Avalanche Forecast

Issued: Feb 25th, 2018 2:05PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Fresh Wind Slabs and lurking Deep Slabs have combined to create dangerous avalanche conditions. After the latest round of heavy snow and intense wind, you can trigger large and dangerous avalanches. Stay safe by avoiding slopes steeper than 35 degrees, obviously wind loaded features, and staying out from under large open slopes.

Summary

Detailed Forecast

Triggering large avalanches is still likely. Many of these could be big enough to kill you and even very large and destructive. The most dangerous areas will be steep slopes where new snow and wind have deposited 2 feet or more of cohesive snow since Friday. Use conservative decision-making and terrain choices. Avoid obvious drifts and deep pillows of wind deposited snow. Minimize your exposure to areas where avalanches commonly run by staying away from open slopes steeper than 35 degrees and out from under large avalanche paths.

With several overlapping avalanche problems this is a challenging time to travel in the mountains. Storm and Wind Slabs eixst in the upper snowpack and very dangerous Deep Slabs lurk below. An avalanche triggered in the upper snowpack could step down to deeper layers, resulting in a very large and destructive slide. While Deep Slab avalanches may be difficult to trigger your chances of surviving one are slim. Your best way to stay safe from these avalanches is to avoid triggering avalanches in the upper snowpack. Excellent travel conditions can still be found on slopes less than 30 degrees that are not connected to steep overhead slopes.

Snowpack Discussion

We’re on the tail end of a major avalanche cycle. Observers reported widespread avalanches throughout the West Slopes over the weekend. One person was killed in an avalanche south of Snoqualmie Pass on Sunday. Our condolences go to the friends and family of the victim. If conditions allow, NWAC staff will visit the accident site. A very large natural avalanche crossed Highway 20 east of Newhalem on Sunday.

A break in the weather is allowing avalanche conditions to ease. Triggering avalanches remains likely Monday. Heavy snowfall and strong winds elevated avalanche danger over the weekend. 2-3 feet of snow accumulated across the West Slopes since Friday. This fell on a variety of surfaces including old snowflakes, surface hoar, and possibly some small facets. Observers have reported inconsistent buried weak layers at the February 23rd interface.

Over a week ago, observers reported a widespread cycle of large to very large avalanches ran with a potent storm snow as well as on facets, buried on the 13th of February. In many locations these weak sugar-like facets sit 3 feet below the surface on or adjacent to a hard crust layer. If you dig that far into the snowpack, you will find a thin layer of sugar-like facets adjacent to a hard crust. You can use snowpits and snowpack tests to confirm the strong over weak snow layers of the Deep Slab avalanche problem. Avalanches, collapses, and whumphs have been triggered on this persistent weak layer well after last weekend’s initial avalanche cycle.

Snowpack test and observations from around the region continue to demonstrate that this layer can fail and produce avalanches. Tests and profiles show the most consistent indication for triggering Deep Slabs at Paradise on Mt Rainier and Stevens and Snoqualmie pass areas. Observers are now reporting a wider range of test results and observations on the 2/13 facets, including some indication of that this weak layer is slowly improving. In the Mount Baker and Crystal area the problematic weak layer is more intermittent. At Mount Baker it can often show up as facets around a series of crusts.

A thin rain crust formed and was buried on 2/17 near and below 4000’ in the Snoqualmie area. Limited information about this snowpack layer has been reported other than it is ski-supportable. 

Below the 2/8 crust there are no significant layers of concern.

Observations

Stevens Pass

On Sunday, Stevens Ski Patrol reported reactive conditions, triggering avalanches up to 3 feet deep in the recent storm snow with control work. NWAC staff observed Wind Slabs forming in open areas below treeline. They also reported tests indicating skier triggering on the 2/13 facets 90cm below the surface.

The most recent Persistent Slab avalanche was triggered on Monday at Stevens Pass backcountry (Stevens Pass-Hollywood Bowl-2-19). It failed on facets just above the 2/5 crust.

Snoqualmie Pass

On Saturday and Sunday, NWAC staff reported multiple locations near Snoqualmie pass that while the 2/13 facet layer is showing rounding, test results indicated that the potential for triggering an avalanche still exists. On Saturday, NWAC observers and Alpental Patrol reported numerous soft slab avalanches in the recent snow.

South

On Sunday, NWAC observer, Jeremy Allen reported blizzard conditions, small triggered slab avalanches in the new snow, and signs of rounding and variable test results on the 2/13 facets.

On Friday NPS rangers in the Paradise area reported 3-4 feet of snow over weak facets.

NWAC Forecaster Dallas Glass observed a large audible whumph caused by a collapsing weak layer last week near Paradise. Snowpack tests in the area indicated the persistent weak layer could fail and produce avalanches up to 4 feet deep.

 

Problems

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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing 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: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.

 

Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.

 

A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.

Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Feb 26th, 2018 2:05PM