Bulletin d’avalanche

Mt Hood

Mar 13 – 14, 2020Northwest Avalanche Center
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Archivé · expiré 7 years ago

Danger d’avalanche

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

Shifting winds are bringing significant snowfall on top of a variety of crusts and weak snow surfaces formed during last week’s clear weather, creating dangerous avalanche conditions. Choose lower angled terrain, under 35 degrees, until you are confident in the bond between the new and old snow.

Problèmes avalancheux

  • Wind Slabs

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

    The wind forecast for Saturday is challenging. Low-level easterly flow is battling upper-level south-west flow. The elevation at which a wind direction switch may be apparent on the mountain is nearly impossible to forecast. One thing is clear: whichever direction the wind is coming from it will easily transport cold, low-density snow to form fresh wind slabs. Wind slabs will be likely at upper elevations, especially near ridge crests. Look for clues about recent wind loading patterns such as the current wind direction, active blowing snow, and textured snow surfaces. Don’t rely on old cornices to clue you into where wind slabs may be--east winds are not the normal wind direction. Where you see evidence of freshly wind transported snow, steer around slopes over 35 degrees.

  • Storm Slabs

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

    Significant amounts of cold light snow are expected Friday night and Saturday. Look for signs that suggest you could trigger an avalanche such as cracks shooting through the snow, layers of snow with very different densities, or loose snow poorly bonded to a crust below. With cold temps and heavy precipitation rates, the possibility of both storm slab avalanches and far-running loose dry avalanches exists. Choose lower angled terrain, under 35 degrees, until you are confident in the bond between the new and old snow.

    Observations around Mt Hood this week have documented weak and faceted snow on or near the snow surface, especially on shaded aspects and in tree-covered areas. Look for this “sugar-like” faceted snow under the fresh storm snow as the old snow surface or around freshly buried melt-freeze crusts formed this week. The weak old snow surfaces and melt-freeze crusts are likely layers on which to initiate avalanches.

Discussion

After 13 days of generally stable avalanche conditions and spring-like weather, a weekend storm will impact Mt Hood. Fresh snow will hide a wind-scoured blue ice layer that has been exposed on many aspects above 6000 ft. Cold, low-density snow may bond poorly to these slopes and a shift to east winds may scour them again. Be skeptical of the bond between the new snow and formerly scoured slopes at upper elevations. These slopes may produce both avalanches and potentially dangerous or difficult travel.

Snowpack Discussion

March 12, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

From March 6th to the 12th the Pacific Northwest experienced a few small storms and a few notable days for people and avalanches. Light to moderate snow accumulated slowly over a period of a few days, and avalanche danger increased from Low to Moderate over the weekend of March 8th. The exception was the West North zone, which won the snowfall competition this week. 18-24” of new snow was observed in the Mount Baker area on the 6th, and 10” of new snow on the 11th. Avalanche danger reached Considerable there for storm slab, wind slab, and loose wet avalanche problems. 

People, complicated avalanche terrain, and lots of avalanches. Bagley Lakes Basin, West North zone. Photo by Nathan Resick, March 8, 2020.

Loose dry slides initiated by skiers in big terrain. No one was caught or carried. East Central zone. March 8, 2020. Matt Primomo photo.

The snow fell with minimal wind, and accumulated slowly over firm suncrusts or wind stiffened surfaces. Thicker cloud cover inhibited the sun in areas where 4” to 12” of snow accumulated over the course of a few days. Cold overnight temperatures allowed the low density snow to weaken. By the 8th, the clouds broke up enough to allow strong radiation to change snow surfaces. When the sun finally did hit the snow, it quickly caused loose avalanches in steep terrain. 

An active weekend for people and avalanches: 

Many areas experienced some form of loose avalanche activity from March 6th to 9th.  Sunday, March 8th was the day that recorded the most widespread avalanche activity across the region. The size of these avalanches were directly related to the amount of new snow that had recently accumulated in that area, and the size of the terrain. 

West North zone on March 8, and West South zone on March 8th. Uphill and downhill tracks got covered by debris. Photos by Zack McGill and Dallas Glass.

We as a community can look back at, and learn from these few days. Numerous human triggered slides occurred, some were slabs but the majority were loose. A number of catch and carries were observed, and many more likely went undocumented. Luckily, no injuries or burials were reported. Observers across the region reported lots of instances where previous tracks got hit with debris, either from human triggered or natural avalanches. 

As the days get longer and folks begin to step into the alpine zone and bigger terrain more frequently, it is important to keep a humble attitude. Continuously evaluate route selection with potential for loose wet and cornice falls in mind. Check the mountain weather and avalanche forecasts for the most up to date info. Enjoy the backcountry, be safe, and let us know what you see out there!

-MP

Solo snowboarder aired into this slope, and went for a ride but was not buried or injured. Slide took out the corner of a skin track. Bagley Lakes Basin, March 8, 2020, Zack McGill photo.

Wind slab triggered by a skier at 7,500ft next to the popular route on Mt St Helens. The skier wasn’t caught or carried. Andy Goodwin photo, March 8, 2020.

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