Avalanche Forecast

Issued: Feb 13th, 2018 10:25AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avalanche conditions will begin to change on Wednesday due to a quick moving frontal system bringing light amounts of new snow to the east slopes of the Cascades. Westerly winds should build fresh and shallow wind slabs near and above treeline. Wind slabs will be more likely to trigger in areas where wind deposited snow bonds poorly to an underlying crust or weak surface snow. 

Summary

Detailed Forecast

Avalanche conditions will begin to change on Wednesday as a quick moving frontal system brings fresh snowfall to the Cascades Tuesday night and Wednesday along with much cooler temperatures.

On average, 2 to 6 inches of new snow may bond poorly to a medley of snow surfaces including weak surface snow in the form of persistent grain types, wind sculpted snow surfaces or various surface crusts. Heightened avalanche conditions will exist on any slope greater than 35 degrees where significant new snowfall bonds poorly to the underlying surface. 

While winds are not forecast to be especially strong with this system, westerly winds should build fresh wind slabs on lee slopes near and above treeline. Avoid steep slopes with wind deposited snow such as below cornices, on wind drifts, and near uneven snow surfaces. Wind slabs will be more likely to trigger in areas where wind deposited snow bonds poorly to an underlying crust or weak surface snow. 

Loose dry avalanches will not be listed as a problem but will be possible on very steep slopes. Be aware of the consequences of a loose dry avalanche near terrain traps such as cliffs, gullies, or open creeks. In areas where shallow loose new snow bonds poorly to the underlying crust, avoid steep icy slopes where it will be difficult to stop a fall. 

Snowpack Discussion

Heading into a more active weather pattern beginning Tuesday night, a relatively stable snowpack exists in most locations along the east slopes of the Cascades. Above treeline and to a lesser extent near treeline, wind affected surfaces or surface crusts exist. In shaded and sheltered areas around Washington Pass, 4-8 inches of soft snow now sits over the most recent crust. On slopes that have recently received direct sun, spring-like conditions have developed.

Significantly less snow has been received in areas further east of the Cascade crest, resulting in limited avalanche concerns.

Warm weather the first week of February has resulted in a strong and consolidated mid and lower snowpack in many locations. While we are tracking several old crusts around the east slopes of the Cascades, there are no significant layers of concern in the snowpack.

In the last week of January, observers reported isolated buried surface hoar on top of the 1/16 crust. This persistent weak layer was found or thought to be the cause of several avalanches. A recent observation (2/11) from the Mission Ridge area confirmed this layer is still present and reactive in some locations. Though it’s no longer a main concern, use caution when traveling in areas further east of the crest where this layer may survive. Snow profiles and snowpack tests are the best means to confirm the presence of this layer.

Observations

North

Over the last several days, avalanche professionals in the Washington Pass area reported no new avalanche activity with debris from the most recent 2/5 avalanche cycle visible in many paths. Recent winds have created a variety of snow surface conditions above treeline.  On Sunday, observations from professionals near Hart's Pass noted surface hoar up to 1 cm thick on all aspects up to a few hundred feet below ridgecrest. 

Central

A recent observation from the Mission Ridge/Lake Clara area. 

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

Valid until: Feb 14th, 2018 10:25AM