Bollettino valanghe

East Slopes North

Northwest Avalanche Center
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Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

In upper-elevations, recently formed wind slabs should be less likely and more stubborn to trigger.  It's been nearly a week since the last very large avalanche released on deeper, weak layers in the Washington Pass area.  Limit your time traveling in terrain connected to large avalanche paths until we can put this problem to bed.  

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    If you're headed to upper elevations on Tuesday, you'll still want to use caution on steep lee slopes directly below ridgelines and below the bases of steep cliffs.  You're most likely to find stiffer wind-drifted snow on NW-SE aspects. These are also the same aspects that are less likely to have a thin surface crust from recent sunshine and cold temperatures.

    With wind slabs becoming less likely and more stubborn to trigger as we move further away from the most recent loading event, you're less likely to see signs of instability like shooting cracks and recent avalanches.  Instead, you'll need to evaluate the consequences of triggering an avalanche in the wrong piece of terrain.  If you're traveling above cliff bands or in rocky chutes, understand that in these specific terrain pieces, even a small avalanche could cause injury or worse.  Steer around smooth pillow-like drifts and slopes directly below recently formed cornices to minimize your chances of triggering an avalanche.  A public observation from Sunday noted a recent natural slab avalanche directly below cliffs on Silver Star. 

    If SW ridgeline winds pick up Tuesday, keep an eye out for blowing snow and the potential for thin new shallow wind slabs on the same lee aspects. 

    On very steep and sheltered shady aspects, you may still trigger a small dry loose avalanche or sluff.  Use care around terrain traps. 

    (11/28/2022)

    Wind slabs will often live under cornices like this. 

    Photo: Willie Webster

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    In the Washington Pass area, it has been nearly a week (12/27) since a very large avalanche released on persistent grains buried early last month. Recent snow profiles suggest that recent natural avalanches involved the Dec 8th buried surface hoar layer (2-3ft below the surface and below the Christmas crust) with at least one avalanche suspected to have run on sugar-like facets from November deeper in the snowpack.

    The chances of a human triggering an avalanche on this layer are unlikely, but the size and consequence of an avalanche breaking this deeply should still shape your travel choices.  You can minimize your exposure to this problem by limiting time spent near large avalanche paths and away from runouts, spacing out while crossing large paths, and maintaining awareness of other parties in the terrain around you. Larger triggers like cornice fall or an avalanche in the more recent snow (see wind slab above) could result in a large and destructive avalanche.

    A weak snowpack structure also exists in the Mazama environs and eastern foothills.  A lack of recent avalanche activity following the recent Christmas period loading event has us watching how the snowpack evolves moving forward but not including it with the deep persistent slab problem further west. 

    (12/28/2022)

    A very large avalanche (D2.5) occurred during the Christmas Storm. Potentially ran on weak, faceted grains from November. Vasiliki Ridge, NW, 7,400ft

    Photo: Jeremy Allyn

Avalanche Discussion

The weather turned colder on Monday with stubborn low clouds in valleys and lower slopes and periods of filtered sunshine up high.  On Sunday, under sunnier and relatively milder temperatures, professionals reported a few dry loose avalanches, but no new slab avalanches or other signs of instability like collapses.  On Friday, observers on Delancey Ridge experienced a large collapse at 6,500ft on a south aspect. They believe it may have been due to a deeper weak layer, possibly older facets from early season. Other observers reported that the new snow was light and right-side up, showing few signs of instability.

We've entered a period of relative tranquility avalanche-wise, following a very active stretch around the Christmas holiday.  Observers reported at least 4 very large, size D2.5-3.5, natural, slab avalanches that ran roughly between December 24th and 27th. One very large (D3.5) debris pile was reported on the east end of Delancey Ridge. This avalanche could have involved weak layers near the bottom of the snowpack, though it hasn't been confirmed.  A crown, reported in the Vasiliki Ridge area on a northwest aspect around 7,400ft was deep enough that it may have also run in the lower snowpack . On Thursday 12/29, observer Willie Webster,  reported a couple of very large avalanches on westerly aspects in the Hairpin Valley. Willie's snow profile nearby found the problem layer to likely be a layer of surface hoar, believed to be buried 12/8, over 3 feet below the surface.  NWAC forecasters also found similar, propagating test results on the same layer of surface hoar near the confluence of the Cutthroat and Early Winters drainages (see Josh's video in the media reel).

Heavy snow and rain brought over 3.50 " of water equivalent to the Washington Pass weather station on December 23-27th. A thick crust was buried on the 25th.  The Christmas crust is now over 2 feet below the surface near Washington Pass.  If you're traveling well to the east of the Cascade crest, near the Silverstar drainage, or near and east of Mazama, you'll find an overall shallow, weaker snowpack with the Christmas crust closer to the surface.  

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