Bulletin d’avalanche

Whitefish Range

Flathead Avalanche Center
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Danger d’avalanche

Mon
Alpin
2 · Modéré
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Warming temperatures and the possibility of sustained sunshine should trigger wet avalanches, especially this afternoon. The danger will rise on slopes facing the sun as the day progresses. Shady slopes will offer drier snow and a generally safer experience.

Problèmes avalancheux

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Today's warming temperatures and the potential for sustained sunshine may initiate wet avalanches in the surface snow. Recent storms have left up to a foot of dry, loose snow on sunny aspects. This week's cool temperatures and the lack of southwest winds have left tree branches covered in snow. As the day heats up, snow falling from trees (tree bombs)may trigger sluffing. Rock outcroppings facing the sun absorb heat and may shed their coating of snow, initiating sluffs. 

    Significant snow is available for entrainment, allowing a small slide the potential to become big enough to bury someone in larger terrain features. As the debris moves downhill, it may gouge into the relatively soft underlying snowpack producing a more dangerous slide. 

    Rollerballs and pinwheels are precursors to a wet slide. If you can make a snowball in the surface snow, this signifies that the surface has become moist. These are signs directing you to move to a different aspect. Stay ahead of the sun today. Choose a route that does not expose you to westerly aspects on your exit. Limit time above terrain traps where even a small slide can pile up deep. Northerly aspects should remain safer and hold higher-quality snow.

     

    (12/28/2022)

    A Loose Wet avalanche displays the classic inverted v shape. Recent warm temperatures and rain probably triggered this slide.

    Photo: Mark Dundas

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2–3

    Observers continue to find weak layers 2-4 feet below the surface in snowpits. Testing of these layers shows a strengthening trend. However, a rider in the northern Whitefish Range unintentionally started a slide on a persistent weak layer Wednesday (Photo to the right/above). Since mid-January, most slides on these layers occurred on mid and upper-elevation slopes facing NW-NE-SE, where the weak layer is beneath a slab stiff enough to propagate. 

    The only way to determine if this snow structure is present is to dig down several feet to identify it. A more straightforward and foolproof method is utilizing the terrain to mitigate this problem. Recent very large avalanche activity occurred during loading events on wind-loaded slopes with rocky terrain features harboring a snowpack of varying thickness. Stay clear of areas with a peppered appearance where the weak layer is closer to the surface and more susceptible to triggering. Convexities or rollovers are locations where the stress is concentrated in the snowpack. Bypass those options and choose lower-angle slopes free of overhead hazards for a safer experience. 

    (3/8/2023)

    A snowmobiler accidentally triggered this slide on a buried weak layer on an east-facing slope. The crown height was 18-24" deep.

Avalanche Discussion

We begin the day with LOW danger on sunny slopes at all elevations. If the ambient air temperature and solar input arrive as expected, we anticipate the danger to increase through the day on these aspects. The danger should reach MODERATE  at low and upper elevations and MAY reach CONSIDERABLE at mid-elevations. If prolonged periods of sunshine do not materialize the danger will reach MODERATE at mid-elevations.

Today's weather may include periods of sustained sunshine, especially this afternoon. Temperatures this morning are warmer than they have been in quite a while. Forecasted high temperatures should reach readings not seen recently. Our area has not experienced a warm-up and subsequent wet avalanche cycle since mid-January. Plenty of loose, dry snow is on and beneath the surface, available for entrainment. We are primed for a large wet loose avalanche cycle.

There is a bit of uncertainty regarding today's cloud cover. Most models point toward afternoon clearing, but that is not a given. If today's forecast were for bluebird skies, I would grab a packable chair and sunhat and park safely to watch a wet loose cycle unfold in large terrain in the Middle Fork. Exciting stuff. Unfortunately, today's forecast comes with uncertainty due to the cloud cover, or lack thereof. 

A snowmobiler triggered a Persistent Slab avalanche  Wednesday in the northern Whitefish Range. This D2 Slab failed on an east aspect and had a crown depth of 18-24 inches. This is the first recorded human-triggered slide on buried weak layers in this range in many weeks. We thank the party involved for submitting this information which helps make our product more accurate. and hopefully, keep the recreationists safer.

On Thursday, NOAA's Climate Prediction Center (CPC) declared an end to the long-lasting La Niña that began nearly three years ago, signaling that ocean temperatures in the equatorial Pacific were warming.

Fireside/Snowpack Wednesday, March 15. Join us for a virtual evening combining two favorite community ed workshops. FAC Forecaster Cam Johnson will kick things off with a State of the Snowpack presentation to help you digest the current conditions and plan your next trip. Then we'll shift to Fireside Chats for open dialogue about human and avalanche factors. A recreationist, forecaster, and avalanche educator all experienced a near miss in the first three weeks of February. Together, Kat, Cam, and Meg will explore the tough decisions, tricky snow, and lessons learned from their experiences. No registration is needed; click here for Zoom. The event will begin at 6 p.m. 

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