Avalanche Forecast

Issued: Apr 19th, 2018 11:10AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

You may still trigger lingering wind slabs in steep terrain, near convex rollovers, or on unsupported slopes. Use visual clues such as cornices and snow drifts to identify and avoid these wind loaded features. As the surface snow becomes wet during the day, expect loose wet avalanche to occur on slopes greater than 35 degrees where more than 4 inches of wet surface snow exist. As the snowpack adjust from winter to spring, do not linger in areas below overhanging cornices, glide cracks, or very large avalanche paths.

Summary

Detailed Forecast

The mountain snowpack continues its transition from winter to spring. As it adjusts, we have received reports of isolated avalanche events including cornice failures, large deep wet slabs, and glide avalanches. Our slow transition to spring and a complex and diverse snowpack makes it very difficult to predict when and where these events will take place. Do not linger below slopes overhung by large cornices, glide cracks, or where a cornice fall may trigger larger deeper avalanches.

Wind slabs, formed earlier in the week, continue to be reported in some backcountry locations. You are most likely to trigger a lingering wind slab at higher elevation in very steep terrain, near convex rollovers, or on an unsupported slope. Use visual clues such as uneven snow surfaces, snow drifts, or cornices to identify and avoided these wind loaded features.

Warm air temperatures and morning sun breaks will allow for wet snow surface conditions to develop on lower elevations slopes. If you see new rollerballs, observer fresh fan-shaped avalanche debris, or experience wet surface snow deeper than your ankle, expect loose wet avalanche to occur on similar steep slopes. Many recent loose wet avalanches have entrained significant amounts of snow, gouged into older snow layers, and run farther than normal. Give this common avalanche problem respect by avoiding slopes greater than 35 degrees if you see signs of loose wet avalanches.

Other springtime hazards exist in the mountains. Recent warm weather and rain has cause many creeks to open. Choose creek crossings carefully if your route causes you to travel across large creeks. 

Snowpack Discussion

Very large slab and glide avalanches have been reported over the last several days. Mt Baker, Alpental Valley, Chinook Pass, and Mt Rainier areas have all reported very large deep avalanches. These events have not been linked to a specific layer. However, they are a reminder that very large triggers, such as cornice falls, may trigger older deep layers that have become wet with recent rain and warm temperatures.

Mild weather Wednesday and Thursday allowed the upper snowpack to gain strength and settle. Sunny aspects and slopes at lower elevations developed melt-freeze crusts during this time frame. Cold dry snow may still be found on shaded slopes at higher elevations.

Significant snow impacted the area Monday and Tuesday with 12-20 inches (30-50cm) falling in most locations. Winds during the storm formed wind slabs in exposed terrain near and above treeline.

In most locations the snowpack consist of various melt-freeze crusts, old wet snow, and strong rounded grains. The exact combination depends on aspect and elevation. Currently we do not have any discernable significant layers of concern within the snowpack.

Observations

Baker

On Thursday NWAC professional observer Lee Lazzara traveled near Heliotrope Ridge. Lee found lingering wind slabs 12-16 inches (30-40cm) near and above treeline. Below 5000 feet, all aspects had formed moist to wet snow on the surface. Lee reported one very large avalanche in the Sisters Range SW of Mt Baker.

Saturday through Tuesday, Mt Baker ski patrol and several public observations reported very large (D3) slab avalanches on Table Mt and the Shuksan Arm. These events tend to be on northerly aspects below 5500 feet.

Stevens Pass

No recent observations

Snoqualmie Pass

Tuesday and Wednesday Alpental ski patrol reported glide avalanches in the Alpental Valley that entrained significant snow and traveled farther than normal.

South

We received several reports of a very large avalanche in the Frying Pan Creek drainage near Mt Rainier Thursday. The avalanche appears to be on a NE aspect around 7500’.

NWAC staff visited the Crystal backcountry Wednesday and Thursday. They reported many large overhanging cornices. Wind slabs in this area had bonded and gained strength. Surface snow on nearly all slopes showed signs of warm air temperatures.

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: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 20th, 2018 11:10AM