Avalanche Forecast
Issued: Feb 19th, 2018 11:48AM
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
Avalanche conditions will remain dangerous Tuesday. You can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 degrees. The combination of Wind Slabs, uncommon Persistent Slabs and a high degree of variability over short distances all points to conservative decision-making and cautious terrain choices. Big terrain will be able to produce big avalanches Tuesday.
Summary
Detailed Forecast
Cold temperatures with a chance of a few light snow showers Tuesday will not significantly improve the current avalanche danger, allowing recent slabs to persist longer than usual.
Even a small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab. Snowpack observations and one serious skier triggered avalanche from along the west slopes of the Cascades Sunday show you can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 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. Avalanches may be surprisingly large and run farther than you expect.
The combination of Wind Slab and Persistent Slab requires very cautious decision-making and terrain selection. Persistent Slab avalanches are not very common in the Cascades and the usual playbook of waiting a day or two for storm instabilities to settle out does not apply.
Snowpack Discussion
Persistent Slab avalanches were reported in Snoqualmie Pass area Saturday and Sunday with a skier caught, carried and injured near the Alpental ski area Sunday. The culprit, weak sugar-like snow has been observed on a hard crust 3-4 feet below the snow surface in almost all of the forecast zones. Professionals from around the region Sunday and Monday found this layering reactive in snowpack tests. You will still be able to trigger these large and dangerous avalanches Tuesday.
An extended cycle of natural and triggered avalanches occurred Wednesday through Saturday on the west slopes of the Cascades. Peak avalanche activity occurred late Friday to Saturday.
Since February 15th, up to 3.5 feet of snow has fallen with extreme winds on Saturday. Rain was reported up to 4000 feet from Stevens Pass south Saturday. If you dig 2.5-4 feet below the surface, you will find recent, firmer 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
NWAC observer, Lee Lazzara was back in the Baker BC Monday. Lee reported a high degree of spatial variability both of the persistent slab depths and also the crust/facet layering structure. Wind slabs were seen on a variety of aspects but were stubborn to trigger. Snowpack tests still indicated the persistent slab was likely to trigger and propagate. Terrain choices were limited due to the observed conditions.
On Sunday, Mt. Baker ski patrol reported 3 separate skier triggered wind slab avalanches up to 2 feet deep outside the ski area. One wind slab avalanche on Hemispheres broke 180 ft wide and caught, carried and partially buried a skier. The skier was able to self-extricate.
Details are sparse but two snowmobile triggered avalanches were reported Sunday, one on a WNW aspect at 5800 feet. One was in the Canyon Creek area, the other in the Glacier Creek area with a crown depth of 3-4 ft.
NWAC pro-observer Lee Lazzrra on Sunday found NE winds significantly redistributing snow near Artist Point with scoured surfaces near ridges. In this area the 2/8 crust was 3+ feet down in non-wind scoured areas. A crust facet sandwich is widespread about 3-4 feet below the surface on all but steep solar slopes. Snowpack tests were limited and inconclusive regarding the potential for a persistent slab avalanche in this area.
Stevens and Snoqualmie Pass
A snowboarder triggered what was likely a large persistent slab avalanche Sunday outside the Alpental ski area near Powder Bowl on a NE aspect near treeline. The avalanche began in recent storm snow and stepped down to about 3 feet, likely the 2/8 crust. The rider was carried 1000 feet and suffered non-life threatening but serious injuries.
On Sunday, NWAC staff performed snowpack tests indicating propagation (Propagation Saw Test) on the facets buried February 13th. These tests were located on W and SW aspects at Snoqualmie below treeline in the Kendall Peak and Rampart Ridge areas. A ski-supportable re-freezing rain crust was noted up to 4200 ft. The depth of the 2/8 crust varied depending on elevation, between 15-30" (40-80 cm) down.
In the Yodelin area near Stevens Pass in snowpack tests Sunday, persistent slabs were deemed likely to propagate on the facets above the 2/8 crust which was 2.5-3 ft (70-90 cm) deep. A slight rain crust was noted 6" below recent low density snow.
On Saturday, 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. They were also able to trigger a small storm slabs on steep slopes within the storm snow.
Mt Rainier
Monday, NWAC forecaster, Dallas Glass traveled above Paradise on Mt Rainier reporting consistently 2 ft of low density snow over 2 ft of firm slabs formed Saturday. The slabs are sitting over a very weak structure of facets on the rain crust from Feb 8th.
On Sunday, an avalanche professional at Mt. Rainier reported 4 feet of snow above the 2/8 crust. Facets were found above the crust but were rounding. Recent wind distribution was significant from Saturday and debris from several from large avalanches that ran on Saturday were noted. Due to continued shower activity Sunday, new storm instabilities were present in the storm snow with natural small storm slab avalanches observed.
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
Valid until: Feb 20th, 2018 11:48AM
