Bollettino valanghe

Stevens Pass

Northwest Avalanche Center
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Pericolo valanghe

Thu
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Dangerous avalanche conditions continue after a long period of heavy snow, rain, and strong wind.  Avoid being on, under, or near slopes 35 degrees and steeper at mid and upper elevations where you may be able to trigger dangerous deep persistent slab avalanches. The most dangerous slopes will be where westerly wind formed drifts near and above treeline.

Problemi valanghivi

  • Wind Slabs

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

    Watch for triggering avalanches in recently drifted snow. Following travel advice for the deep persistent slab avalanches will also keep you from triggering wind slab avalanches. Avoid leeward slopes where the snow looks thick and pillowed. Watch for wind features in the snow and areas of deeper snow adjacent to shallower areas. Feel for stiffer snow under your skis or feet as signs of drifting. Avalanches will be largest and most dangerous at the highest elevations. You may also be able to trigger small wet avalanches on lower elevation slopes as the day warms.

    Westerly wind drifted cold snow onto wet, rain-soaked surfaces on Monday night as temperatures plummeted. This may have created a poor bond for newly formed wind slabs. Avalanches in the recently drifted snow may still be easy to trigger. Like the deep persistent slabs, it's a good idea to give the snow time to adjust and bond.  The photo to the right shows an impressive avalanche triggered in wind drifted snow on Tuesday.

    (12/22/2020) On Tuesday, avalanche workers intentionally triggered a fresh wind slab avalanche on an ENE aspect at 5,000ft near Stevens Pass. The crown was 200ft wide and 18-24" deep.

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    Deep persistent slab avalanches could be very large and easily could bury or kill you. These dangerous and surprising avalanches can catch even the most experienced backcountry travelers. With uncertainty about how easily you can trigger these avalanches, it's important to give them time to heal. You may get little warning before an avalanche occurs. The best way to stay safe is to avoid where these avalanches exist. Keep your distance from slopes 35 degrees and steeper that are open enough and tall enough to produce large avalanches. Make sure you're not standing below or adjacent to steeper slopes. Use clues to stay out of avalanche paths, like trees stripped of uphill branches or openings in vegetation that continue up-slope.

    The most dangerous slopes will be at the highest elevations, where snow stayed mostly dry and where southwest to west wind drifted snow over the past few days. Avoid any areas that appear to be covered with thick drifts snow. You can find a thin weak layer of small facets (and in places surface hoar) above a crust buried about 3-4 feet below the surface. The layer became active on December 14th and was involved in a widespread cycle of large, natural avalanches on Dec 18th through the night of the 19th. What we know about these avalanches is that most occurred on a variety of aspects above 5,400ft. 

    (12/18/2020) Part of a remotely triggered persistent slab avalanche (D2) that was ~500ft wide and 15"-20" deep. Lanham creek, 5800ft, NE.

Avalanche Discussion

The past week has been the most active and dangerous period of the season to date. On Tuesday, observers reported a cycle of avalanches near treeline in the 8" of snow that fell since Monday.  This came on the heels of a prolonged cycle of natural avalanches in the zone up to D3, (big enough to break a tree or bury a car) some with crowns over 4 feet deep. Some of these propagated widely, and most are suspected to have run on facets and surface hoar above the prominent December 9th crust. Smaller avalanches that ran on Sunday and Monday only involved recent storm interfaces and/or wet surface snow. On Monday, I caught a foggy glimpse of a couple of crowns in terrain adjacent to Stevens Pass Mountain Resort that ran on Friday and Saturday nights, including a size D2 on a northwest aspect around 5,400ft in Big Chief Bowl. On Sunday very large slab avalanches ran on Lichtenberg Mtn (both SW and SE) at 5600ft with crown depths estimated at 1-2 meters (up to 6 feet). On Saturday, observers reported a widespread cycle of slab avalanches mostly on northerly aspects above 5,000ft, including the northeast bowl of Arrowhead and much of the Rooster Comb, above Lake Susan Jane. On the 17th, a party remotely triggered a large D2 avalanche on a northeast aspect at 5800ft above Lanham Lake with a crown about 500ft wide. On the 16th, another party triggered a size D2 avalanche on a NE aspect at 5450 feet near the top of a peak northwest of Josephine Lake and Nasikelt peak. 

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