Avalanche Forecast

Issued: Jan 8th, 2016 10:02AM

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

Watch for shallow wet snow on steep sun exposed terrain and be mindful of cornices as these are likely to weaken during the warmest part of the day.  Older wind slabs may still be lurking in isolated higher elevation areas.

Summary

Detailed Forecast

Mild daytime temperatures and sunshine Saturday should give way to increased clouds late in the day and a chance of a few showers late. This should allow for shallow surface snow melt, breaking down existing surface crusts through the day where they existed. Cooler settled and mostly stable powder is likely on steeper shaded slopes. 

Small loose wet avalanches should be possible on direct, steep, solar aspects. 

Also, watch for any older cornices as the sun and warming may weaken these, especially near midday.

We have lowered the likelihood of triggering an avalanche on an older wind slab, however, it would still be wise to watch for these conditions on specific terrain features. Most likely suspects are steep slopes below ridges or cross loaded ribs, mainly near or above treeline and avoid slopes where being caught in a small slide could have larger consequences, such as slopes above trees, cliffs or where the terrain would funnel snow into a trap, like a creek bed or natural depression.

Snowpack Discussion

 

The deep storm snow from late December is now well settled and appears rather homogeneous in the numerous recent snowpits dug throughout the region. Fair weather following Christmas allowed surface hoar and near surface faceted snow to form extensively.  Snowfall buried this layer January 3rd and 6-12 inches accumulated by early this week along with some wind loading and wind redistributed wind slabs noted. Several avalanches occurred on this layer early in the week.

Mild weather, sunshine and light winds over the past several days with above freezing temperatures in many areas have allowed for this layer to bond and strengthen, making avalanches unlikely. Extensive testing Friday, January 8th by Ian Nicholson on Chair Peak in the Alpental Valley, found this layer to be gaining significant strength from earlier in the week and no failures were noted on the buried layer, now difficult to distinguish about 6 inches below the surface on a NE slope near 5500 ft.  

High pressure returned Thursday and Friday, providing plenty of sunshine and mild temperatures, well above freezing in many areas. This weather has allowed for the recent 6-10 inches of storm snow to further consolidate while sunshine and warming has caused shallow wet snow conditions during the warmest part of the day, followed by surface crusts overnight and mornings. Surface crusts were found on most aspects Friday, causing challenging travel conditions. Thin melt-freeze crusts were even found on steep north slopes. 

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: North, North East, East, South East, South.

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.

Aspects: South East, South, South West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 9th, 2016 10:02AM