Bulletin d’avalanche

Mt Hood

Feb 24 – 25, 2019Northwest Avalanche Center
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Danger d’avalanche

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

Monday's storm may tip the scales from dangerous avalanche conditions to High danger. You can trigger avalanches in the new snow, and in isolated areas, new snow avalanches may step down to deeper layers to release very large avalanches. There are many overlapping avalanche problems, so choosing simple terrain and steer clear of slopes capable of producing large avalanches.

Problèmes avalancheux

  • Wind Slabs

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

    If E-SE winds increase as forecast Sunday night through Monday, winds will easily transport low density snow and form fresh and sensitive wind slabs. Winds may be strong enough to form wind slabs on open slopes below treeline. If the easterly winds materialize, wind slabs may form on unusual aspects. 

    If this sounds like a lot to navigate safely, you are probably on to something. Avoid slopes steeper than 35 degrees where wind slabs have likely formed, using visual clues like textured snow, blowing snow and fresh drifts. Feel for firm or hollow snow. If you can't tell which slopes have been loaded, choose lower angled terrain. 

  • Storm Slabs

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

    The faster the snow piles up Monday, the more likely you will trigger a storm slab. Storm slabs from Sunday have not had enough time to heal, increasing your odds of triggering a large avalanche down to recent storm layers. You are most likely to trigger storm slabs on unsupported and very steep slopes. Look for signs of natural slab avalanches and use small inconsequential test slopes to gauge the storm slab potential. You can avoid storm slabs by sticking to lower angled and moderate slopes. 

    In areas that don't have a storm slab problem, loose dry avalanches are likely in very steep terrain. Think about sluff management on very steep slopes and what would happen if a small but fast moving loose dry avalanche pushed you into or over a terrain trap.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1–2

    The 2/8 persistent weak layer/crust combination we have been tracking is only getting deeper with each passing storm and there haven’t been any reported avalanches on this layer in over a week. However, the snowpack structure remains the same and with deep persistent slabs, long stretches of time may pass between triggered avalanches.

    You are unlikely to experience warning signs like cracking or whumphing given its depth. It will probably take finding a shallower spot in the snowpack where you can trigger an avalanche on this layer or a smaller avalanche that steps down to this interface.

    Continue to let this low-likelihood/high consequence avalanche problem shape your terrain choices, steering you away from terrain capable of producing large avalanches.

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.

 

Discussion

Snow and Avalanche Discussion

Storm slab avalanches were very sensitive to skier triggering on Sunday morning during Mt. Hood Meadows avalanche control. The new avalanches were shallow in depth but broke widely across the terrain. Natural shallow slab avalanches were also observed on a variety of aspects. 

Prior to Sunday's new snow, professionals digging in the snow and looking for the 2/8 weak snow/crust combination in the Mt. Hood Meadows area were finding it roughly 4 feet down on N-E aspects and in a rather narrow elevation band around 5300’-6000'.  While this is very useful and specific information, we can't extrapolate this to the larger Mt. Hood area to fine-tune the suspect aspects. 

With Mt. Hood in the bulls-eye of the storm track, the avalanche danger will largely be driven by new snow concerns over the next few days. However, continue to think about the lingering deep slab potential when choosing terrain.  

With all the low-density snow piling up the risk of a Tree well/Snow immersion and Suffocation accident continues. Unfortunately, there was a tragic tree well/SIS fatality at the Timberline ski area on Saturday. 

Régions couvertes