Avalanche Forecast

Nearly every zone in the Cascades contains a similar potentially dangerous snowpack structure. Each area has subtle differences, but they appear more alike than different. The most recent and compelling piece of information we have concerning the persistent slab is from Stevens Pass. On Saturday, two snowboarders triggered a persistent slab avalanche in Tunnel Creek outside the boundary of Stevens Pass ski area. Both were caught, carried and partially buried in the event. We will post more information as it becomes available.

Human triggered persistent slab avalanche form Tunnel Creek: Photo: Stevens Pass Ski Patrol

NWAC has not received observations from the Mt. Hood Backcountry, making it difficult to refine the forecast. Please help your forecast by submitting an observation.

The epic storm cycle that wrapped up Wednesday produced several deep avalanches averaging around 4 feet that were triggered Wednesday and Thursday by both skiers and explosives near and below treeline.

On Thursday and Friday, Mt. Hood Meadows Patrol identified a layer of weak snow (1-2 mm facets) above a hard crust as the weak layer/bed surface combo on northerly aspects below treeline. This weak layer/crust combination is likely most reactive near treeline and in the upper treeline elevations, but information is still limited. Stay safe by keeping your terrain selection simple.

By Saturday evening, 7” of storm snow had accumulated during the day and was being blown around by moderate to strong winds with more snow and transport anticipated.

The risk of tree well and snow immersion suffocation continues in areas with deep snow. Don't travel alone and always keep in contact with your partner. You can learn more about deep snow safety here.

If the sun comes out on Sunday, expect roller balls and loose wet activity that will entrain the surface snow and shed off rocks and steep slopes.

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