Avalanche forecast

Swan Range

Flathead Avalanche Center
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Avalanche danger

Sat
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

A surprise storm increased the danger in the Swan Range yesterday. This new load increases the chance that a rider could trigger an avalanche on a weak layer buried about 2 feet below the surface. Stick to lower-angle slopes free of overhead hazards on the north side of the compass. New slabs will be thinner and more manageable on sunny and windward slopes.

Avalanche problems

  • Persistent Slabs

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

    The Noisy Basin Snotel (6040 feet) picked up 1.1 inches of SWE and 6 inches of snow during the day yesterday. This new load arrived during a 12-hour period and increased the stress on a layer of weak surface hoar and facets buried about 2 feet below the surface.

    Fresh slabs may be reactive to the weight of a rider on slopes facing NW-NE-SE. Slabs will be thicker and more dangerous as you climb in elevation. If you want to recreate on the north half of the compass, choose lower-angle terrain free of overhead hazards. Windward and sunny aspects do not seem to harbor the weak snow layer and will generally be safer. Stick to slopes with clean runouts that minimize the chance of trauma should something go wrong.

    (1/11/2023) SHsu only in very isolated exposed clearings Probably more widespread in more open terrain.

  • Storm Slabs

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

    An unexpected storm has brought a new snow avalanche problem to the Swan Range. Slabs may be found on all aspects but will be thicker and more dangerous on slopes facing NW-NE-SE above 6500 feet. As you gain elevation, evaluate new snow totals and look for evidence of blowing snow. On sunny and windward aspects, utilize hand pits and test slopes to see how well the fresh snow is adhering to the underlying harder layer. On leeward aspects, choose lower-angle terrain free of overhead hazards to minimize triggering a much larger avalanche. Feel for denser snow and cracking beneath you to identify a slab. 

    (12/23/2022)

    Thin storm slabs released from the weight of a skier in today's new snow. Slabs were thin and harmless in the terrain we traveled.

    Photo: Mark Dundas

Avalanche Discussion

A surprise storm dropped 1.1 inches of SWE and 6 inches of snow yesterday at the Noisy Basin SNOTEL (6040 feet). Mid-elevation weather stations in the Whitefish Range and southern Glacier Park received virtually nothing. Unfortunately, the Tunnel Ridge station remains down, and we don't know if there was "spillover precipitation" in the Flathead Range. Occasionally, moisture-laden storms on southwest flow do not drop all of their precipitation in the Swan but instead drop what is left on the next formidable barrier to the east, such as the Flathead Crest. 

Swan Range: The new load increased the danger in the Swan yesterday afternoon, and I suspect natural sluffing and Loose Dry slides occurred, along with possibly a few storm slabs. There were no observations from the Swan yesterday, and we don't know if the new load tipped the balance and triggered larger slides on the buried weak layers. Today, the natural avalanche activity is over; however, the chance of triggering a slide on a buried weak layer has increased from earlier in the week. Due to uncertainty with the buried weak layer, and a subsequent conservative mindset, we have bumped the danger up today as we let the snowpack adjust to this loading event. The danger is CONSIDERABLE on slopes above roughly 6500 feet, harboring the buried weak layer (NW-NE-SE). Expect the risk to decrease to a MODERATE rating tomorrow in this terrain. We added the storm slab to the problem list on all aspects at mid and upper elevations. This has increased the danger of the windward and sunny aspects to MODERATE.  Storm slabs generally heal quickly; we may drop this rating to LOW tomorrow.

Whitefish Range, Flathead Range, southern Glacier Park: Light winds and the lack of precipitation keep the avalanche danger steady in the Whitefish Range and southern Glacier Park. Probably the Flathead Range as well. In these areas, slabs resting on weak snow are not being tested like in the Swan Range. Slabs remain thin, and the risk of triggering a slide on a buried weak layer may decrease slightly during these calm conditions. 

  • Blase was in the side country of WMR searching for buried surface hoar, which he and his partner did not find. Multiple pits at mid-elevations revealed weak interfaces in the upper ~75 cm of the snowpack, but none propagated in tests.  They were surprised to trigger several sluffs on steep, north-facing, mid-elevation slopes involving the 4 cm of new snow. These ran surprisingly far and could be a hazard in areas with more snow above the 1/19 rime crust.
  • The USGS was in John F. Stevens Canyon, where they found buried decomposing surface hoar on sheltered N and NE aspects at 6800' down to 5500' ~20cm below the surface. They stomped small fresh, touchy cornices around 6800 feet with no meaningful results. The surface hoar in their snowpits was unreactive in tests, but there is not much of a slab above it yet.

Today and Saturday will offer benign weather conditions of seasonable temperatures, light winds, and mostly dry conditions. This weather pattern will continue to help our Persistent Slab problem improve slightly in many areas and allow the snowpack time to adjust to the recent loading event in the Swan. The next weather system will impact the area overnight Saturday into Sunday with up to 6 inches of new snow in favored locations. This storm may help build a thicker slab on top of the near-surface weak layer in the Flathead and Whitefish Range and may be another loading event that will test our buried weak layers.

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