Avalanche Forecast

An extended period of instability began on the 11th with natural avalanches reported through the 13th on a wide range of aspects and elevations. Common characteristics of these avalanches are very widely propagating crowns 2 to 3 feet deep. While some avalanches ran in storm layers in the upper snowpack, many have run on a layer of faceted snow over a crust. Dangerous conditions and deep snow have limited our ability to gather more detailed information from near and above treeline.

Observers in the Salmon la Sac area on Friday found 90cm of snow over the facet/crust layer of concern at 4,800ft. On Thursday, a straight up scary snowpack structure was found in Icicle Creek with roughly 100 recent slab avalanches, some over 1,000ft wide. Low elevation slopes near towns and cities are now locations of concern for avalanches. On Tuesday, I found a very weak basal structure with a thin snowpack, particulary on shaded aspects in the Wenatchee foothills. Expect similar conditions near the towns of Peshastin and Leavenworth. In addition to avalanches, all the deep snow is building concern for Snow Immersion Suffocation, and roof avalanches.  Travel with partners, keep them in sight, and don't linger under roofs. 

A natural avalanche in the Icicle Canyon. Grindstone Mtn, SE, 6,600ft. Photo: Matt Primomo

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 Equivilent (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