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Avalanche Forecast

Archived

Mar 9th, 2018–Mar 10th, 2018

Alpine
Natural avalanches possible, human triggered probable.
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 possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

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Several overlapping avalanche problems will create dangerous avalanche conditions Saturday. Avoid steep slopes over 35 degrees where avalanches start. Expect changing conditions through the day as warming and a strong sun effect the snow surface making it easier to trigger all avalanches.

Detailed Forecast

A new persistent slab avalanche problem has been observed in the Stevens Pass area and in locations along the Eastern Cascades. Newly buried surface hoar and facets have resulted in several avalanches on a variety of aspects (E, SE, S, NW) between 4400-5400 feet. Current observations have not found this layer in other Western Regions. However, use caution. If you experience sudden collapses, shooting cracks, or whumping noises, avoid all nearby avalanche terrain.

You will be able to trigger wind slabs on lee slopes and cross-loaded features. Use visual clues such as snow drifts, uneven snow surfaces, and fresh cornices to identify and avoid nearby slopes where the wind has deposited snow.

Expect new rollerballs, pinwheels, and small loose wet avalanches to occur on sunny aspects throughout the day. These conditions will occur first on steep rocky slopes receiving direct sunshine. Stay off of steep slopes where you see signs of wet surface snow conditions.

Persistent and deep persistent slab avalanche may still occur. To avoid these low likelihood - high consequence situations, stay out of large open slopes that may harbor this difficult to predict and manage avalanche problem. While it is difficult to trigger these types of avalanches, smaller surface avalanches such as loose wet and wind slabs may step down, failing into deeper older layers.

Snowpack Discussion

Several new avalanches were reported Friday. Small natural wind slab avalanches were observed near Alpental Ski area on steep NE facing terrain at 5400’. A cornice triggered wind slab was observed in the Crystal Backcountry on a E slope at 6400’. Several very large new crowns estimated to be 4-5' tall were observed in the Sourdough Mountains NE of Mt Rainier. These avalanches were seen on NE facing slopes around 6500’. Most of these avalanches occurred Thursday night into Friday morning.

New snow fell across the western regions of the Cascades Thursday night and Friday. Significant changes in snow totals were experienced with elevation. Wet heavy snow and rain was observed in up to 5500’ Crystal, 4400’ Snoqualmie, and 4300’ farther north. Above these elevations, 12-20 inches of new storm snow accumulated.

Generally SW winds transported the snow in all regions forming new wind slabs on lee slopes and cross-loaded mid-slope features.

This new snow has fallen on a variety of old snow surfaces including settled cold snow and new melt-freeze crusts. There is potential in some locations that surface hoar and/or near surface facets were buried.

Several older persistent weak layers exist within the snowpack. On E-S-W aspects a thin facet-crust combo (2/23) can be found. Snowpack test results show this layer healing but it has been reactive in some snowpack tests. An older deeper and more widespread persistent weak layer has been observed for several weeks. Weak sugary facets (2/13) sit just above a firm crust formed and buried in early February (2/8). This crust is generally found about 3-4 feet below the snow surface.

There are no other significant layers of concern below the 2/8 crust.

Observations

Baker

An avalanche professional at Heather Meadows reported significant wind transportation of snow. He did not find a buried persistent weak layer below the recent storm snow in the locations he observed.

Snoqualmie

An avalanche professional at Alpental reported rain up to 4400 feet Thursday night. Two new natural wind slabs were observed in steep NE facing terrain.

SouthNWAC professional observer Jeremy Allyn traveled in the Crystal Backcountry Friday. He found significant wind transportation of the new snow above 6000’. A cornice failure triggered a widely propagating wind slab 2 feet deep on an E aspect of East Peak. Observations generally demonstrated the new snow was bonding well to the old snow surface.

Crystal Mountain ski patrol reported several new large crowns seen across the White River valley in the Sourdough Mountains. Avalanches occurred on NE aspects and were estimated to be 4-5 feet deep. Wind transported snow was observed on the upper mountain.

Problems

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.