Avalanche Forecast
Issued: Jan 20th, 2015 9:49AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Evaluate snow and terrain carefully Wednesday especially closer to the Cascade crest. Watch for isolated wind slabs sensitive to human triggering on lee slopes near and above treeline and sun affects on steeper solar slopes.
Summary
Detailed Forecast
Freezing levels on Wednesday should begin to rise on Wednesday. Increasing high and mid clouds should be seen in the afternoon. Recent storm snow should continue to settle, but filtered sunshine and increasing temperatures in the afternoon may activate loose avalanches predominately on steeper southerly slopes. Especially watch near and above treeline, where more snow is available for entrainment and could funnel you into a terrain trap.
Expect a higher regional likelihood of finding sensitive wind slab on N-SE aspects near the Cascade crest Wednesday. Approach wind-loaded slopes with caution and practice conservative decision making especially when pushing into the near/above treeline band.
In the Washington Pass area, storm slabs releasing down a the Jan 5th crust/facet interface deserves your attention. Watch for cracking and collapsing while traveling as obvious instability clues and generally be conservative near avalanche terrain. There's plenty of good skiing to be found on lower angled slopes even in the alpine. This zone will have a higher danger rating than the other east slopes zones.
As a general backcountry travel safety note, tread carefully at lower elevations and on wind scoured aspects where terrain hazards (exposed rocks, trees, streams, etc.) are present.
Snowpack Discussion
Strong southwest flow carried a wet front across the Cascades Saturday night. This front tapped subtropical moisture but cold air trapped on the east side kept 48 hr water amounts through Mon AM of half an inch to 2 inches mainly in the form of snow. Snow was most significant in the northeast and central east Cascades with about 12-24 inches at Holden and in the Washington Pass area again through Mon morning.
The North Cascade Mountain Guides in the Washington Pass Monday reported the storm snow had formed a soft but cohesive slab and collapsing was observed on a facet/crust combo down 70 cm to the Jan 5th crust. This layer was reactive in snowpit tests with propagation and one sizeable avalanche in the Cutthroat Drainage may have released down to this layer.
We have no new information about the non-reactive PWL in the Jove Peak area from last week, but following this storm cycle it is likely more than 1 meter down and even harder for a human to trigger. Sensitive soft wind slab has been reported in the Stevens Pass area, and this avalanche problem likely extends into the neighboring area along the east slopes.
An avalanche class near Mission Ridge on Saturday reported via the NWAC Observations that facets between crust layers in the mid pack gave a Q1 shear but did not show a tendency to propagate via an ECT. The Mission Ridge ski patrol Sunday reported a surface rain crust with no natural or ski cut avalanches.
The south-east Cascades and central-east Cascades away from the crest will likely have a lower avalanche danger.
The overall snowpack remains well below normal for this time of year and some windward or southerly aspects have little if any snow cover.
Problems
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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
Expected Size
Valid until: Jan 21st, 2015 9:49AM
