Bollettino valanghe

East Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Wed
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

A potent storm will bring significant snow and winds to the Cascades, quickly overloading a weak snowpack and bringing very dangerous avalanche conditions. We expect very large avalanches to occur naturally and be very easily triggered. Avoid all avalanche terrain, and be knowledgeable of the terrain above you, as avalanches may run into lower elevations from above.

Problemi valanghivi

  • Persistent Slabs

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

    If we haven't already reached a tipping point, a storm bringing 8-14" of snow with strong winds will undoubtedly push the snowpack over the edge. Triggering large persistent slab avalanches that break widely 2-4ft deep will be very likely, and we expect natural avalanches to run on these deeply buried weak layers as well. These avalanches may propagate widely across entire bowls and ridgelines. The most concerning slopes will be at mid and upper elevations, particularly in leeward terrain where the winds drift the snow deeper. With this rapid loading event, you should treat any steep slope as suspect. Avoid traveling on or under steep terrain. Choose low-angle slopes well away from the runouts of large avalanche paths. Since last Friday, there have been six close calls and one fatality involving this layer of weak-faceted snow over the Early February Crust. The most recent close call was Saturday near Longs Pass, where a skier triggered a very large avalanche and was caught and carried but miraculously only partially buried and uninjured. (Other avalanche incident details in the discussion) Many of these observations indicate that the terrain had already been skied/ridden once or multiple times before the avalanche occurred. It might not even be the first person on the slope that triggers the slide. While this reactivity is unusual for Washington, it is typical for this avalanche problem. This problem requires patience, conservative terrain selection, and maintaining wide margins between you and steep overhead terrain.

    (3/9/2024)

    A very large persistent slab avalanche triggered by a skier near Long Pass. They were the second skier on the slope when the avalanche occurred. The skier was caught, carried, and partially buried but uninjured. 

    W aspect, 6400ft, crown depth 2-3ftHS-ASu-R3-D3-O

  • Storm Slabs

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

    Areas at the western portion of the zone may pick up over a foot of new snow with 6-8" further east. This snow will fall on variable surfaces, including slick crusts, weak faceted snow, and surface hoar. New snow may form a weak bond with the old snow surface, and avalanches may be easy to trigger. Moderate to strong winds accompanying this storm will form deep wind slabs near ridgelines, the base of cliffs, and steep gullies. Trigging a small storm or wind slab could release a much larger persistent slab avalanche. Choose very conservative terrain in forested areas and avoid slopes over 30 degrees. Elevations below about 4000ft have little new snow over a thick crust and will likely have much less potential for large avalanches. 

    (1/18/2024)

    Example: Natural storm slabs failing on steep convex rollovers in gladed terrain.  

    Photo: Katie Warren

Avalanche Discussion

NWAC will be closed on Tuesday, March 12th, as we gather and grieve following the death of one of our forecasters, Matt Primomo. We will not issue regular avalanche or mountain weather forecasts on Tuesday. We will return to normal operations on Wednesday, March 13th. We appreciate the community's understanding and support during this time.

Discussion:The heaviest snowfall will occur overnight, but winds will continue into the day on Tuesday. With this active persistent slab problem, give extra wide margins in your routes, steering far away from avalanche terrain.

Outlook (Day 2): While we expect natural avalanche activity to taper off by Wednesday, the snowpack will hang in a delicate balance and be ripe for human triggering of very large avalanches. Sunshine and warmer temperatures will also add to this messy situation with the potential for large wet loose avalanches on southern aspects. 

This pattern of persistent slab problems can be challenging for backcountry decision-making. It takes conservative terrain management and diligent patience. Stepping into steep terrain at mid and upper elevations is simply just a game of chance; you may get away with it, or you could trigger a very large avalanche that is likely to injure or kill you. The structure is especially apparent in the eastern Cascades, where the snowpack is generally more shallow than at the crest. So, if you're venturing into these eastern zones, be prepared to take a step back in your typical terrain selection and understanding of the snowpack.

On Thursday, forecasters toured near Blewett Pass. We could see the recently triggered avalanche in Owl Creek (Blewett Pass) from a distance, and they found variable test results that showed the persistent slab problem lingering. Tests and layer characteristics indicated that weak layers are slowly gaining strength. Our snowpack is complicated due to the intermittent dry spells in February. Besides the early February Crust, there are also potential layers of surface hoar and faceted grains from the 14th, 18th, and 24th. Widely propagating avalanches could fail on any of these layers. Approach the mountains cautiously and enjoy some low-angle powder until these weak layers gain strength.

Summary of Recent Avalanche Incidents: In addition to the avalanche incident on Saturday, there have been nearly daily reports of close calls on this persistent weak layer. On Thursday, snowbikers remotely triggered two very large avalanches in the Teanaway, and there was a close call with a skier who was caught and carried in an avalanche on the S aspect of Van Epps Peak. Earlier in the week, snowmobilers triggered large avalanches around the Blewett Pass area, and a skier was partially buried with minor injuries in an avalanche on a NW aspect of Jim Hill in the neighboring Stevens Pass zone. Last Saturday, a snowmobiler was buried and did not survive an avalanche near Darland Mountain in the East South zone. 

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