Avalanche Forecast

Issued: Apr 21st, 2018 11:19AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Loose Wet and Cornices.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Warm temperatures and sunny skies will quickly weaken the snowpack Sunday. When you see new rollerballs, observe fresh fan-shaped avalanche debris, or experience wet surface snow deeper than your ankle expect to be able to trigger loose wet avalanches on similar slopes.  Do not linger in areas overhung by cornices that may fail without warning with Sunday’s warm sunny weather.

Summary

Detailed Forecast

Expect warm air temperatures and sunny skies to allow wet avalanche conditions to quickly develop Sunday. New rollerballs, fresh fan-shaped avalanche debris, and wet surface snow deeper than your ankle all indicate that you may be able to trigger a loose wet avalanche on similar slopes. Expect these conditions to develop first on E-S aspect but become prevalent in all locations as the day continues. Don’t underestimate these loose wet avalanches. Avalanche observations form the past week have reported small loose wet avalanches gouging into older snow layers, growing large, and traveling further than normal.

Cornices have grown large over the winter in many locations. As the temperatures warm and the sun comes out, these hanging blocks of snow will begin to droop and fail. It is very difficult to determine when and where this will occur. Do not linger in locations where cornices are above you.  Choose travel routes and techniques that minimize your exposure this difficult to assess overhead hazard.

We have received reports of isolated but very large deep wet slab avalanches over the last week in many of the Cascade West zones. This is a reminder that older deeper weak layers may still exist in the snowpack. These layers have become wet with recent warming. In most cases these avalanches appear to have been triggered by a cornice fall or a loose wet avalanche. Do not hangout in locations below very large avalanche paths where these low-likelihood high-consequence avalanches could occur.

Snowpack Discussion

Generally cold temps and radiational cooling should allow for a strong overnight freeze of the snowpack Saturday night. Minimal new snow was reported at NWAC weather stations Saturday. Locally higher amounts may have occurred in areas closer to the Cascade crest.

In most near and below treeline locations the surface snow consist of a melt-freeze crust. The thickness and strength depends on aspect and elevation. On shaded slopes above treeline pockets of cold dry surface snow may still be found. The upper snowpack is a combination of frozen melt-freeze crust, weak large melt forms, and strong rounded grains. Areas of weak older snow can be found in some locations, however we do not have adequate information about the distribution of these layers.

We have very limited information about older persistent weak layers on the east slopes of the Cascades. You are most likely to find a reactive persistent weak layer on shaded aspects or at higher elevations above recent rain events. Large cornice failures are the most likely way to trigger such a layer. If you have any information on snowpack conditions consider sharing your observations with NWAC on our public observations page.

Two common persistent weak layers seen in the Northeast Cascades are:

  • 3/25 and 3/22: Buried surface hoar layers found on shaded aspects near and above treeline.
  • 2/13 Facets above a firm and thick melt-freeze crust can be found in many locations near and above treeline. This layer is thought to be dormant at this time but may re-awaken with a substantial weather input.

Lower elevations further east of the crest have melted back for the season and no longer present an avalanche risk. 

Observations

An avalanche professional traveling in the Stuart Range this week reported a widespread loose wet avalanche cycle Thursday with warm air temperatures and clear skies. Above 6500’ dry snow was still found including lingering wind slabs. This resulted in a skier triggered wind slab on an E aspect at 7800’. Several large and very large (D2-D3) slab avalanches from earlier in the week were also observed.

Problems

Loose Wet

An icon showing Loose Wet

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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Aspects: East, South East, South, South West, West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Cornices

An icon showing 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 lips 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.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 22nd, 2018 11:19AM