Avalanche Forecast
Issued: Mar 6th, 2018 11:01AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Persistent Slab avalanches claimed lives along the east slopes of the Cascades over the weekend. The snowpack remains complex and scary. Avoid complex terrain and sit out this low likelihood - high consequence problem; ensure a wide buffer between where you travel and open slopes over 35 degrees as well as large avalanche paths. Also watch for lingering wind slabs at higher elevations Wednesday.
Summary
Detailed Forecast
Fatal avalanche incidents along the east slopes of the Cascades occurred this weekend in the Teanaways near Long's Pass (Saturday) and north of the Methow Valley at Setting Sun Mountain (Sunday). Persistent Slab avalanches were reported both at the Long's Pass and Setting Sun incidents.
The snowpack on the Cascades East slopes remains complex and scary. It's currently easier to trigger dangerous Persistent Slab avalanches on the East slopes of the Cascades than the West Slopes of the Cascades. There are multiple weak layers to avoid, complicating the snowpack. A Moderate avalanche hazard tells you that large avalanches may be triggered in isolated areas. You will not be able to identify deeper instabilities just by looking at the surface snow.
Several layers of persistent grain types that have been reactive in tests are likely to remain reactive over the next week or perhaps longer. Several older weak layers of facets exist in the snowpack, but are still shallow enough that you can trigger them. In many areas with a deeper snowpack, you may get little warning of a Persistent Slab avalanche. While these avalanches are difficult to trigger, they are also very difficult to predict. A resulting avalanche will likely be large enough kill you. Put a wide margin of terrain between you and any slopes 35 degrees and steeper where you suspect the Persistent Slab problem. Continue to be cautious and stay away from steep, open slopes connected to large avalanche paths. If you experience collapsing or audible whumphs, avoid any nearby avalanche terrain.
Wind Slabs are trending toward unlikely, but may still exist in isolated areas especially above treeline where there have been few recent observations. Generally avoid areas of recently drifted snow, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist well below ridge-lines.
Small loose wet avalanches are possible on Wednesday on steep solar slopes and below treeline. Avoid steep sunny slopes near terrain traps if the surface snow becomes moist and watch for signs of natural pinwheeling and rollerballing as a clear signal to change aspects.
Snowpack Discussion
Tuesday was mostly sunny with freezing levels rising to 4000-5000'. The last snowfall was up to 4" of new snow through Sunday evening along the east slopes of the Cascades with gusty winds transporting snow. On solar aspects this new snow sits on a melt-freeze crust formed Saturday.
The last major snow storm was Wednesday 2/28 through Thursday 3/1; dropping up to 1.5' of snow with significant accumulations as far east as Mission Ridge. Moderate to strong southerly flow during the storm built wind slabs. Several days of settlement has allowed these wind slabs to become stubborn to trigger.
Several potential persistent weak layers exist in the snowpack along the East Slopes of the Cascades. Two common layers that have been reported in many locations 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.
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
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.
On Sunday, North Cascades Heli observed a recent avalanche (likely from Friday) which released mid-slope and featured a deep crown. They suspected the avalanche to involve the 2/13 layer.
On Sunday, NCMG was in the Washington Pass area near and below treeline where they observed no new avalanches and no results with ski tests.
On Saturday, NCMG traveled in the Cuthroat area and observed small wind slab avalanches in steep terrain that had run naturally on Friday. The 2/23 crust was not observed on north facing terrain above 5600'. The 2/13 layer down 3' (85 cm) at 6100' on a NNW aspect showed mixed results in tests. Widespread collapses and a stubborn small persistent slab release was reported on Vasiliki ridge from a third party.
Central
On Sunday, 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.
On Friday, NWAC observers traveled in the Bean Creek area north of Cle Elum. On both south and northeast slopes, they reported large and small column tests indicating potential for human triggering on the 2/13 facets. This weak layer was 3-4 feet below the surface. They also found the 2/23 facets about 2 feet below the surface on a south aspect at 5450 ft and several reactive layers of preserved snow crystals within the upper 1.5' of the snowpack.
On Wednesday 2/28, an avalanche professional in the Chiwakum Mountains reported collapses and whumps on the 2/13 buried facet layer. Depth to the layer was highly variable (1-3 feet). Another traveler triggered an avalanche almost 3 feet deep on a small steep slope near McCue Ridge.
Problems
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
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 7th, 2018 11:01AM
