Avalanche Forecast
Issued: Mar 8th, 2018 11:48AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Significant snow and wind have brought a return to dangerous avalanche conditions. Watch out for Storm and Wind Slabs in the upper snowpack. Persistent Slab avalanches claimed lives along the east slopes of the Cascades in the past 2 weeks. Avoid steep, complex terrain and large avalanche paths to reduce the threat of the low likelihood - high consequences of Persistent Slabs; ensure a wide buffer between where you travel any large avalanche terrain.
Summary
Detailed Forecast
Today, you can easily trigger avalanches in the upper layers of the snow due to new snow, wind, and light rain at low elevations. New snow may have fallen on a variety of surfaces that could form a poor bond with the new snow. Watch for cracking, wind stiffened snow, freshly formed drifts, and small test slopes where the new snow easily slides as clues that you could trigger an avalanche. Steer around fresh wind features, convex rolls, and slopes holding a foot or more of newly fallen, cohesive snow that are 35 degrees and steeper.
Avalanches in the upper snowpack and heavy snowfall are making it easier to trigger deeper and dangerously wide avalanches. Persistent weak layers lurk deeper in the snowpack. The signs of Persistent Slab avalanches may not be obvious. These low likelihood, high consequence avalanches are very difficult to manage. The best way to stay safe is to avoid the slopes where you can trigger them. Take a day or two to choose more cautious terrain before returning to the kinds of slopes you traveled on prior to this storm. 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.
Snowpack Discussion
As of Thursday night almost a foot of snow has fallen on the east side of the Cascade crest. Expect less snow as you move further east. Crusts, surface hoar, and small facets could all create a poor bond for the new snow. Asses this new interface with small test slopes and quick tests. In areas of higher snow totals both Storm and Wind Slabs are adding to the underlying weak layers in the North East zone.
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. Several potential persistent weak layers exist. Two common layers that have been reported 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 2/13 (or 16) 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 triggered avalanches and more recent collapsing and whumphing. The exact depth of these layers depend on aspect, elevation, and proximity to the Cascade crest.
Observations
North
Mid-week, observers on Washington Pass reported a mix of sun crusts and surface hoar/near surface facets on snow surfaces 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.
Last 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.
Last Saturday, NCMG traveled in the Cuthroat area and observed the 2/13 layer down 3' (85 cm) at 6100' on a NNW aspect. Widespread collapses and a stubborn small persistent slab release was reported on Vasiliki ridge from a third party.
Central
On Wednesday, an observer in the Icicle drainage identified the mid-February facets over a crust on a northwest aspect.
Observer Matt Primomo reported a large slab avalanche on a southeast aspect at 7,500ft in Icicle creek that may have occurred last weekend.
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.
Last 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.
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
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: 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 9th, 2018 11:48AM
