Boletín de aludes

Stevens Pass

Feb 26 – 27, 2019Northwest Avalanche Center
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Archivado · caducó 8 years ago

Peligro de aludes

Thu
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Lo esencial

Expect cold temperatures and east wind with wind slabs and lingering weak layers keeping avalanches possible. You can trigger wind slab avalanches on steep slopes at upper elevations. It's now difficult to trigger older weak layers, but the resulting avalanche could still be very large. Choose well-supported terrain and minimize your exposure to large open slopes where avalanches can start.

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1

    Watch for drifts and pillows of snow on leeward sides of ridges and wind exposed terrain near and above treeline. You can see textured wind surfaces and feel stiffer slabs as you travel through these areas. Steer around lense-shaped wind features. It's especially important to avoid triggering wind slab avalanches, right now, as they could break into deeper layers and result in a dangerous deep persistent slab avalanche. 

    Gusty east wind efficiently drifted the recent, light snow since Sunday. Any snow still available may continue to form drifts on Wednesday. You will be able to trigger wind slabs most easily where they're resting on slick crust/facets layers on sunny slopes or on softer layers (including facets) on shaded aspects. The most likely pieces of terrain to encounter a wind slab avalanche will be near high elevation ridges on the leeward slopes and wind exposed areas in the pass corridor and drainages that funnel east wind over the Cascade Crest. Features such as Lichtenberg Mountain, Jim Hill, Rock Mountain, and other high peaks harbor this type of terrain.

  • Deep Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    2

    It's now difficult, but not impossible, to trigger avalanches on facets buried in early February. As the weak layer gains strength, the evidence that you can trigger these avalanches is getting harder to find. If you're in the wrong place, you may get more "evidence" than you bargain for. Due to uncertainty and the potential consequences of triggering one of these wide-breaking avalanches, I'm still avoiding large start zones and steep, unsupported features. 

    Avoid large, open slopes 35 degrees and steeper, and minimize your exposure to large avalanche terrain. Steer clear of steep, unsupported slopes with rock features and large upper elevation start zones at that received recent wind-loading. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative uphill routes and descent options to minimize your risk.

    The February 8th facet/crust layer can be found about 2-4 feet beneath the surface on all aspects and elevations. On the 20th, this layer is responsible for two very large triggered avalanches near Crystal Mountain. The last reported avalanche on the weak layer in this zone occurred on the 16th in the Tunnel Creek drainage.

Discussion

Snow and Avalanche Discussion

East wind has drifted storm snow from the past week. These storms buried a mix of surface hoar and near surface facets on shaded aspects, and melt-freeze crust on southerly aspects. We haven't received reports of slab avalanches on these layers, but they could make it easier to trigger wind slab (or lingering loose dry) avalanches. 

Wind transported snow suspended above fresh drifts. Monday, 2/25, 5,000ft, SSE, Lichtenberg Mountain, Stevens Pass. Photo: Josh Hirshberg

Snowpack Discussion

February 24th, 2019

The Status Quo

As we look at the avalanche conditions the phrase “Status Quo” keeps coming to mind. We use this term in the avalanche industry to note periods of no substantial changes, where observations continue to support the current avalanche danger, and our terrain use doesn’t appreciably change. We’ll look at recent avalanche conditions with this lens.

Avalanches

Following several large winter storms just prior to Valentine's Day, we saw very active avalanche conditions. With a few notable exceptions, most of the avalanche activity occurred more than a week ago. This lack of activity has allowed us to gain some confidence with the persistent weak layer buried on February 8th. You can see this reflected in the progression in the likelihood of persistent slab avalanches from “Likely,” to “Possible,” to “Unlikely.” The timing of this progression has been different depending on the zone.

The recent exceptions were two very large avalanches in the Crystal Mountain area on February 20th. During avalanche mitigation work, Crystal Mountain Ski Patrol triggered a slide on a SE aspect at 6300 ft in a less often skied portion of their ski area. On the same day, a skier triggered a persistent slab on a W aspect at 6300 ft in the backcountry adjacent to the ski area. Both were 3-4 ft deep. It’s hard to ignore these two obvious signs of instability.

Feb 20, 2019: SE 6300’: SS-AE-D3-R3-O. Photo: Crystal Mt Ski Patrol

Snowpack

Snow profiles and snowpack tests have been coming in from nearly every region. This is great! It helps us put together a more complete picture of the snowpack structure. Observers continue to report a layer of facets 2-5 feet below the snow surface and just above a crust. These facets are showing signs of rounding (gaining strength). In the past week, snowpack tests have become more variable. Some tests are indicating triggering an avalanche and crack propagation (necessary for a slab avalanche) are becoming less likely. That said, we are still seeing other tests that indicate that triggering deep persistent slabs remains a possibility. More tracking of this trend is needed to concretely illustrate the trend.

The Feb 8 facet layer is 3-5ft deep in the Snoqualmie Pass area.  Photo: Susie Glass

Weather

Our recent weather systems have not been big water producers, especially by Cascade standards. However, the colder than normal temperatures have produced light, low-density snowfall. Wind transported snow have been the main driver of slab avalanches in the new snow. These storms have been large enough to keep the avalanche danger elevated, but not enough for major spikes in danger.

Recent low-density snow drifted by the wind to form fresh slabs Photo: Dallas Glass

Looking Ahead

At this point avalanches on persistent weak layers can’t be fully ruled out on specific aspects or elevations in most zones. Your chances of triggering an avalanche on a persistent weak layer are lower than they were on February 13th, but the change from day-to-day will remain slow and incremental. As a result, you’ve seen slow changes in the avalanche danger in most zones. Due to the serious consequences and the uncertainty that these avalanches present, we will continue to operate with a “Status Quo” mindset. Don’t let this lack of change lull you to sleep.

When will the conditions change? Not tomorrow, but possibly within the next week or two. Keep checking the forecast. Make sure to read the Snow and Avalanche Discussion and the Avalanche Problem text so you can stay up to date with any changes as they arise.

 

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