Avalanche Forecast

 Stevens Pass has received over 50 inches of snow since February 8th. Strong east wind drifted snow on the 14th, adding more stress and fresh slabs to a snowpack that was already unstable. On Friday, you can expect light snow, some breaks in the clouds, and lighter west wind. The calmer weather will allow the likelihood of natural activity to decrease. It will still be easy to trigger large and dangerous avalanches. In addition to avalanches, all the deep snow is building concern for Snow Immersion Suffocation, or tree well hazards. Travel in the mountains with partners and keep them in sight.

See the Regional Synopsis for an overview of recent storm and avalanche activity. Local ski patrols, highway workers, and backcountry travelers all reported extensive avalanching on the night of the 11th, into the 12th, with triggered slides and a few naturals continuing into the 13th. 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 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. Big Chief Mtn, Highlands Bowl, SE, ~5400ft, remotely triggered. Skyline Ridge, E, 5250ft. Arrowhead Mtn, Zephir paths in the Weyhauser clearcut, N, ~3500ft.

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

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.