Avalanche Forecast

The avalanche danger Wednesday will increase as winds and snowfall increase throughout the day.

Fresh Wind Slab avalanches will become likely to trigger as they build on lee aspects near and above treeline. Watch for signs of snow movement such as tracks filling in and blowing snow. Fresh wind slabs will generally be small and easily triggered. Older, more stubborn but larger wind slabs will linger underneath shallow new snow. Given the variety of avalanche concerns in the snowpack, give any slopes that may be loaded by wind a very wide berth. Assume that a wind slab has the ability to step down to deeper layers. 

A new persistent weak layer has been identified within the upper snowpack on the West Slopes of the Cascades, this time driven by a slick sun crust within the upper snowpack (see the Snowpack Discussion below for more details). Triggering large avalanches on this interface 1.5-3' down will be possible on Wednesday. The distribution throughout Cascades of this layer is still being confirmed, but it is likely to exist on steep solar slopes that have not been wind-stripped of snow near and below treeline. In some locations, faceted snow above the crust may provide a weak layer that enhances the propagation of slabs on this layer.

The Deep Slab problem has become difficult to trigger, but it exists on all aspects and should remain in the back of your mind should you venture into open or steeper terrain. Cornice fall, wind slab or persistent slab all may step down to trigger a large or very large avalanche on this layer of facets sitting on the 2/13 crust. While Deep Slab avalanches may be difficult to trigger, your chances of surviving one are slim. Your best way to stay safe from these avalanches is to avoid triggering avalanches in the upper snowpack. Excellent travel conditions can still be found on slopes less than 30 degrees that are not connected to steep overhead slopes.

With all avalanche problems present, use conservative decision-making and terrain choices. Minimize your exposure to areas where avalanches commonly run by staying away from open slopes steeper than 35 degrees and out from under large avalanche paths.

The snowpack in the Cascades is complicated. The good news is that the avalanche danger had decreased markedly as we move beyond the dangerous avalanche conditions that developed Sunday and in some cases lingered into Monday. The bad news is that many of these problems may re-awaken with increased snow/wind loading or with a large trigger such as a cornice failure. 

Winds over the weekend stripped start zones in some areas of snow and created wind-affected snow conditions in exposed areas at all elevations some parts of the Cascades. Additional wind transport was observed on Tuesday with moderate winds behaving unpredictably and at times gusting to strong. Up to 4 inches of fell during the day Tuesday. 

Heavy snowfall and strong winds elevated avalanche danger over the weekend. 2-3 feet of snow accumulated across the West Slopes since Friday. This fell on a variety of surfaces including thin sun crusts, old snowflakes, surface hoar, and possibly some small facets. 

This slab of storm snow now sits on a melt-freeze crust/facet combo buried by storm snow on 2/23. This interface sits 1.5-3' down and exists as a persistent weak layer that is possible for humans or cornice fall to trigger. The layer produced avalanches on solar aspects Sunday and Monday which took three lives in two separate accidents near Snoqualmie Pass. NWAC field staff traveled to both accident sites to investigate the avalanches in preparation for forthcoming accident reports. Our condolences go to the friends and families of the victims. The interface also produced large avalanches that were human, natural, or cornice-triggered in the Stevens and Crystal Backcountries on these same steep sun-affected slopes.

A very large natural avalanche crossed Highway 20 east of Newhalem on Sunday.

A thin rain crust formed and was buried on 2/17 near and below 4000’ in the Snoqualmie area. Limited information about this snowpack layer has been reported other than it is ski-supportable. 

Over a week ago, observers reported a widespread cycle of large to very large avalanches ran with a potent storm snow as well as on facets, buried on the 13th of February. In many locations these weak sugar-like facets sit 3-4 feet below the surface on or adjacent to a hard crust layer. If you dig that far into the snowpack, you will find a thin layer of sugar-like facets adjacent to a hard crust. You can use snowpits and snowpack tests to confirm the strong over weak snow layers of the Deep Slab avalanche problem. Avalanches, collapses, and whumphs have been triggered on this persistent weak layer well after last weekend’s initial avalanche cycle.

Snowpack tests and observations from around the region continue to demonstrate that this layer can fail and produce avalanches. Tests and profiles show the most consistent indication for triggering Deep Slabs at Paradise on Mt Rainier and Stevens and Snoqualmie Pass areas. In the Mount Baker and Crystal areas the problematic deep weak layer is more intermittent. At Mount Baker it can often show up as facets around a series of crusts.

Below the 2/8 crust there are no significant layers of concern.

Observations

Stevens Pass

On Tuesday, an avalanche professional in the Grace Lakes area of the Stevens Pass found wind blowing and forming shallow wind slabs and recent storm snow stabilizing. He also found consistent results in snowpack tests indicating the 2/13 facet layer down 3.5 feet was still likely to propagate on various aspects . Another avalanche professional reported two recent large avalanches in Tunnel Creek on south aspects near or below 4800'; one skier triggered and the other natural and likely both from Monday. Both slides likely ran on the sun crust buried 2/23.

On Monday, professional observations in the Stevens Pass area confirmed that the deep persistent layer we have been tracking was buried about 4 ft below the surface was still likely to trigger based on multiple PST snowpack test results. Storm layer instabilities were gaining strength rapidly. The crowns of numerous large natural avalanche that likely released during the storm Sunday were visible along Skyline and Windy Mtn near the pass.   

On Sunday, Stevens Ski Patrol reported reactive conditions, triggering avalanches up to 3 feet deep in the recent storm snow with control work. NWAC staff observed Wind Slabs forming in open areas below treeline. They also reported tests indicating skier triggering on the 2/13 facets 90cm below the surface.

Snoqualmie Pass

On Saturday and Sunday, NWAC staff reported multiple locations near Snoqualmie pass that while the 2/13 facet layer is showing rounding, test results indicated that the potential for triggering an avalanche still exists. On Saturday, NWAC observers and Alpental Patrol reported numerous soft slab avalanches in the recent snow.

South

On Tuesday an avalanche professional identified two recent large avalanches in the Crystal Backcountry at 6400' and 6600' on SE and E aspects respectively. In both cases the slides were triggered by a large natural trigger (cornice or ice fall).  One avalanche stepped down 2' to the 2/23 sun crust/facet interface found elsewhere. 

On Sunday, NWAC observer, Jeremy Allen reported blizzard conditions, small triggered slab avalanches in the new snow, and signs of rounding and variable test results on the 2/13 facets.