Avalanche Forecast
Issued: Apr 29th, 2017 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Cornices.
NWAC,
Northwest Avalanche Center
Avoid steep, exposed wind loaded terrain above treeline where fresh wind slabs may be reactive to human trigger Sunday. Watch for wet and weak surface snow from daytime warming and any afternoon sunshine. Continue to give cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices.
Summary
Detailed Forecast
A front is expected to move across the Cascades Saturday night. This should cause a period of moderate rain and snow with snow levels about 5500 feet and a period of strong SSW winds. Showers should taper Sunday and end with gradual clearing late Sunday. Wind should shift to westerly and diminish through the day Sunday.
Some shallow fresh wind slabs are likely on lee slopes, mainly NW-SE facing above treeline by early Sunday. Watch for and avoid areas of firmer wind transported snow on NW-SE aspects and other cross loaded features on other aspects in areas of varied terrain.
Near and below treeline, cooling should allow for previously wet surface snow to begin re-freezing, diminishing the threat of loose wet slides. However, watch for wet surface snow deeper than a few inches, pinwheels and initial small loose wet avalanches that indicate the potential for loose wet avalanches.
Cornices are still large, so give them a wide safety margin. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give potential cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices. See a blog post regarding cornices here.
Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches are difficult to predict and can release at any time, not just during the heat of the day.
Note that this forecast applies up to the Cascade crest level and does not apply to higher elevations on the volcanoes.
Snowpack Discussion
A cool, snowy pattern has been seen in late April mainly for the Cascade west slopes and especially in the Mt Hood area.
Last week from Monday 4/17 to Thursday 4/20 the NWAC weather station at Mt. Baker picked up about 3 inches of water equivalent (WE) while other NWAC stations along the Cascade west slopes picked up 1 - 2 inches of WE. Much of this WE fell as snow above 5000 feet in the north, 5500 feet in the central and 6000 feet in the south Washington Cascades.
The most recent storm cycle began a week ago Saturday. In the 6 days ending Friday morning most NWAC stations along the Cascade west slopes have had 1.75 - 2.75 inches WE. Much of this WE fell as snow above about 4500 feet in the north, 5000 feet in the central and 5500 feet in the south Washington Cascades.
Snow levels were even a little lower by Thursday morning. Stevens Pass had 7 inches of snow due to convergence and Paradise had 8 inches of new snow for the 24 hours ending Thursday morning. By Saturday, the recent storm snow had settled to about 2-5 inches and undergone melt freeze cycles.
Recent observations
NWAC observer Lee Lazzara was at Paradise on Thursday. He found wind transported snow and sudden or resistant planar easy compression tests on N-SE slopes in the near treeline. He noted that a couple small cornice chunks had triggered small wind slab avalanches. The upper snowpack on solar slopes near treeline and on all slopes below treeline consisted of crusts and consolidated rounded grains.
Lee was on Mt Rainier again Friday, 4/28. Finding sun warming surface snow above 6000 feet and clouds and light showers below 6000 feet. Shallow recent storm cycle, wind slabs of 4-6 inches were found to be reactive with compression tests, becoming less reactive with daytime warming.
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, North West.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 30th, 2017 11:00AM
