Avalanche Forecast
Issued: Apr 13th, 2017 11:54AM
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
Watch for fresh wind slab on lee slopes in steeper terrain primarily above treeline. Sunbreaks can quickly create loose wet avalanche conditions on steeper solar aspects. Give cornices a wide safety margin.
Summary
Detailed Forecast
A cool upper trough will sit over the Pacific Northwest on Friday with shower activity increasing in the afternoon and evening. Sunbreaks are most likely during the late morning through mid-day before convective clouds build in the afternoon. The Mt. Baker area will maintain the Considerable rating above treeline due to ongoing wind transport and the potential for larger wind slab in this zone/elevation band.
Watch for firmer wind transported snow, mainly on NW-SE slopes and above treeline on Friday. Fresh wind slab may have locally bonded poorly to graupel layers received Thursday.
After a cool night Thursday, snow levels will remain low on Friday. This should generally restrict the loose wet potential to the shallow amounts of new snow found on steeper solar aspects near and below treeline. This time of year even brief sunbreaks combined with subtle daytime warming can quickly activate loose wet avalanches.
Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. There have been numerous recent cornice failures with some being very large in the Washington Cascades. Five people were tragically killed by a cornice release in British Columbia on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.
Snowpack Discussion
Weather and Snowpack
Heavy rain received in mid March has left behind a well consolidated snowpack with one or more strong melt freeze crusts in the upper portion of the snowpack in the near and below treeline elevation bands.
A series of strong spring storms occurred during the first week of April with high water amounts seen particularly on the volcano stations (Baker and Paradise) and less in the Cascade Passes.
A front crossed the Northwest on Sunday night and Monday, depositing 8 inches of storm snow at Mt Baker and about 6 inches on Mt Rainier with only a few inches in the Passes by Tuesday morning. Moderate winds built fresh wind slab above treeline on steep lee terrain features.
Mostly sunny weather Tuesday helped storm snow begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow.
A weather system passing through the area Wednesday night produced half an inch to 1 inch of water across west slopes of the Cascades through Thursday morning. This translated to a few inches of new snow around 4000 feet with 4-6 inches at the higher NWAC stations atop Alpental, Paradise, Chinook Pass and Green Valley at Crystal. Showers increased during the day on Thursday with light additional snow accumulations mixing with graupel during more intense showers.
Recent Observations
North
NWAC's Lee Lazzara traveled to the Hidden Lakes Peak area in the North Cascades Sunday, 4/9. Recent storm snow ranged from about 1 ft near treeline to about 2 ft above treeline. Recent wind slabs were noted as well as some fresh wind transported snow on many cross loaded features. Storm snow remained cold and dry on steeper shaded terrain at higher elevations. Wind slabs appeared to require a bit more settling time before venturing into steeper committed terrain. Solar affects caused many natural small loose wet avalanches on steep solar slopes.
Lee and Jeremy Allyn were in the Mt Baker backcountry Tuesday, 4/11. Storm snow in non-wind affected terrain ranged from 8-12 inches and on steeper and shaded slopes provided good ski conditions. The winds were strong enough Tuesday above treeline to actively transport recent storm snow and continue to build fresh wind slab on lee terrain. Cornices were once again observed to be very large with at least 5 large cornice failures noted, likely releasing in the past day or two. These cornices pulled out D2 sized slabs on the slopes below.
Mt. Baker pro-patrol reported one natural wind slab off of Mt. Herman on Thursday. In area the new snow was not reactive to ski cuts at any elevation or aspect. Despite a steady W-SW wind atop Pan Dome Thursday, little loose surface snow was available for transport near and below 5000'. Fresh wind transported snow was observed above treeline outside the ski area.
Central
NWAC's Jermey Allyn was in the Alpental Valley, Sunday 4/9. Recent storm snow of about 6-8 inches was hanging in there on shaded terrain, over a very strong crust. On all but the steep shaded terrain at high elevations, shallow surface snow became wet and cohesive by midday as solar affects took hold. There was no evidence of wind slab in this terrain, but no observations were made in true above treeline zones.
South
NWAC's Matt Schonwald was in the Cement Basin area near Crystal Mountain on Sunday 4/9. North slopes had about 25-30 cm of Fist hardness snow well bonded to an underlying crust, with shallower recent snow on other aspects. Warming was helping settle recent snow, but was creating loose wet surface snow on West facing slopes.
WSDOT Chinook Pass crews checked in Wednesday and Thursday. On Wednesday, shallow loose wet avalanches were sensitive to ski cuts below 5000' feet and running well. On Thursday, cooler temperatures helped limit the loose wet potential to below 4800' on solar aspects. Isolated pockets of wind slab could be found on lee slopes below ridges. About 14-18" on storm snow sat above the most recent crust in non-sun/wind affected terrain.
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.
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, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 14th, 2017 11:54AM
