Boletín de aludes

Stevens Pass

Mar 13 – 14, 2019Northwest Avalanche Center
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Archivado · caducó 8 years ago

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Expect wind slabs to be more stubborn on Thursday, but still quite possible. As the temperatures warm up, loose wet avalanches may become easy to initiate, even with cloud cover. Avoid recent, firm wind slabs on steep slopes and be alert to changing snow surfaces.

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1

    The recent storm snow landed on a variety of surfaces from surface facets and surface hoar on shaded aspects to melt freeze crusts on southerly aspects. The moderate westerly winds built slabs on exposed open slopes. The snow may fracture quite wide on shaded aspects where the recently buried layer is weak and faceted. Any avalanches on the old surface may step down a foot or so to layers from early March. Check for a strong, cohesive layer of snow sitting over weaker snow. If you see obvious clues to instability such as shooting cracks and wind loaded texture, it's a good bet to stay off of steep wind loaded slopes.

  • Loose Wet

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1

    The clouds may or may not be thick enough to keep the snow surface cool and dry. If the sun comes through, a "greenhouse effect" may occur, and loose wet avalanches may begin to run by late morning. These will begin first on east and southeasterly aspects, and continue onto south and southwest aspects as the afternoon rolls on. Look for signs of loose wet instability such as wet, heavy surface snow, rollerballs, and fan shaped avalanches. The larger the slope, the large these slides may become. If you notice these clues, heed the warning signs and avoid going underneath steep slopes where loose wet slides may run naturally. 

Discussion

Snow and Avalanche Discussion:

The recent storm buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Stevens Pass received about 8" of snow with less than 0.5" of water equivalent out of Monday/Tuesday's storm. A few new, small loose wet avalanches were observed by mid morning on the 13th. An observer was able to easily initiate small (D1) loose dry avalanches on steep west to north to northeast aspects around 5,000ft. On the 12th, Professional control teams reported numerous dry loose avalanches (up to D1.5)  in the morning, and WSDOT triggered one large (D2) wind slab on a northwest aspect in wind exposed terrain above the highway. This highlights where some of the most dangerous conditions may be found, places where thick wind slabs overlie weak snow.

For Thursday, expect warming temperatures to begin changing the snow surfaces. 

Snowpack Discussion

March 10, 2019

February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.

Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.

A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg

Recent Avalanches

Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.

A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo. 

Moving Forward

As we move further into March, there are two points to consider:

  • The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.

  • Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.

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