Avalanche Forecast

Snow and Avalanche Discussion

On Wednesday, a skier triggered a large hard slab off the NW aspect of Mt McCausland near Stevens Pass and went for a 600ft ride. He was the second skier on the slope, pulled his airbag, and ended up on top of the debris. Luckily the skier was ok and got away with just losing a pair of poles. This was in a couloir feature at 5,600ft in the near treeline elevation zone, and the crown was 18-24" deep. We will pass along more info as it becomes available. While this event is in a different forecast zone, similar conditions make this avalanche involvement extremely relevant.

A very small shot of new snow will not be enough to increase avalanche danger around the East-South zone Thursday. There are two pertinent pieces of information on our minds.

• Recent east winds reshaped the snow surface in many areas. You may find wind affect snow on a variety of aspects and elevations.

• Two shallow weak layers have been reported in the upper snowpack. You may find thin layers of facets and/or buried surface hoar preserved in the upper few feet of the snowpack. So, far there have not been any avalanches confirmed on either of these layers. That may change as winds drift snow into slabs over these weak snow layers.

A snow profile highlighting buried weaklayers in the surface snow in the nearby White Pass area. Photo: Glass

Fresh wind slabs formed over the past two days is the likeliest slab avalanche problem to encounter Wednesday. It doesn’t take much to blow the light, dry surface snow into wind slabs. Over the past few days the winds impacted the East-South zone from a variety of directions. As a result you may find reactive wind slabs on several aspects, especially at higher elevations. Keep you eyes open for areas where the wind deposited snow into thicker and firmer drifts. If you see smooth pillow-shaped features, fresh cornices, or find firmer snow surfaces, steer away from nearby slopes greater than 35 degrees.

The adjacent West-South zone saw the most recent activity on our deep persistent weak layer. However, even that activity is now more than a week old. So, why are we still talking about this avalanche problem? On Sunday, 2/24, observations from the Tieton River drainage identified the deep persistent weak layer identified in adjacent zones about 3 feet down above 5000'.  Nearby Crystal, Paradise, and White Pass areas all confirmed the presence a layer of weak sugar facets 3-5 ft below the snow surface, so the problem remains extensive. Many snowpack tests continue to show this layer can fail and produce avalanches. During conditions like this, be leery of places where the snowpack is thinner, the terrain is complex, or potential triggers are large. If you are planning to travel in large open avalanche terrain, stop and consider this low likelihood but high consequence scenario. Other weak layers closer to the surface where identified in the Tieton Drainage, but were lacking a denser slab on top. Snow depths jumped from about 2 ft at 3000' to 6 ft above 5000'.

Keep in mind that any terrain, even low elevations, that had a shallow snowpack at the beginning of February has transformed into potential avalanche terrain, even sagebrush country. Snowstorms over the last few days continue to incrementally load and stress the snowpack. Avalanches that release in the new snow may step down to deeper layers and lead to more destructive and deadly avalanches.

Read the East Central zone forecast for the most complete information on the complicated snowpack confirmed north of I-90. 

D2 Persistent Slab avalanche in Swakane Canyon (East Slopes Central forecast zone) that ran on basal facets near the ground. North at ~1900ft. Unknown when it occurred, perhaps remotely triggered by a skier ascending the rib nearby. Observed on 2/22. Photo: Steven Gnam

Forecast Schedule and No Rating

At this time, we do not have enough specific snowpack information to issue an avalanche hazard rating for the East Slopes South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season. When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

February 24th, 2019

The Status Quo

As we look at the avalanche conditions the phrase “Status Quo” keeps coming to mind. We use this term in the avalanche industry to note periods of no substantial changes, where observations continue to support the current avalanche danger, and our terrain use doesn’t appreciably change. We’ll look at recent avalanche conditions with this lens.

Avalanches

Following several large winter storms just prior to Valentine's Day, we saw very active avalanche conditions. With a few notable exceptions, most of the avalanche activity occurred more than a week ago. This lack of activity has allowed us to gain some confidence with the persistent weak layer buried on February 8th. You can see this reflected in the progression in the likelihood of persistent slab avalanches from “Likely,” to “Possible,” to “Unlikely.” The timing of this progression has been different depending on the zone.

The recent exceptions were two very large avalanches in the Crystal Mountain area on February 20th. During avalanche mitigation work, Crystal Mountain Ski Patrol triggered a slide on a SE aspect at 6300 ft in a less often skied portion of their ski area. On the same day, a skier triggered a persistent slab on a W aspect at 6300 ft in the backcountry adjacent to the ski area. Both were 3-4 ft deep. It’s hard to ignore these two obvious signs of instability.

Feb 20, 2019: SE 6300’: SS-AE-D3-R3-O. Photo: Crystal Mt Ski Patrol

Snowpack

Snow profiles and snowpack tests have been coming in from nearly every region. This is great! It helps us put together a more complete picture of the snowpack structure. Observers continue to report a layer of facets 2-5 feet below the snow surface and just above a crust. These facets are showing signs of rounding (gaining strength). In the past week, snowpack tests have become more variable. Some tests are indicating triggering an avalanche and crack propagation (necessary for a slab avalanche) are becoming less likely. That said, we are still seeing other tests that indicate that triggering deep persistent slabs remains a possibility. More tracking of this trend is needed to concretely illustrate the trend.

The Feb 8 facet layer is 3-5ft deep in the Snoqualmie Pass area.  Photo: Susie Glass

Weather

Our recent weather systems have not been big water producers, especially by Cascade standards. However, the colder than normal temperatures have produced light, low-density snowfall. Wind transported snow have been the main driver of slab avalanches in the new snow. These storms have been large enough to keep the avalanche danger elevated, but not enough for major spikes in danger.

Recent low-density snow drifted by the wind to form fresh slabs Photo: Dallas Glass

Looking Ahead

At this point avalanches on persistent weak layers can’t be fully ruled out on specific aspects or elevations in most zones. Your chances of triggering an avalanche on a persistent weak layer are lower than they were on February 13th, but the change from day-to-day will remain slow and incremental. As a result, you’ve seen slow changes in the avalanche danger in most zones. Due to the serious consequences and the uncertainty that these avalanches present, we will continue to operate with a “Status Quo” mindset. Don’t let this lack of change lull you to sleep.

When will the conditions change? Not tomorrow, but possibly within the next week or two. Keep checking the forecast. Make sure to read the Snow and Avalanche Discussion and the Avalanche Problem text so you can stay up to date with any changes as they arise.