Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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Pericolo valanghe

Fri
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

An atmospheric river is headed our way on Thursday. Rising temperatures will change the precipitation type from snow to rain and result in multiple rounds of natural avalanches. You should avoid all avalanche terrain until this storm has run its course.

Problemi valanghivi

  • Storm Slabs

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

    First up on the docket for this storm is snow. Although the amount of accumulation will vary around the zone, it will likely be heavy and wet in all locations, overlaying lighter snow that currently exists on the surface. The resulting upside-down snowpack is the perfect set-up for Storm Slabs. This unstable snowpack structure combined with high precipitation rates and some gusty winds will produce very dangerous avalanche conditions. Thursday will not be a day to mess around in the mountains - stay out of avalanche terrain until this storm plays out.

    (1/10/2023)

    Three natural wind slab avalanches from the past few days. Expect more widespread natural activity during this atmospheric river event.

    Photo: Andy Harrington

  • Wet Slabs

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

    As the day wears on and freezing levels creep ever higher, precipitation is likely to change over to rain. Whenever this occurs, expect another round of natural avalanches. We are calling them Wet Slabs due to the potential failure points in the snowpack. But make no mistake, there will likely be a combination of overlapping wet avalanche problems. It should start with smaller Wet Loose avalanches as a precursor to the bigger and more dangerous Wet Slabs. Categorize these slides however you want, the take-home point is that very dangerous avalanche conditions are expected to develop in the mountains and you should avoid anywhere an avalanche can start, run, or stop.

    (12/23/2022)

    A wet slab avalanche at Mount Rainier National Park from our previous atmospheric river event over Christmas. Similar avalanches are expected during this event.

    Photo: NPS Rainier Rangers

Avalanche Discussion

It’s going to be a messy day in the mountains of the West South zone on Thursday. While forecasted water numbers may not be very impressive by atmospheric river standards, it’s still a good shot of precip with rising freezing levels. Combine that information with known weak grains in the snowpack (1/4 buried surface hoar) plus multiple rounds of recent storms resulting in natural avalanches, and we feel that very dangerous conditions will develop during this event. This forecast isn’t without uncertainty, of course. Rain should reach the top of the peaks in the zone (that aren’t a volcano) by day’s end, but intensities may diminish by that time. That’s part of the uncertainty. Even if we’re wrong and multiple rounds of avalanches don't occur, it probably won’t be a very nice day to be in the mountains, so best to sit this one out and let mother nature do its thing. 

There may be as much as 3 feet of snow over the easily identifiable Christmas Crust. The 1/4 buried surface hoar layer is generally about 1 foot above this interface. There are also plenty of recent storm interfaces in the upper snowpack, some of which showed up consistently in snowpack tests this past week. We anticipate peeling back the snowpack onion layer by layer as the storm slowly warms and changes precipitation type over the next 24 hours. Several of these layers could be activated and we are considering all of the snow above the Christmas Crust in play until the duration of this storm. Which layers failed, during what part of the storm, was the slab wet or dry, and did the storm play out as we predicted? None of that really matters. The important take-home point is that we expect several rounds of natural avalanches and you should avoid avalanche terrain.

Regioni coperte