Avalanche forecast

West Slopes South

Northwest Avalanche Center
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Avalanche danger

Wed
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

This week's stormy weather will peak on Tuesday, resulting in dangerous avalanche conditions across the zone. A significant concern is that weak layers deep in the snowpack may wake up and produce very large and destructive avalanches. New snow problems will be widespread near the surface, so avoid all terrain over 35 degrees.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    High snowfall rates, fluctuating freezing levels, and moderate winds will all aid in slab development on Tuesday. Slabs will be larger as you ascend in elevation or in wind-loaded terrain. They could overlay weak grains or a firm crust at the 3/9 interface, making them fail easily. Use small test slopes, uptrack tests, and quick hand-pits to gauge the storm snow’s reactivity and bonding. When in doubt, avoid slopes over 35 degrees where triggering an avalanche is more likely.

    (3/10/2024)

    A skier-triggered slab mid-slope on a northerly aspect in the Bullion Basin area.

    Photo: Andy Harrington

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    We're getting a good shot of water across the zone. If our persistent weak layers are going to wake up, Tuesday and Wednesday may be the days. If you've been ignoring this problem and traveling in large open terrain up to this point, it's time to dial it back for a day or two. If these layers become activated, the resulting avalanches will be 3-6 feet deep and likely unsurvivable.

    The snowpack tests and recent avalanche activity are there to support this problem. Last time we had a significant loading event, the West South produced multiple very large (D3) avalanches. These same layers have been wreaking havoc in neighboring zones for over a week. You won't see any warning signs or get feedback from the snowpack. Even if you've traveled on a slope recently, that doesn't mean it's safe. You should avoid large open slopes where avalanches like this can start, run, or stop.

Avalanche Discussion

NWAC will be closed on Tuesday, March 12th, as we gather and grieve following the death of one of our forecasters, Matt Primomo. We will not issue regular avalanche or mountain weather forecasts on Tuesday. We will return to normal operations on Wednesday, March 13th. We appreciate the community's understanding and support during this time.

Discussion:

The finale of this storm parade should be the most significant we've seen this week. Higher accumulations are forecast for the volcanoes compared to the crest, but substantial snowfall is expected throughout the zone. This storm will further increase the depth over the 3/9 interface and stress our buried weak layers.

On slopes that typically receive direct sunshine, the 3/9 consists of a melt-freeze crust of varying thickness. It could be a good sliding surface or result in poorly bonded storm snow. Weak grains, such as near-surface facets and surface hoar, were observed on shadier aspects before this storm parade. Where preserved, they could allow avalanches to fail easily or propagate widely. We expect the majority of avalanche activity to occur above the 3/9 interface. On Sunday morning, NWAC forecasters in the Crystal Mountain area reported the snow was poorly bonded to the 3/9 on most aspects and elevations. 

The Early February Crust (EFC) continues to produce concerning test results. Sudden collapses, propagation, and confirmed weak grains all point toward a scary layer 3-6 feet below the surface on Tuesday. This storm has us on edge, wondering if these weak layers will wake back up. If you've been ignoring this problem recently, you should take it seriously for the next few days. Check out the avalanche occurrence database to understand how giant and destructive these slides have been in the West South. In the neighboring East South, they resulted in a fatality. Let's not add to that total. Please keep things simple for a few days.

Outlook (Day 2):

On Wednesday, expect similar avalanche conditions to those found on Tuesday. The storm should be winding down, but the snowpack will need time to adjust to the recent load. Storm snow related avalanches will remain likely and should be confined to the snow above the 3/9 interface. The wildcard will be sunshine. With significant sun breaks possible, wet avalanches could follow. Look to the snow surface to help determine if you are dealing with wet or dry avalanche problems. If the surface is getting moist and sticky, head for shadier slopes.

Those deep persistent layers won't magically heal in a day. Please continue to give them space by avoiding large open slopes. 

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