Bollettino valanghe

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Northwest Avalanche Center
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3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

On Sunday, a brief and intense storm will bring dangerous conditions, and you will likely encounter avalanches. The biggest and most dangerous avalanches will be at upper elevations in the new snow. Expect a chance for enough rain near Washington Pass for wet avalanches below 4,000ft. 

Problemi valanghivi

  • Storm Slabs

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

    Expect triggered, and maybe even natural, avalanches in the new snow. Use caution near and on slopes steeper than 35 degrees. Avoid triggering avalanches by sticking to lower-angle slopes and steering around steep convexities and rollovers. Recent avalanches and cracks shooting through the snow are red flag indicators of the danger. You can also perform quick tests on small slopes to see how easily the snow slides before committing to bigger terrain.

    Storm slab avalanche conditions will build rapidly on Saturday night. Above 4,000ft, the snow should remain dry. However, warming temperatures will likely create an "upside-down" or strong-over-weak layering. The rapid load will stress the underlying recent snow. While avalanches may break within the new snow, some slides could be up to 2 feet deep.

    (12/7/2023)

    A shallow crown (20cm) from a small triggered wind slab avalanche in Spire Gulley. NE, 7,300ft

  • Loose Wet

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

    Wet avalanches can be powerful. Even smaller avalanches will be strong enough to knock you over or push you into trees and rocks. Be aware of the instability, and don't be surprised by the difference in how much more forceful the avalanches will be where you find wet snow. 

    Up to about 4,000ft, you could encounter wet avalanches of a couple different forms. The snowpack characteristics may seem more like moist storm slabs rather than the classic wet loose slides. We are uncertain about how these avalanches will behave based on the timing and extent of less frozen precipitation.

    (12/7/2023)

    Wet loose debris from 12/4-5 in the Liberty Bell group

    Photo: Willie Webster

Avalanche Discussion

The current storm has some elements and uncertainties that will shape the avalanche conditions as they play out. Cold temperatures and east flow late on Saturday are already bringing snow to the foothills and towns of the Methow Valley. As the storm shifts, the big question is how much it will warm up and when/where will the precipitation shift to something less frozen. Even where that happens, rain may not be a significant factor in much of the winter recreation terrain. Lower elevation terrain, closer to the Cascade crest will be most prone to receiving up to 0.25" of water as rain. As a comparison, the last storm brought roughly 2" of rain to Washington Pass.

Prior to the weekend, observers assessed the snowpack. Near Washington Pass, professionals found snow depths of ~2.5 feet below 5,000ft. Higher up, they reported up to 4 feet of snow on the ground, including new snow totals (since 12/6) of up to 1 foot at upper elevations.  On Thursday, Willie Webster traveled throughout the Highway 20 corridor.  The entire snowpack at the aptly named Rainy Pass had been wetted all the way to the ground. Any weak layers that had existed there are likely no longer an issue. In the Hairpin Valley, Willie found less impacts from rain including the November 30th surface hoar intact and producing propagating test results and whumphing collapses. This was likely an isolated instability that subsided as the 12/6 Hanukkah crust froze and gained strength.

The November 30th surface hoar may still be intact in cold valley bottoms east of Washington Pass. Even in these areas, you can find the weak layer in the middle of the snowpack, capped with a layer of 6-10" of melt forms that add strength to the snowpack. I suspect that even where the surface hoar is found, it likely is no longer a culprit weak layer for avalanches.

From November 30th through December 6th, the weather station at Washington Pass recorded nearly 5" of water equivalent, with the last 2" falling as rain from December 4-6th. Rain levels may have spiked around 8,500ft on the 5th while heavy rain fell. Observers reported a widespread cycle of wet avalanches up to size D2.5, along with dry slab avalanches that occurred prior to the warm-up. Observations and access to the mountains during this time were limited.

If you got out, please let us know what you found in the mountains by submitting an observation.

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