Avalanche Forecast

We aren’t seeing the same obvious signs of unstable snow that were found earlier in the week around Snoqualmie Pass, but there is plenty of evidence that our persistent slab problem is still alive. Alpental patrol triggered a few larger persistent slabs with explosives on Saturday. Some of these caught our attention because they occurred at lower elevations (4000’) and on more stubborn slope angles (30-35 degrees). Snow profiles and snowpack tests from Snoqualmie Mountain and Roaring Ridge continued to highlight the Feb 8 facet and/or surface hoar layer. It's this layer of unstable snow that has our attention and continues to shape how we are traveling in the mountains.

As the weather continues to improve, if you travel to higher elevations or more remote trailheads, know that we have a higher degree of uncertainty in these areas. Proceed cautiously, and ease into the terrain.

There are two other hazards to consider Sunday.

• When the sun comes out, you may see rollerballs and small loose avalanches from steep, rocky slopes. Don’t let these easy to predict avalanches catch you off guard.

• The snow out there is deep, especially around tree wells. Be careful when traveling near small trees. Keep your partners in sight, and maintain voice contact.

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