Avalanche Forecast

Issued: Mar 14th, 2018 11:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Persistent Slabs, Loose Wet and Wind Slabs.

NWAC, Northwest Avalanche Center

Cascades - East.

You may be able to trigger avalanches Thursday in specific locations such as steep rock sunny slopes, wind deposited features, or where weak older snow still exists in the snowpack. Take time to identify and avoid areas in the terrain where these avalanches may occur. Even though it is becoming more difficult to trigger persistent slab avalanches, a smaller avalanche may step down into these older layers.

Summary

Detailed Forecast

Expect new rollerballs, pin wheels, and loose wet avalanches on sunny slopes as the sun creates wet unstable surface snow. This will occur first on steep rocky slopes receiving direct sun. The exact aspect will change throughout the day as the strong March sun moves across the sky. Stay off of any slope where you see signs of wet surface snow. Even though loose wet avalanches are expected to be small, they may carry you into areas where even small avalanche can have large consequences such as over cliffs and into gullies.

Small winds slabs will continue to be reactive at higher elevations. Simple visual clues such as snow drifts, wind textured snow, or firm surface snow with cracking will help you locate wind loaded slopes as you travel. Use these visual clues to identify and avoid slopes greater than 35 degrees where wind loading has occurred.

Weak old snow continues to exist in many locations in the Eastern Cascades. The exact depth and type of persistent weak layer is depends on your location. Snow profiles and snowpack tests can confirm the presence of these layers, but they cannot prove its absences. You can avoid this difficult to predict and manage avalanche problem by staying off of large open slopes where these larger avalanches may occur. This is a classic low likelihood-high consequence scenario. Be aware that tracks on a slope does not mean that it is safe.

Cornices may grow weak and fail during periods of day time warming and strong direct sunshine. Choose travel routes and techniques to limit your exposure to any overhead cornices.

Snowpack Discussion

In general 1-4 inches of snow fell across the Eastern regions of the Cascades Tuesday night and Wednesday. This modest storm was accompanied and followed by moderate to strong winds that redistributed the snow forming shallow wind slabs in exposed terrain near and above treeline. In locations receiving sun Wednesday, moist to wet surface snow quickly formed.

Older weak persistent layers continue to linger in the snowpack. Observations from the Eastern areas of the Cascades have found these layers reactive and 3 fatalities occurred on persistent weak layers in early March. The exact persistent weak layer depends on your location.

Commonly seen persistent weak layers are:

Surface hoar and facets buried on 3/8. This layer is typically found 8-14" below the snow surface near and below treeline.
A more wide spread persistent weak layer is found just above a firm crust layer buried on 2/5. This layer of weak sugary facets (2/13) can be found 2-4 feet below the snow surface.
Other shallow and very weak snowpacks may be found further east of the crest or in areas with less snow.

Observations

North

Monday and Tuesday North Cascade Heli reported several very small wind slabs avalanches.

On March 7th, NCMG generally found the 2/5 crust down 30-35" in the Hairpin valley with a 0.5 cm layer of 1.5 mm rounding facets. They also found a layer of 5 mm surface hoar buried on March 8th down 3-12" due to variable wind transport.

Central

No recent observations

Problems

Persistent Slabs

Release of a cohesive layer of soft to hard 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 Slabs.

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Wind Slabs

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.

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 15th, 2018 11:00AM