Avalanche Forecast
Issued: Mar 9th, 2018 10:45AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Persistent Slabs.
NWAC,
Northwest Avalanche Center
You can trigger a large wind slab on steeper lee slopes, a large loose wet avalanche on slopes affected by the sun, or a large persistent slab in this complex and dangerous snowpack. Any of these avalanches could injure and kill you. Persistent Slab avalanches are a low likelihood - high consequences problem that claimed lives the Cascades in the past 2 weeks. Don't assume that tracks on a slope mean it is safe and avoid steep, complex terrain where you might trigger one.
Summary
Detailed Forecast
With recent storm instabilities settling, but March sunshine coming out in full force, on Saturday you can trigger avalanches in the upper layers of the snowpack due to wind slabs, loose wet snow, or a shallow persistent slab.
Wind slabs, forming Thursday through Thursday night, and are most likely to be found on NW-N-NE-E-SE-S aspects and are likely to remain reactive on Saturday. Avoid lee slopes steeper than 35 identified near fresh cornices, scoured slopes, or wind-drifted snow.
A strong March sun will quickly warm up sunny slopes creating wet surface snow. This will happen first on steep rock slopes receiving direct sunshine. Expect new roller balls, pinwheels, and small to large loose wet avalanches to occur. Stay off of any steep slope where you see signs of wet surface snow conditions. Limit your exposure to slopes where the sun might weaken an overhanging cornice.
Persistent weak layers lurk throughout the mid-to-upper snowpack. They can be triggered directly below the recent storm snow (2/8) or deeper in the snowpack. This low likelihood, high consequence avalanche problem is very difficult to manage. Signs of Persistent Slab avalanches may not be obvious. The best way to stay safe is to avoid the slopes where you can trigger them. Avoid large avalanche paths, start zones, and unsupported slopes steeper than 35 degrees. Tracks on a slope don’t mean that the slope is safe. In two recent fatalities in the East Central Cascades, the slopes had numerous tracks on it before the avalanches were triggered. While these avalanches may give little warning, the consequences could be un-survivable. In some cases, the 2/5 layer may be transitioning to a deep slab layer making it very difficult for humans to trigger, but a wind slab or loose wet avalanche can still trigger the layer.
Snowpack Discussion
Much less active weather on Friday allowed recent storm-related instabilities some time to gain strength. Generally clearing skies by Friday afternoon allowed some moistening of snow surfaces on any aspects receiving direct sunshine, but with cool temperatures not enough for loose wet avalanche activity.
Generally 6-14" of new snow fell along the east slopes of the Cascades Thursday through Thursday night, with lesser amounts further east, such as Mission Ridge where 2" of snow were reported. Winds heavily transported the snow forming wind slabs and other wind-affected snow surfaces.
The past two weeks brought three fatal avalanche accidents across the East Slopes of the Cascades. All of these were triggered on persistent grain types. A new weak layer has been added to our problem list along the east slopes. Crusts, surface hoar, and small facets have all been identified and persistent grains have been buried intact (3/8) beneath the recent snow. Assess this new interface with small test slopes.
Two common weak layer that we've been tracking for a longer time are a facet/crust combination buried on 2/23 and a facet/crust layer buried on 2/13.
The Setting Sun avalanche released on faceted grains above a thin crust. Faceted grains comprised the bottom 1.5' of the snowpack, making the shallow 51" snowpack here very different from areas closer to the Cascade Crest. Very weak shallow snowpacks such as this may be found in areas farther east of the Cascade Crest.
The upper (shallower 2/23) layer can be found 1-2 feet below the snow surface on steeper slopes that have received direct sun. Small weak facets have been found in other regions surrounding a thin sun crust.
The 2/13 facet/crust combination is typically found 2-4 feet below the snow surface and above the 2/5 firm crust. This layer has two confirmed skier triggered avalanches and more recent collapsing and whumphing.
The exact depth of these layers depend on aspect, elevation, and proximity to the Cascade crest. A high level of uncertainty remains surrounding these layers.
Observations
North
On Thursday, North Cascades Heli reported a D1.5 (small/large) avalanche that occurred naturally where a wind slab relased on a 40 degree SE aspect at 6800 feet near Washington Pass. The avalanche likely failed above the 3/8 crust buried 12-18" down. They also found that the storm snow was very touchy on the 3/8 developing Persistent Weak Layer.
On Thursday during the storm, NCMG generally found the 2/5 crust down 30-35" in the hairpin valley with a 0.5 cm layer of 1.5 mm rounding facets. They did not see the Feb 23 facets, but found a layer of 5 mm surface hoar buried on March 8th down 3-12" due to variable wind transport. Wind slabs were reactive on N-NE-E-SW-S aspects.
Mid-week, observers reported a mix of sun crusts and surface hoar/near surface facets prior to snow on 3/8.
Monday NWAC and NCMG professionals visited the Setting Sun Mt accident site. They found the large avalanche had released on a WNW aspect at 6900 ft. The hard slab avalanche had released on 1.5 mm rounding facets.
Central
On Friday, Mission Ridge Pro Patrol reported 2" of new snow with extremely high winds forming slabs that were not reactive on test slopes or column tests. The recent 2" of snow sits on a layer of 3-4 mm surface hoar buried 3/8. The 2/5 crust was down 20" and is still reactive.
On Wednesday, observers in the Icicle Creek drainage identified the mid-February facets over a crust on a northwest aspect and reported a large slab avalanche on a southeast aspect at 7,500ft from the previous weekend.
On 3/4, NWAC forecasters Dallas Glass and Josh Hirshberg were in the Long's Pass area of the North Fork of the Teanaway drainage where they traveled up to 5700' on S-W-NW aspects. They found the 2/13 persistent layer down 3' everywhere they dug. A new breakable surface crust formed from direct sunshine Saturday on S and SW aspects, but due west aspects had settled powder without the crust. Winds continue to transport snow with NW winds loading SE slopes in that location.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 10th, 2018 10:45AM
