Lawinenlagebericht

Southern Wallowas

Wallowa Avalanche Center
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Lawinengefahr

Hochgebirge
4 · Gross
Waldgrenze
4 · Gross
Unterhalb
3 · Erheblich
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Kernpunkte

The Wallowa Mountains are under an avalanche warning for the next 24 hours.

Significant accumulations of new snow have created dangerous avalanche conditions near treeline and above on all aspects. Strong southerly winds have built thick unstable wind slabs on leeward terrain features near ridgeline and mid-slope. The best advice for the next few days is to avoid avalanche terrain. Stay off and out from underneath slopes steeper than 30 degrees. 

This forecast is sponsored by The Trailhead in Baker City. 

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    The Southern Wallowas above 5500' have received about .8" of water or 8-10" of new snow in the last 48 hours. Over the next 24 hours, the mountains above 5500' could pick up another 12-16" of storm snow. This is a significant load on the snowpack in a short amount of time, which has likely created an unstable storm slab problem. Near treeline and above human-triggered avalanches are likely. This new snow is not bonding well to the underlying snow surface. The weak interface consists of slick melt-freeze crusts on slopes facing the sun that got a substantial amount of heat late last week, and on W-N-E aspects there may be a reactive layer of buried surface hoar to contend with. The best way to avoid triggering a storm slab avalanche is to stay away from potential trigger points - avoid convex rollovers, areas below and above cliffs and rocky areas, steep slopes below cornices, and steep slopes with slick melt-freeze crusts resting underneath the new storm snow . Practice conservative travel techniques by spreading out and only have one rider on the slope at a time. The best advice is to avoid avalanche terrain. Stay off and out from underneath slopes > 30 degrees.

    (2/27/2024)

    Human-triggered wind slab on Poster Ridge outside the Anthony Lakes ski area boundary at 8400' on a East aspect. 

    Photo: Jeff Long

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Very Likely
    Größe
    1–2

    Strong southerly winds the past 36 hours have built unstable wind slabs in the alpine. Reactive wind slabs, 1-3 feet thick, are likely to be found on slopes facing NW-N-E.  Human-triggered wind slab avalanches are very likely near treeline and above. The effects of recent wind-loading may present as smooth hard pillows lurking below ridgelines or sub-ridges mid-slope, as cornices hanging imposingly over steep slopes, or if settled powder snow gives way to stiff punchy windboard. These are all the signs of potential wind slab. Look for and avoid steep slopes (>30 degrees) with large overhanging cornices, smooth textured wind pillows, and/or deposition zones with convex rollovers.  Something to be mindful of...the sustained 20-30mph winds will have transported snow farther down the slope than you may suspect. You may find reactive wind slabs at lower elevations. Be cautious around leeward terrain features mid-slope like sub-ridges and gullies. 

Avalanche Discussion

One of the godfathers of snow science, Ed LaChapelle, noted "snow does not respond well to rapid change." The same can be said for some of the people we know...but...I digress. What Ed was talking about is any big change in the mechanical or thermal properties of the snowpack will likely lead to the snowpack becoming unstable. By mechanical, we mean stress in the form of weight,  and adding a bunch of new snow to the snowpack in a short amount of time can stress the snowpack to its breaking point. We are at that breaking point.

A general rule of thumb around these parts is if you get more than 12" of snow or 1" of water (SWE or Snow Water Equivalent) in a 24-hour period you are going to have an unstable snowpack. 

Our area mountains have received copious amounts of new snow since Monday. This new snow is falling on very weak snow surfaces. This includes slick melt-freeze crusts on more south-facing aspects to buried surface hoar on more northerly terrain. The calm and clear nights at the end of last week facilitated the creation of massive surface hoar crystals up to a centimeter or more in length. What's particularly impressive is how widely distributed the surface hoar is across the mountain ranges of NE Oregon and that folks were finding it at the highests elevations. There is the potential of buried surface hoar existing on W-N-E aspects from 6000 to 9000 feet in elevation. I will caveat this by mentioning that one of our forecasters, Nick Burks, has been out in the Southern Wallowas and Northern Wallowas the last two days hunting specifically for buried surface hoar, but didn't find any. I will quote the famous astrophysicist, Carl Sagan "The absence of evidence is not evidence of absence." Carefully evaluate the slopes you are choosing to ride for the presence of buried surface hoar, but when in doubt, choose mellower terrain < 30 degrees. 

Once buried, surface hoar becomes a notorious and super-reactive avalanche problem. If surface hoar is continuous across a slope it has the propensity to carry fractures over long distances. This can produce large and destructive avalanches. When out touring or riding pay attention to signs of buried surface hoar including shooting cracks propagating out from your skis or sled a half dozen feet or more. If you hear a whoomph - that is a collapse of the surface hoar layer beneath you. If you were on a slope steep enough to slide, that whoomph might have been the precursor to setting off a slab avalanche. Buried surface hoar can produce sympathetic or remotely triggered avalanches - this means you can be on a slope < 30 degrees but collapse the weak layer and have a fracture propogate out to a slope steep enough to slide... bringing the slab avalanche down on top of you. Not a good deal. Avoid and stay out from avalanche terrain to avoid this problem. It is very hard to assess in the field, better to be conservative and cautious out there, and come home at the end of the day. 

As Jerry Garcia sang, "I'm going where those chilly winds don't blow." For the next few days, that's going to be a hard thing to accomplish around here. A strong cold front will be working its way across our mountains through late Thursday into early Friday morning. Winds will be strong out of the south during this time. If any of you have been fortunate to hear one of us WAC forecasters babble at you at an awareness talk, wind is the prime mover of snow. Wind can load slopes ten times faster than snow falling out of the sky. If you are going to play in the mountains in the wintertime, then you need to be paying attention to wind intensity and duration. If wind speeds pick up much above 12 mph, then you are going to get snow drifting from one side of the mountain or ridge onto the other side loading slopes, creating stress, and making the snowpack unstable. Wind slabs can remain reactive for several days after a significant wind event. Slopes in the alpine are going to be very touchy for the remainder of the week. Best to stick to lower angle terrain in more sheltered areas below treeline to avoid triggering wind slabs. 

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