Avalanche Forecast

New snow amounts from vary widely across the region. Between Feb 22nd and 25th the Salmon la Sac area has received 1" of snow water equivalent with up to 1.5 feet of snow. On the 25th, Mission Ridge reported 13 inches of snow in 24 hours and drifts up to 4 feet deep at 6,400ft. In Icicle Canyon, you may find less recent snow, less significant drifting with an overall stronger snowpack. Observers continue to report very shallow and poor snowpack structure on northerly aspects in sagebrush country. Slab avalanches remain possible on this weak basal structure, and a recent avalanche was observed in Swakane Canyon on a North aspect at 1,900ft on this same layer (see photo below). 

An observer on Blewett Pass on Saturday was able to trigger a small wind slab avalanche on a north aspect at 4800ft. He also reported a large natural slab avalanche in steep terrain on a north-northwest aspect at 3000ft. This avalanche may have run on old, faceted snow, and likely had freshly drifted overlying slabs. This is the most recent report of a possible persistent slab avalanche on Blewett Pass.

D2 Persistent Slab avalanche in Swakane Canyon 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

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.