Avalanche Forecast

Issued: Feb 3rd, 2018 12:11PM

The alpine rating is considerable, 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

Stormy conditions and a rising snowline will increase the avalanche danger on Sunday. Fresh wind slabs will be increasing in size and sensitivity above treeline. Loose wet avalanches will become more likely to trigger in areas and elevations that see a switch from snow to rain Sunday. Avoid slopes where small avalanches may have large consequences such as above cliffs, rocks, creeks, and gullies.

Summary

Detailed Forecast

Light to occasionally moderate precipitation near the Cascade crest, increasing crest level west winds, and a rising snow line will increase the avalanche danger on Sunday at all elevations. 

Loose wet avalanches will become more likely to trigger in areas and elevations that see a switch from snow to rain Sunday. You will be able to trigger loose wet avalanches on steeper slopes during and immediately following rain events. Avoid slopes where small avalanches may have large consequences such as above cliffs, rocks, creeks, and gullies.

Moderate to strong winds will continue to form fresh wind slabs on a variety of aspects above treeline. Fresh cornices, snow drifts, plumes, and blowing snow all indicate where wind slabs are forming. Fresh wind slabs will be increasing in size and sensitivity Sunday.

Large cornices exists along ridgelines. Warming temperatures and rain will make these massive blocks of snow more likely to fail. It is very difficult to predict when and where cornices will fall. Avoid traveling on or below cornices.

The avalanche hazard will be lower in areas receiving less precipitation further east of the crest.

Snowpack Discussion

Moderate to strong westerly winds at crest level mixed warm temperatures down to valley floor along the east slopes of the Cascades on Saturday, with valley or lower elevation sites such as Lake Wentachee, Tumwater and Mazama popping up to near or above 50 degrees. The freezing level hovered around 5500 feet in the northeast Cascades, increasing to 6000-7000 feet further south and east. Light precipitation noted near the Cascade crest quickly diminished further east. Wet snow conditions were noted near and below treeline along the Hwy 20 corridor. Continued snowpack settlement was also noted in these elevation bands at NWAC and Snotel sites. 

8" of new snow was reported in the Washington Pass area Friday while for areas south of Holden, rain fell up to about 6000 feet. Very touchy wind slabs formed and developed in the Washington Pass area Friday and again Saturday near and especially above treeline.

A natural avalanche cycle has been reported in many areas from Monday’s (1/29) warm and wet weather. Rain occurred along the east slopes as far north as Holden, WA.

Scattered observations from the last week of January found 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. Extra caution should be taken when traveling in areas further east of the crest where this layer may survive. Snow profiles and snowpack tests are the only means to identify and locate this layer.

Observations

North

On Saturday, a professional in the Washington Pass area reported about 6" of new snowfall over the preceding 24 hours. Very touchy wind slabs were present near and especially above treeline with moderate to strong W-SW winds transporting new and recent snow onto lee slopes. Storm slabs were generally unlikely to trigger. Moist surface snow conditions were present below 5800'. The 1/16 crust was 4 to 5 feet down. Similar conditions were reported Friday in this area. 

Central

An avalanche professional traveling in Icicle Creek Thursday found 6-8” of settled snow well bonded to the 1/29 crust. Observations demonstrated a strong upper snowpack. No buried surface hoar was found in this location.

On Tuesday, Mission Ridge Pro Patrol identified several layers of concern in this regionally shallower snowpack. These layers should be watched during future loading events. Similar basal weak layers were found in the nearby shallow snowpack of Blewett Pass.

South

No recent observations

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.

Likelihood

Likely

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

Likely

Expected Size

1 - 1

Valid until: Feb 4th, 2018 12:11PM