Bulletin d’avalanche

Stevens Pass

Feb 15 – 16, 2019Northwest Avalanche Center
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Archivé · expiré 8 years ago

Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

We're on the tail end of a major avalanche cycle resulting in many large and wide-breaking avalanches. Less stormy weather on Friday will bring a decrease in the danger rating, but the serious risk of triggering dangerous avalanches remains. If you venture in the backcountry be certain of your ability to avoid slopes steeper than 30 degrees and put plenty of space between yourself and large avalanche paths.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    On Thursday, easterly wind drifted snow at upper elevations, formed fresh slabs, and further stressed an already tenuous snowpack. Avoid any slope over 30 degrees where you see recently drifted snow. Today, you can't simply avoid or manage the storm instabilities. Any avalanche triggered in the recent snow could result in a larger and more deadly persistent slab avalanche. In sheltered areas you could still trigger large storm slab avalanches in the snow that fell in the past week. The upper layers may slide easily.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2

    The snowpack has seen a rapid and major change. Give the snow some time to adjust. Continue to stick to slopes under 30 degrees and smaller terrain features. Stay far out from under big avalanche paths and make sure not to get too close to start zones. Persistent slabs can break widely across terrain features, come down on top of you, and can be triggered from a long distance away. These avalanches can easily bury or kill you. They are difficult to predict and break in ways much wider and in more surprising ways than avalanches due to snow and wind alone.

    On the 13th, during a tour in the Skyline area of Stevens Pass, we avoided even smaller open slopes over 30 degrees that I normally wouldn't consider posing a hazard. On the tour we saw; 1) Widespread collapsing of the snow underfoot and a "whumphing" sound that traveled across a slope, shaking trees over 60 feet away. 2) Avalanches with crowns over 200ft wide, and some on neighboring ridges that were remotely triggered, 3) A layer of weak, sugary facets and surface hoar resting on a crust 3-4 feet below the snow surface. Watch for these signs indicating that you can trigger a Persistent Slab avalanche.

Discussion

Snow and Avalanche Discussion

 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

Snowpack Discussion

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.

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