Stevens Pass

A hefty slab of storm snow rests on a weak layer over a crust. 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 large avalanche paths.

Stevens Pass has received over 52 inches of snow since February 8th. The calmer weather has allowed natural avalanche activity to settle down. However, It will still be easy to trigger large and dangerous avalanches. In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, or tree well hazards, and roof avalanches. Don't linger beneath roofs, and travel in the mountains with partners and keep them in sight. 

See the Regional Synopsis for an overview of recent storm and avalanche activity. An extended period of instability began on the 11th with natural avalanches reported through the 13th. Local ski patrols, highway workers, and backcountry travelers all reported extensive slides. Common notable characteristics of these avalanches are very widely propagating crowns about 3 feet deep. 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. Visibility, deep snow, and dangerous conditions have limited observations. 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 Weyhauser clearcut, N, ~3500ft. Nason Ridge S, 5,100ft.

February 15, 2019: Lots of instability has been observed, even at elevations. Near Coles Corner, Hwy 2. Photo: Matt Primomo

Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experienced cold and very stormy 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).

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.