Stevens Pass

A thick slab of storm snow rests on a weak layer over a crust. The likelihood of triggering large avalanches may be slowly decreasing, but overwhelming evidence indicates that the snowpack remains unstable on Stevens Pass. If you venture into the backcountry, be certain of your ability to avoid slopes steeper than 30 degrees and put plenty of space between yourself and big, steep terrain. 

The calmer weather has allowed natural avalanche activity to settle down. However, if you are a gamblin' man, you may still trigger large and dangerous avalanches. In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. Don't linger beneath roofs, travel in the mountains with partners and keep them in sight. 

On Saturday, three separate professional parties got test results that indicated propagation on the layer of concern on Stevens Pass. Check out one of the profiles here. On Friday the 15th, a skier remotely triggered a slab from approximately 50ft away at 4,800ft in an open area between old growth trees near Lanham Lakes. The slide took out the entire clearing, edge to edge, and ran on facets over a crust. Many steep slopes ran during the cycle on the 12th and 13th. While some avalanches ran in storm layers in the upper snowpack, the largest and most concerning avalanches appear to be running on a weak layer of facets (and in some places surface hoar) buried on February 8th. Many areas may have been reloaded by additional snow and wind. Common notable characteristics of these avalanches are very widely propagating crowns up to 3 feet deep. A few notable natural (unless otherwise noted) slab avalanches suspected to have run on the February 8th facets are: Tye Peak, SE, 5200ft up to 300ft wide. The entire Highlands Bowl on Big Chief Mtn was triggered remotely. Skyline Ridge, E, 5250ft. Arrowhead Mtn, Zephir paths in the Weyerhauser clearcut, N, ~3500ft. Nason Ridge S, 5,100ft.

February 15, 2019

Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experience cold and very storm weather. Significant snowfall has added up in all forecast zones. Records from Snoqualmie Pass DOT avalanche workers back to 1973 show that February 11-12th set a record for the most snow recorded in a 24hr period at that location. The table below shows storm totals starting February 8th through the morning of the 13th

5 day totals ending morning of Feb 13th

Water Equivalent (inches)

24hr storm totals

(inches)

Difference in Height of Snow (inches)

Hurricane Ridge

1.97

N/A

+ 30

Mt. Baker

1.94

44

Washington Pass

1.66

NA

+ 16

Stevens Pass

2.71

49

Snoqualmie Pass

3.91

80

Mission Ridge

1.86

38

Crystal

2.91

59

Paradise

4.55

N/A

White Pass

N/A

57 (4400ft)

+ 26 (5800ft)

Mt. Hood Meadows

4.70

43

Heavy precipitation brought many mountain regions to their tipping point. Avalanches ran readily with a peak of snowfall intensity. For Stevens Pass, Snoqualmie Pass, East Central, West South, Mt Hood, and possibly West Central zones we have good confirmation that this cycle happened from the night of February 11th through the 12th. In other zones, snow totals haven’t been significant enough for widespread avalanche cycles, or we lack data (like in the East South zone).

A natural persistent slab avalanche (D2.5) on a southeast aspect at 6,600ft. Grindstone Mtn in Icicle Canyon. Likely ran 2/12. Photo: Matt Primomo

The high rates of precipitation drove avalanches in the storm snow. Notably, a persistent weak layer of facets and surface hoar was buried in most zones on February 8th. Storms produced a widespread and prolonged cycle of avalanches on the February 8th interface, involving a variety of aspects and elevations. Local ski patrols, highway workers, and backcountry travelers reported extensive avalanching with widely propagating crowns and very sensitive conditions. With less stormy weather, observers have just begun to get a sense of the extent of the avalanche activity. Triggering persistent slab avalanches will be a concern for backcountry travelers in zones where the February 8th weak layer is active for at least the near, if not distant future. Stay tuned for more updates.

Large surface hoar near Snow Lake Divide on February 7, 2019 just before it was buried on the 8th. Photo: Jeremy Allyn