Avalanche Forecast

Issued: Feb 25th, 2019 10:17AM

The alpine rating is below threshold, the treeline rating is below threshold, and the below treeline rating is below threshold.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Abundant recent snow and strong shifting winds will maintain dangerous avalanche conditions Tuesday. Wind slabs formed over the past few days will be slow to settle. You can trigger wind slabs on steep open slopes on a variety of aspects as strong winds have occurred from multiple directions. Choose conservative, simple routes with minimal exposure to avalanche terrain.

Summary

Discussion

Fresh wind slabs formed over the past two days is the likeliest slab avalanche problem to encounter Tuesday, however, there are a number of overlapping avalanche problems that should guide your travel choices. 

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. 

Simplify the situation by avoiding large open slopes 35 degrees and steeper, and minimize your exposure to avalanche terrain. Especially steer clear of steep, unsupported slopes. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative up routes and descent options. When dealing with persistent slab issues, remember a handful of sled or ski tracks on a slope doesn't mean that it won't release for you.  

In non-wind affected terrain, loose dry avalanches will continue to be the most identifiable and easiest to manage avalanche problem. They may release naturally on steep sunny slopes or be human triggered on shaded aspects. Think about sluff management on very steep slopes and what would happen if a small but fast moving loose avalanche pushed you into or over a terrain trap.

Recent observation: 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'. Other weak layers closer to the surface where identified but were lacking a denser slab on top. Snow depths jumped from about 2 ft at 3000' to 6 ft above 5000'. 

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.

Snowpack Discussion

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.

 

Valid until: Feb 26th, 2019 10:17AM