Avalanche Forecast
Issued: Jan 2nd, 2018 12:45PM
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
Shallow wind slabs may linger in wind exposed terrain near and above treeline while small loose wet avalanches are possible on steep solar slopes. In the Mission/Blewett area, be aware of the low likelihood of triggering a persistent slab in isolated areas on non-solar aspects above 5000'. You can dig snowpits to confirm the existence of basal facets well before entering avalanche terrain and adjust your travel to respect this low likelihood/higher consequence problem.
Summary
Detailed Forecast
After a cloudy start, partly to mostly sunny skies are expected Wednesday afternoon with continued mild temperatures. The exception once again will apply to the lower slopes where cooler temperatures and low clouds will persist due to temperature inversions.
Lingering wind slabs should be far less sensitive to triggering on Wednesday and confined to higher terrain. Continue to watch for areas of recently wind transported snow such as fresh cornices, snow drifts, and uneven snow surfaces. Identify and avoid locations where recent wind loading occurred.
Wind slabs can be deceptively difficult to manage in the terrain. Take a moment and read our recent blog post by NWAC Pro Observer Jeremy Allyn on wind slabs.
Small loose wet avalanches are possible on steep solar slopes in areas that experience warm temperatures and afternoon sunshine on Wednesday. Be aware of the consequence of even a small loose wet avalanche around terrain traps.
In the Mission/Blewett area, be aware of the low likelihood of triggering a persistent slab in isolated areas on non-solar aspects above 5000'. Varied surface roughness combined with an overall shallow snowpack made for a low likelihood of finding the right terrain to support a persistent slab avalanche. However, you can dig snowpits to confirm the existence of basal facets well before entering avalanche terrain and adjust your travel to respect this low likelihood/higher consequence problem.
Despite all this new snow, early season hazards still exist. Many creek beds have still not filled in for the winter.
Snowpack Discussion
Cool weather east of the Cascades has slowed the stabilization of wind slabs formed during the recent active weather pattern. However, on Monday and to a lesser extent on Tuesday, warmer air finally worked it's way aloft, with higher elevation stations such as Lyman Lake, Mission Ridge and Dirty Face warming near or above freezing while lower elevations stayed in the freezer (see graph below).
Winds toward the end of the last storm cycle in late December formed firm and initially reactive wind slabs on a variety of aspects.
Storm totals from across the east slopes of the Cascades show 1 to 2 feet of snow (highest storm totals Lake Wentachee and Holden area) fell during the series of storms ending Saturday Dec. 30th.
Snowdepth decreases substantially the further east of the Cascade crest one travels. In many areas below treeline, there is not enough snow to present an avalanche danger.
Observations
North
North Cascade Mountain Guides were in the Washington Pass area Friday through Sunday. By Sunday, sensitive storm slab layers were gaining strength and becoming unlikely to trigger. Wind slabs were thought to be stubborn but were still avoided or approached cautiously above treeline. One natural cornice release was observed on Sunday.
Central
A NWAC forecaster was in the Diamond Head area of Blewett Pass on Tuesday and traveled on non-solar aspects up to 5800 feet. 2-3 mm basal facets were rounding but present above 5000' on NW-N-E aspects. Large column tests showed the 2 ft (60 cm) slab above this layer was reactive and likely to propagate. However, varied surface roughness combined with an overall shallow snowpack made for a low likelihood of finding the right terrain to support an avalanche. Just like the nearby Mission Ridge area, this layer will need to be watched moving forward. Other pertinent observations from the area include a rain crust observed up to 5400 feet. Snowdepths were quite low in this area, making backcountry travel difficult at lower elevations, wind scoured areas and on solar aspects.
Mission Ridge Pro Patrol reported very sensitive wind slabs observed around the area Saturday. A natural avalanche cycle was observed from Friday night. Avalanche control work within the ski area Saturday produces 1-4 ft wind slab avalanches. The larger avalanches released to ground on 1 mm basal facets.
South
No recent observations
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: All aspects.
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 3rd, 2018 12:45PM
