Bollettino valanghe

Stevens Pass

Mar 12 – 13, 2019Northwest Avalanche Center
Vedi sulla mappa
Archiviato · scaduto 8 years ago

Pericolo valanghe

Thu
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
Apri la mappa completa

In sintesi

Wind slabs may be easy to trigger and large at upper elevations. Loose wet avalanches may run on steep slopes that face the sun. Be alert of quickly changing conditions as the sun and temperatures begin to change the snow. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    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 slightly warmer temperatures associated with the storm, along with fairly strong windds likely built slabs in upper elevation and exposed areas. The recent heavy wind loading transported snow-both new and old, onto leeward 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. Slides may run quite a distance, encompassing much of a terrain feature at upper elevations. Check for thick, cohesive layer of snow sitting over a weaker snow layer. If you see obvious signs of instability such as shooting cracks and recent slab avalanches on small steep slopes, it is time to stay off of, and out from underneath steep open slopes.

  • Loose Wet

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

    With new snow sitting over a crust, and the strong sun of mid March about to make an appearance, expect loose wet avalanches to become common 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 heavy wet 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:

We only received about half of the amount of snow and water equivalent out of the storm as anticipated on Stevens Pass. The new snow was found to easily shear off the old surface with tilt tests early in the morning of the 12th. The winds increased during the day, and likely built reactive wind slabs at the higher and more exposed areas. The recent storm buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Dangerous conditions may be found where thick wind slabs overlie weak, faceted snow. The sun should make an appearance on Wednesday, and with it, a rapid change in snow surface conditions. 

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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