Bollettino valanghe

Stevens Pass

Northwest Avalanche Center
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Pericolo valanghe

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

In sintesi

Heightened avalanche danger exists at upper elevations where you can trigger avalanches in recently drifted snow. At these same elevations, it may still be possible to trigger a very large and deadly avalanche on isolated slopes. The best way to deal with the uncertainty of triggering avalanches on deeply buried weak layers is to stick to slopes less than 35 degrees.

Problemi valanghivi

  • Wind Slabs

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

    Watch for triggering avalanches in recently drifted snow. 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. Watch for cracks shooting through the snow and small test slopes that slide easily as signs that you can trigger an avalanche. Avalanches will be largest and most dangerous at the highest elevations. Sticking to windward, or non-loaded, slopes and terrain under than 35 degrees in slope angle.

    On Thursday, observers on Lichtenberg Mtn found sensitive drifts up to 30cm deep on cross-loaded southeast slopes near treeline. Westerly wind drifted cold snow onto wet, rain-soaked surfaces on Monday night as temperatures plummeted. This created a poor bond for wind slabs that formed on the 21st with westerly wind. The photo to the right shows an impressive avalanche triggered in wind drifted snow on Tuesday.

    (12/24/2020) Cracks in recently drifted snow. On Thursday, drifts and pockets of wind slab were easy to trigger. Lichtenberg Mtn, SE, 5,000ft

  • Deep Persistent Slabs

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

    Deep persistent slab avalanches could be very large and easily could bury or kill you. These dangerous avalanches can catch even the most experienced backcountry travelers. Triggering one is unlikely but not impossible. 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.

    On Thursday, observers in two different locations, both SE above 5,000ft, were surprised to still find test results (CTM (SP), ECTP) indicating that you can trigger these avalanches. You can find a thin weak layer of small facets (and in places surface hoar) above a crust buried about 2-4 feet below the surface. The weak grains have been rounding and gaining strength over the past week. The layer became active on December 14th and was involved in a widespread cycle of large, natural avalanches this past weekend. These avalanches occurred on a variety of aspects above 5,400ft. 

    (12/24/2020) Debris from a natural size D2.5 slab avalanche that likely occurred on the night of Dec 19th. Lichtenberg Mtn, SE, 4,100ft.

Avalanche Discussion

Expect light snow for Friday. A crust buried on the 21st has made for slick and challenging travel in the backcountry. The recent pause in the weather is a good time to recap recent avalanche activity:

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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