Avalanche Forecast

Issued: Feb 21st, 2018 11:37AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Persistent Slabs and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

While it is still possible to trigger a large avalanche in larger terrain, there are still excellent travel conditions in more protected lower angle terrain of lower consequence. The ongoing Persistent Slabs problem is uncommon and still requires terrain choices with a wide margin of safety. 

Summary

Detailed Forecast

Cold temperatures with light snow showers at times and generally light winds are expected Thursday. This weather should do little to change the current avalanche danger.

The very cold temperatures are helping preserve the weak interface, where present, at the buried crust layer from 2/13, maintaining the possibility of triggering a large avalanche in open big terrain steeper than about 35 degrees.  This layer varies throughout the terrain but ranges in general from 2-3 feet or more below the surface, making it more difficult to trigger with the weight of a skier or boarder, but not impossible. This uncommon avalanche problem in our snow climate requires a step back in terrain choices. Under these conditions, be prepared not to see avalanches but this doesn't mean there is not inherent danger. 

The good news is that great snow conditions can be accessed in smaller more protected, lower angled terrain with lower exposure risk. 

Shallow fresh wind slabs may be found near and above treeline so watch for wind stiffened or wind sculpted surface snow.

Avoid large terrain, especially areas exposed to avalanches that could come from above. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches. A small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab.

The persistent slab problem is likely more sensitive on shaded, non-solar aspects, but can not be ruled out on any aspect. 

Snowpack Discussion

Cold temperatures and light snow showers since Sunday have not significantly changed the overall danger since Sunday. Since Sunday most areas have received 2-8 inches of cold low density snow. In most areas this surface snow is low cohesion and lacks the ability to propagate a slab avalanche.

The low cohesion snow is very susceptible to wind transport and there have been some reports of fresh but shallow wind slabs forming in exposed terrain near or above treeline.   

The storm snow late last week in many areas fell on weak, near surface facets or surface hoar that developed during a fair weather period on a rain crust. These weak sugary facets over a hard crust, created both a smooth sliding surface and a very weak layer above, now loaded with a slab in some areas.

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 may 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 and requires a change in terrain choices.

Persistent Slab avalanches were reported in Snoqualmie Pass area Saturday and Sunday with a snowboarder caught, carried and injured near the Alpental ski area Sunday. The culprit,  weak sugar-like snow has been observed on a hard crust 2-4 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 Wednesday.  

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. 

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 a large persistent slab avalanche Sunday outside the Alpental ski area near Powder Bowl on a NE aspect near treeline. The persistent slab avalanche released on the smooth crust buried 2/13. The rider was carried 1000 feet and suffered serious but non-life threatening 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 consistent 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. In similar terrain Tuesday, Dallas heard an audible, very loud whumpf in the area and also triggered another whumpf indicating a collapse of a layer below, often a weak layer.

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

An icon showing 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: North, North East, East, South East, West, North West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Wind Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Feb 22nd, 2018 11:37AM