Bollettino valanghe

Stevens Pass

Feb 17 – 18, 2019Northwest Avalanche Center
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Archiviato · scaduto 8 years ago

Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
Apri la mappa completa

In sintesi

Recent human triggered slab avalanches have occurred on Stevens Pass. This is bullseye information that conditions remain dangerous. Travel conservatively, and stay out of complicated terrain. Put lots of space between yourself, and big, steep slopes. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2

    A problematic snowpack structure can be found at a wide variety of aspects and elevations, and is well pronounced even near and below treeline. Uncertainty exists in the alpine elevation band. More time is needed before I'd trust any steep slopes. How steep is too steep? I'm not sure, but recent activity suggests that the steeper the slope, the more likely you are to get one to release. A slab of 2 to 3 feet deep rests on a few inches of facets over a crust, a persistent weak layer that makes me cringe. This is not the time to go into complex terrain or onto unsupported slopes. Measure your slope angles, and don't get lulled to complacency if you don't get obvious clues to instability. Stay far away from steep slopes that may be connected to where you are. These persistent slabs have been breaking widely across terrain features, even in open areas in the trees. It is a good time to keep it reeled in, and don't let complacency catch you off guard.

    Professional guides and astute backcountry travelers are still avoiding big terrain and unsupported slopes. Even steep slopes below treeline are worth going out of your way to avoid. Why? Numerous observations of: 

    1. Recent skier triggered avalanches.

    2. Widespread collapsing, which makes a "whumphing" sound as air escapes from the collapsed weak layer, sometimes shaking trees a long distance away. 

    3. A layer of weak, sugary facets and surface hoar resting on a crust 2-3 feet below the snow surface. Often weak snow is found beneath this uppermost crust as well.

    4. Snowpack tests that continue to indicate propagation. 

Discussion

Snow and Avalanche Discussion

On Saturday afternoon a party of two snowboarders dropped south into the backcountry off Cowboy Ridge, riding down towards Tunnel Creek. At 5,400 ft on a Southeast aspect, they triggered a D2 avalanche on a 40 degree slope, 150ft wide with an average crown depth of 26”. It ran about 400 vertical feet on facets over a crust, breaking through the crust, and stepped down to facets over another crust a few inches deeper. Both members were partially buried, one up to his his head, the other up to his waist. They were able to self rescue without injuries, and hiked up the bedsurface where they re-entered the ski area boundary.  

This is as a very relevant near miss. A good reminder that large and dangerous avalanches may still be triggered on this layer of concern. Additionally, an observer reported a rumbling collapse on a Northwest aspect at 4,100ft on Sunday. On Saturday, three separate professional parties got test results that indicated propagation on the layer of concern on Stevens Pass. Check out one of the profiles here. On Friday the 15th, a skier remotely triggered a slab from approximately 50ft away at 4,800ft in an open area between old growth trees near Lanham Lakes. The slide took out the entire clearing, edge to edge, and ran on facets over a crust.

Bottom line, if you are a gamblin' man (or woman), you may still trigger large and dangerous avalanches. In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. Don't linger beneath roofs, travel in the mountains with partners and keep them in sight.

Snowboard triggered slide in Tunnel Creek. Photo: Dan Veenhuizen 2/17/2019

 

 

Snowpack Discussion

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

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