Avalanche Forecast

Issued: Feb 26th, 2018 11:58AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Storms over the weekend created dangerous avalanche conditions that are gradually improving. Evaluate snow and terrain carefully as recent storm layers gradually stabilize. It may still be possible to trigger a dangerous Persistent Slab avalanche. Stay away from large open slopes steeper than 35 degrees that are suspect for these avalanches.

Summary

Detailed Forecast

The avalanche danger Tuesday will continue to gradually decrease with cool temperatures and generally light snow showers expected.

You could still trigger an avalanche Tuesday, but avalanches are becoming more difficult to trigger as the extensive storm and wind slabs deposited last weekend gradually settle and stabilize. Continue to watch for areas that received significant wind loaded snow, such as steep slopes below ridges, mainly facing North to Southeast. 

Triggering large avalanches has become unlikely. However, the most dangerous areas will be steep slopes where recent 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 different types of overlapping avalanche problems this continues to be a challenging time to travel in the mountains. Wind and Storm Slabs are gradually stabilizing but certainly not out of the question, while very dangerous Deep Slabs lurk below. A likely way to trigger one of these deep slabs would be for an avalanche triggered in the upper snowpack to 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

The avalanche danger has improved markedly as we move beyond the dangerous avalanche conditions that developed Sunday and unfortunately claimed 3 lives in two separate accidents near Snoqualmie Pass. NWAC field staff traveled to both accident sites to investigate the avalanches in preparation for forthcoming accident reports. Our condolences go to the friends and families of the victims. 

A very large natural avalanche crossed Highway 20 east of Newhalem on Sunday.

While avalanche conditions are improving, triggering avalanches remains possible Tuesday. 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 thin sun crusts, 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 Monday, professional observations in the Stevens Pass area confirmed that the deep persistent layer we have been tracking was buried about 4 ft below the surface was still likely to trigger based on multiple PST snow test results. Storm layer instabilities were gaining strength rapidly. The crowns of numerous large natural avalanche that likely released during the storm Sunday were visible along Skyline and Windy Mtn near the 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

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

Unlikely

Expected Size

1 - 2

Valid until: Feb 27th, 2018 11:58AM