Avalanche Forecast
Issued: Apr 21st, 2018 11:19AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Cornices and Wind Slabs.
NWAC,
Northwest Avalanche Center
As the snow warms up, things fall down. Expect avalanche conditions to change quickly Sunday as warm temperatures and sunny skies affect the snowpack. New rollerballs, fresh fan-shaped debris, and wet snow deeper than your ankle all indicate you may trigger loose wet avalanches on similar slopes. Do not hangout in areas where cornices loom overhead. They may fail without warning.
Summary
Detailed Forecast
Wet avalanche conditions will quickly develop Sunday as warm air temperatures and sunny skies affect the snowpack. When you see new rollerballs, observe fresh fan-shaped debris, or experience wet surface snow deeper than your ankle, you may be able to trigger loose wet avalanches on similar slopes. Do not underestimate these loose wet avalanches. Observations from this week have shown that many small slides have gouged into older snow layers, grown large, and traveled farther down slope than normal.
Many cornices have grown large over the winter. This time of year, warm sunny weather causes them to droop and eventually fail. This warm-up may be significant enough to cause this to occur. Do not linger in areas where cornices are above you. Choose travel routes and techniques that limit your exposure to these potential ticking time bombs.
We have received reports of isolated very large wet slab and glide avalanches from most areas over the past week. Many of these avalanches appear to have been triggered by cornice falls or loose wet avalanches. During significant warm-ups such as this, minimize your time in very large avalanche paths by choosing routes and techniques that reduce your exposure to this low-likelihood but high-consequence scenario.
Small lingering wind slabs from Saturday may still be found at higher elevations. You are most likely to trigger a wind slab below cornices, near convex rollovers, or on unsupported slopes. Use visual clues such as cold dry surface snow, fresh cornices, and snow drifts to identify and avoid these wind loaded features
Other springtime hazards exist in the mountains. Creeks have begun to open, running high and fast with snowmelt. Use caution and choose your creek crossings carefully if you are traveling near or over large creeks.
Snowpack Discussion
In general 1-3 inches (3-8cm) of new snow fell in most locations Saturday. Winds during the storm drifted snow at higher elevations. Near and below treeline, this new snow fell on a melt-freeze crust. The thickness and strength of the curst depends on aspect and elevation. On shaded slopes above treeline, this new snow fell on strong settled snow.
The upper and mid snowpack is quite variable depending on aspect and elevations. In general, the upper snowpack consists of frozen melt-freeze crusts, weak large wet grains, and strong rounded grains. The 4/1 melt-freeze crust which may be the culprit in some of these recent very large wet slab avalanches is generally found 4-6 feet (120-200cm) below the snow surface.
Very large slab and glide avalanches have been reported over the last several days. Mt Baker, Stevens Pass, Alpental Valley, Chinook Pass, and the Mt Rainier area have all reported very large deep avalanches. They are a reminder that very large triggers, such as cornice falls or other avalanches, may trigger older deep layers that have become wet with recent rain and warm temperatures.
Observations
Stevens Pass
NWAC avalanche forecaster Dallas Glass traveled in the Stevens backcountry Saturday. Dallas found 1-3" (3-8cm) of new snow. He was able to trigger loose wet avalanches on steep slopes greater than 35 degrees. In areas where the slopes were longer, these small avalanches entrained additional snow and ran further than he expected.
NWAC professional observer Matt Primomo traveled in the Stevens backcountry Friday. A layer of weak large grain wet snow was observed near treeline (5800’). This weak layer was 14-30” (35-75cm) below the snow surface. Numerous loose wet avalanches were observed in the area from the last several days.
Snoqualmie Pass
NWAC avalanche forecaster Dallas Glass traveled in the Alpental Valley Friday. He found weak large grains of wet snow in the upper 30” (75cm) of the snowpack. He was able to trigger loose wet avalanches on small test slopes near and below treeline. This corresponded with numerous recent natural loose wet avalanches on all aspects.
Tuesday and Wednesday Alpental ski patrol reported glide avalanches in the Alpental Valley that entrained significant snow and traveled farther than normal.
South
NWAC professional observer Lee Lazzara traveled near Paradise Saturday. Lee found small shallow isolated wind slabs near treeline. While they were generally too small to pose a risk, at higher elevations they may be larger and more widespread.
We received several reports of a very large avalanche in the Frying Pan Creek drainage near Mt Rainier Thursday. The avalanche appears to be on a NE aspect around 7500’.
NWAC staff visited the Crystal backcountry Wednesday and Thursday. They reported many large overhanging cornices. Wind slabs in this area had bonded and gained strength. Surface snow on nearly all slopes showed signs of warm air temperatures.
Problems
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: East, South East, South, South West, West.
Elevations: All elevations.
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
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
Expected Size
Valid until: Apr 22nd, 2018 11:19AM
