Bollettino valanghe

West Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Tue
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
4 · Forte
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In sintesi

Large, dangerous storm slab avalanches that could bury and kill you are expected to naturally occur during the day on Monday, possibly even running down to lower elevations. Around 2 inches of water, or 2 feet of snow, will fall by the end of the day in this storm, and during more intense periods of snowfall or wind natural avalanche activity will elevate even more. A 4000ft freezing level means we'll also have wet avalanche activity mixed in at lower elevations, so to sum it up, Monday is a day to steer around all avalanche terrain, including the flatter runouts and gullies of avalanche paths at lower elevations. 

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    2.5

    Storm slab avalanches are what can get you into trouble on Monday. A fluctuating temperature, varying at about a 4000ft snowline, means that the snowpack may be heavy, or possibly upside down in nature for most elevations.  Upside down means that lighter, dryer, low density snow is sitting underneath heavier, wetter snow, and this is a good recipe for avalanche activity. Whether the storm layers are upside down or not, we're getting enough water that this isn't a good time to get out in the mountains.

    Not only is it wise to avoid steep slopes, but take extra care to ensure that you aren't traveling beneath large avalanche paths as well. This means you could be on flat ground, but if there's an opening in trees leading to steep slopes above you, you are probably still in an avalanche path. Subtle low-elevation gullies crossing hiking trails or roads may lead to large overhead avalanche paths that could run naturally during the day. If you get out, stick to low-angle, well-treed terrain and avoid all avalanche paths.

    This is an old example of a very large avalanche which ran from higher elevations, but down through a tight gully, spilling onto the road several thousand feet below, from Mt Ross on Highway 20 near Newhalem.  You may see activity similar to this on Monday. Photo: Andrew Kiefer 

  • Loose Wet

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

    It's going to be moist at lower elevations on Monday. We might get rain up to about 4000ft, but you can expect the rain line to oscillate by several hundred feet around that elevation during the day. Moist or wet snow, especially newer layers near the surface, could run on steep slopes during the day. You may just see rollerballs, or enough snow may lose cohesion that wet loose avalanches could run in the new snow layer.

    These avalanches would likely be on the smaller side, but depending on how much moisture is present in the new snow layers, they could grow large. Wet loose avalanches could initiate and then release a slab once enough mass builds up.  As is the case with the storm slab avalanche concern, simply steering clear of all avalanche paths is the best way to go about your day.  

    (2/4/2023)

    A D1.5 Wet Loose avalanche on the north side of Mt. Snoqualmie was observed on 2/4, similar to what you might see on Monday. 

    Photo: Ian Nicholson

Avalanche Discussion

We expect avalanche activity to ramp up as the day progresses and more precipitation falls. Moderate to strong winds will also play a factor, where thicker wind-deposited drifts of snow are expected to be more reactive, however enough activity is expected in non wind-affected areas that differentiating between the two types of slab avalanches is not something that will make a difference for Monday. 

In neighboring zones it's been just over a week since we've heard about large avalanches failing on deeper layers in the snowpack, subtle weaknesses near crusts. We don't expect a large number of avalanches to occur down at this layer, which could possibly be 4 or 5 feet deep, but we're keeping it on our radar through this storm as the final test of added weight on this layer. If an avalanche does occur, it will be very large and destructive, easily traveling below treeline. 

The  West Central zone is centered perfectly in the convergence zone for the incoming storm. Forecasts are calling for over an inch of water, or a foot of snow, overnight, and then similar amounts, possibly a little more, during the day Monday. Precipitation will continue at a high rate through Monday night as well, before backing off into Tuesday. As is oftentimes the case, weather and precipitation amounts will likely vary throughout the zone, so evaluate conditions and adjust the forecast as needed for your travels. 

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