Avalanche Forecast

Issued: Apr 12th, 2018 11:01AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Modest new precipitation amounts will limit the development of avalanche hazards in the Cascade East Central and South zones Friday. You are most likely to experience developing avalanche hazard in locations closer to the Cascade crest. If you see signs such as blowing snow, new drifts, or fresh cornices, avoid wind loaded slopes greater than 35 degrees.

Summary

Detailed Forecast

Expect avalanche hazard Friday to develop in areas close to the Cascade crest where precipitation amounts will be higher. Farther east, avalanche hazard will be limited by lack of new precipitation.

If you see blowing snow, fresh cornices, or snow drifts stay, off of wind loaded slopes greater than 35 degrees. You are most likely to trigger a wind slab in exposed terrain below cornices, and on convex rollovers. Use visual clues such as uneven snow surfaces and drifts to identify wind loaded terrain.

At lower elevations, rain can quickly develop wet surface snow conditions. Avoid slopes greater than 35 degrees if you see new rollerballs, observe fan shaped avalanche debris, or experience periods of rain. Be cautious if you are traveling in terrain where even a small loose wet avalanche can have large consequences.

Snowpack Discussion

Several days of warm temperatures and cool nights has allowed recent wet snow instabilities to refreeze and gain strength. The upper snowpack is generally a combination of various melt-freeze layers and strong rounds.

In the mid and lower snowpack older persistent weak layers can still be found in some locations. You are most likely to find weak older snow at higher elevations and on shaded aspects where recent warm temperatures and rain have had minimal effects. You can use snowpack tests to identify buried persistent weak layers, however they are not a decision making tool. We have very limited information on the distribution of and sensitivity of any buried old weak layer. If you have any observations on these layers, please share them with NWAC on our public observations page.

Two common persistent weak layers seen in these zones 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

On Wednesday, NWAC professional observer Matt Primomo traveled in Hawkins Mountain area. Below 5500 ft about 1 ft of wet surface snow was making Loose Wet avalanches possible on steep slopes. Above about 6000 ft firmer surface snow was limiting the Loose Wet avalanche potential. There were widespread large wet snow avalanches following the weekends storms. The upper two feet of snowpack has undergone a spring transition to melt-freeze grains.

On Monday, a public observation from the Entiat River drainage reported wet snow in the upper 16 inches (40 cm)  of the snowpack on a NE aspect at 6800 ft. Below this, colder drier snow was found with buried surface hoar 2 feet down (60 cm). This layer was reactive in snowpack tests, suggesting propagation was possible. The 2/8 crust was 3 feet (1 m) below the snow surface and breaking down. S through E aspects had a more springlike and consolidated snowpack. 

Problems

Wind Slabs

An icon showing 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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

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.

Elevations: Below Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Apr 13th, 2018 11:01AM