Forecast

Archived

Mar 8th, 2014–Mar 10th, 2014

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

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.

A decreasing avalanche danger is expected on Sunday. Wet snow avalanches should still be the main concern Sunday.

Detailed Forecast

Another potential major wet loose avalanche cycle is likely during frontal passage and heavy rain or snow at warmer temperatures Saturday afternoon and night. Cornice failures are also likely during that period.

But rain or snow showers decrease rapidly on Sunday morning at slightly cooler temperatures. Significant new snow is not expected or should be very limited. A decreasing avalanche danger is expected on Sunday.

The main concern will be lingering wet loose snow avalanches. This is most likely on steep slopes that do not release on Saturday night. Watch for further pinwheeling of surface snow and wet snow still deeper than a few inches which often precede this type of avalanche. Loose wet avalanches may still entrain deeper wet snow and be large and destructive.

New small shallow of wind slab might be possible above treeline. The lack of new snow should limit this new concern. Watch for signs of firmer wind transported snow.

Small shallow new areas of storm slab might be also be possible above treeline. But again the lack of new snow and the cooling trend should greatly limit this concern.

Snowpack Discussion

Past

An extended mild and dry period occurred the latter half of January generally forming a crust in all areas.

Very cold weather followed in early February which produced light low density snowfall and facets just above the late January crust.

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

Warm and mainly dry weather at the end of February led to wet snow avalanches and melt/freeze crusts throughout the region.

The current storm cycle began last weekend with initially cool temperatures and poor bonds to the end of February crust in many areas. This storm cycle is turning out to be almost as wet as the last one! Water equivalents and snowfall for the current storm cycle will be about 9-11 inches but only about 1.5-6 feet of snowfall ending Sunday morning. This has been producing many avalanches all week including reports from Holden Village, Stevens Pass, Tumwater Canyon, Alpental and Snoqualmie with pass closures at times last week.

5 March wet loose avalanche on to the ground on Spiral Butte at White Pass. Photo John Stimberis/WSDOT.

Recent

A short break in the current storm cycle was seen on Friday causing some further consolidation and temporary stabilizing.

NWAC observer Jeff Hambelton on Friday reported many small wet loose avalanches in the Mt Baker area. He found a moderate SP shear at 45 cm at a recent interface.

Alpental ski area reports a good refreeze of the wet snow surface on Friday night and temporary relative stability on Saturday.

The late January crust and faceted layers from early February are now very deeply buried near and west of the crest at about 2-3 meters and have been getting thoroughly tested by the recent storm cycles and heavy precipitation. There are no known avalanches to this layer in this area since 22 February at Chair Peak at Snoqualmie. We are close to dropping the deep persistent slab concern in this area due to the deep depth and lack of activity of the layer. However, we will wait until the current storm cycle comes to an end. Will try to track this layer as we move into the spring. Here is a link to more information about deep persistent slabs which are 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.

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.

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.