Lawinenlagebericht

West Slopes Central

Northwest Avalanche Center
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Lawinengefahr

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

It's remaining wet and warm through Friday and large, destructive natural avalanches are still going to be running. Most of your avalanche concerns will revolve around wet snow, however at higher elevations, large dry slabs are building in size, and natural occurences are expected. Steer around steep slopes and the lower-angle terrain beneath large avalanche paths. 

Lawinenprobleme

  • Wet Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Very Likely
    Größe
    2–2.5

    High snowlines and precipitation will continue through Friday, and large avalanches will run. From our limited observations on Thursday we gathered that many large (D2) avalanches ran naturally in wet, heavy surface snow in neighboring zones. The avalanches picked up a lot of mass as they traveled downhill, ending in several-foot-thick debris piles. Water will percolate further down into the snowpack, potentially causing larger and deeper avalanches on Friday. Since the initial wetting and natural avalanche cycle occurred Thursday, we don't think avalanches will occur quite as frequently on Friday as they did the previous day, despite possibly being larger.

    Glide avalanches will probably be waking up on steep, smooth faces, in addition to concerns of large sagging cornices breaking.  In below treeline areas, most snow surfaces were old melt forms that could withstand the rain, although towards the upper reaches of the BTL some low density snow was present. This would have been impacted by the rain and warming with natural avalanches Thursday.  Much of what has run at the lower elevations, at least naturally, probably already has. A human triggered avalanche could still probably occur at lower elevations. Keep in mind that concerns remain regarding large avalanches running from higher elevations down to lower elevations. Avoid traveling in avalanche terrain on Friday. 

    This is an old photo of a very large natural wet snow avalanche that ran down a gulley to below treeline.  This was on the southeast aspect of Mt. Ross and ran several thousand feet and reached Highway 20 near Newhalem. Avalanches could be similar to this one on Friday. Photo: Andrew Kiefer

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Very Likely
    Größe
    2.5

    The top of our above treeline elevation band is probably hanging onto dry snow for the duration of this storm. There could be about 4 feet of snow at higher elevations by the end of the day Friday. Slopes that haven't slid yet are ripe as more and more snow weights the slope. These avalanches could be as deep as the new snow layer, or possibly even deeper, breaking where we saw several natural avalanches on Tuesday. Winds are likely influencing this snow so even thicker slabs exist that are probably more reactive than non-wind-loaded slopes. Temperatures will finally begin a slow cooling in the afternoon, giving us slightly lighter and fluffier snow, however the majority of this storm's snow has come in upside-down. This means lighter snow from earlier in the storm is beneath the more recent heavier snow on top, indicating an unstable snowpack. This is a recipe for avalanches that will remain very likely to avalanche with a human trigger until the snowpack has had a bit more time to adjust. 

    (2/17/2021)

    A large storm slab avalanche which is similar to one that might occur on Friday.  Photo: NWAC Observations

    Photo: NWAC Observation

Avalanche Discussion

Observations are very limited and we have a lot of uncertainty about how the snowpack will react to all this water and added weight, so this is a day to travel very conservatively in the backcountry. It's been a busy week with natural and human-triggered wind slab avalanches reported from the weekend through Tuesday until this recent atmospheric river arrived. We quickly transitioned to mainly wet snow avalanche problems. Freezing levels rose at the start of the storm, and rain quickly caused a natural avalanche cycle Thursday, which we suspect will sustain through Friday as the precipitation continues to fall. A slow cooling begins Saturday, as the snowpack will finally begin to drain. 

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