Avalanche Forecast
Issued: Feb 17th, 2018 12:10PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Travel in avalanche terrain above treeline is not recommended. Very dangerous avalanche conditions exist due to recent heavy snowfall, extreme winds, and weak old snow. Stay far away from slopes steeper than 30 degrees and out from under any terrain where avalanches could come from above.
Summary
Detailed Forecast
You can trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than 30 degrees. Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Put a wide buffer of terrain between where you travel and any steep slopes. Use extra caution in areas where avalanches could come down from above. Persistent Slab Avalanches are not common in the Cascades. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches.
Avoid areas where the wind has recently drifted snow. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a Wind Slab avalanche. Triggering a Wind Slab could dig into deeper layers and result in a very large Persistent Slab. This combination of Wind Slabs and Persistent Slabs requires very cautious decision-making and terrain selection. Only travel in the backcountry if you are sure of your ability to avoid avalanche terrain, if necessary.
Snowpack Discussion
Observers triggered dangerous Persistent Slab avalanches at Snoqualmie Pass and Hurricane Ridge in the Olympic National Park on Saturday. The culprit weak, sugar-like snow has been observed adjacent to a crust 2-4 feet below the snow surface in almost all of the forecast zones. An extended cycle of natural and triggered avalanches occurred Wednesday through Saturday on the West Slopes. The peak avalanche activity likely occurred late Friday to Saturday. While conditions are easing, you can still trigger dangerously large avalanches on Sunday.
On Friday, a party sustained non-critical injuries in an avalanche on a popular travel route below Mushroom Couloir in the Alpental Valley. Skiers and riders triggered numerous slab avalanches in the Bagley Lakes area of the Mt Baker backcountry. On Thursday backcountry travelers triggered or were caught in avalanches at both Snoqualmie and Stevens Passes.
Since February 15th up to 2.5 feet of snow has fallen with extreme winds. Rain was reported up to 4000 feet from Stevens Pass south Saturday. Dangerous conditions and difficult travel limited observations. If you dig 2-3 feet below the surface, you will find recent, hard snow sitting on a thin layer of weak, sugar-like facets adjacent to a crust. This strong slab over weak facets is the key recipe for Persistent Slab avalanches.
Observations
Mt Baker
Saturday, an observer in the Mt Baker backcountry reported remotely triggering an avalanche from low-angle terrain and test results indicating propagation. On Friday, an NWAC observer reported numerous triggered slab avalanches in steep terrain around Bagley Lakes, with some up to 300 feet wide. A crust facet sandwich is widespread about 3 feet below the surface on all but steep sunny slopes.
Snoqualmie Pass
At Snoqualmie Pass, Alpental Ski Patrol reported ski cuts triggering large avalanches that were 2 feet deep and ran on facets. These avalanches were surprising, with widely propagating crowns, and involved audible collapsing of the snowpack. One slide was roughly 300 feet wide. On Friday, NWAC observers reported snowpack tests indicating propagation (Propagation Saw Test) on the same facets that were buried February 13th. They were also able to trigger a small storm slabs on steep slopes within the storm snow.
Mt Rainier
On Friday, Forecaster Dallas Glass toured near Paradise on Mt Rainier. He reported 2.5 feet of recent snow on a crust and fresh wind drifts up to 3 feet deep. While no avalanches were reported cracks were observed in recently drifted snow.
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: All elevations.
Likelihood
Expected Size
Valid until: Feb 18th, 2018 12:10PM
