Avalanche Forecast

Recent avalanches have had very widely propagating crowns 2 to 3 feet deep. They've been running on a layer of faceted snow over a stout crust, or underneath this crust where it is thin and decomposed. Limited information has come in from near and above treeline. Mild weather with some sun has created a melt freeze crust on solar aspects over the past few days. The warm weather is helping to heal instabilities at the very low elevations and particularly on southerly aspects. The snowpack is staying cold in the shade. This is where best snow conditions can be found, but it is also where the structure is most concerning.  

Observers on shaded aspects of Tumwater Mountain on Saturday reported a number of small collapses, and one very large one. Observers on Blewett Pass are finding very weak facets underneath a decomposed crust on northerly aspects above 4,000ft. On Friday, reports of numerous tree shaking collapses came in from north aspects in Squilchuck State Park. Observers in the Salmon la Sac area on Friday found 3ft of snow over the facet/crust layer of concern at 4,800ft, but few signs of instability. A concerning snowpack structure can be found in Icicle Creek on the north half of the compass, where recent storm snow rests on a few inches of facets over a freezing rain crust. Here, roughly 100 recent slab avalanches were observed on the 13th, some were over 1,000ft wide. Low elevation, shaded slopes near towns and cities are still locations of concern for avalanches. A very weak basal structure with large faceted grains near the ground can be expected on shaded aspects in the Wenatchee foothills. Roofs may avalanche on Sunday if they haven’t already. Don't linger under roofs.

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