Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

Since March 2nd, we’ve documented four very large (size 3 of 5) natural avalanches. These all occurred on east and north-facing slopes in rocky, complex terrain with variable snow depths. This is the type of terrain to avoid. Hedge your bets on planar or concave slopes that appear to have uniform snow depths and clean runouts. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Weak snow buried 2 to 4+ feet deep is the culprit for recent very large natural avalanches in the last five days. These avalanches failed in steep, rocky terrain at upper elevations on north and east aspects. Common trigger points to avoid are thin areas in the snowpack due to wind scouring, or convex rollovers where the slope goes from less steep, to steeper. Choose simple terrain with runouts free of rock bands, trees, or depressions. Choosing lower-angle slopes is the most reliable way to avoid this problem. This avalanche problem may give no warning signs until it is too late. 

    (3/6/2023)

    Crown faces and debris from one or more large natural avalanches on the east face of Mt. Penrose in the Flathead Range. The observer estimated the crown face at 5 feet thick and the date as Sunday. 3/5.

    Like several other recent natural events in the Flathead Range, this avalanche appears to have been triggered by a natural cornice fall that triggered a hard slab that released on old snow, possibly facets around a crust from mid-January or late December.

Avalanche Discussion

Weather:

Since March 2nd, we’ve received 4 to 12 inches of snow throughout the forecast area. The northern Whitefish Range and the Flathead Range received the upper end of snow totals. The southern Whitefish and Swan Range (believe it or not) received the lower end of those numbers. Since then, winds have shifted directions from southwest to east, transporting snow onto varying aspects. The sun warmed southerly aspects and left a breakable sun crust. Today, solar aspects may warm enough to produce small wet loose avalanches. Rollerballs or a wetting snow surface are signs to change to shaded aspects.

Snowpack:

To no surprise, the Flathead Range has received the most observed natural avalanche activity. A weak layer of facets near a crust proved its untrustworthiness after a relatively small loading event on March 2nd. Avalanches failed roughly 3 to 5 feet thick and were large enough to destroy a wood-framed house. The northern Whitefish Range, which received similar snow totals, also has a poor snowpack structure. So far, we have received no reported avalanches from this zone in the past week. Weak layers in the Swan Range are now buried anywhere from 3 to 6 feet deep. We have not received a report of an avalanche failing on a persistent weak layer since February 10th in the Swan Range. 

Observers have noted isolated cracking in small drifts downwind of ridgelines. Expect slabs to be larger the higher you climb, or in areas where you see obvious signs of wind drifting. 

Terrain:

The Flathead Range/Glacier National Park forecast zone is the recent hotspot for Persistent Slab avalanche activity. In this zone, give yourself a wide margin for error since recent avalanches have propagated wide distances and broke into lower-angle terrain, as noted in this observation. The challenging part about this type of avalanche problem is that feedback isn’t so obvious. Signs of instability are not a theme, yet natural avalanches occurred recently. A rider hitting a thin spot in the slab, or a cornice fall can trigger one of these unsurvivable avalanches. This is the type of avalanche problem where we won’t receive the feedback until it is too late. Treat upper elevation terrain in the Whitefish Range the same. 

Generally safer avalanche conditions exist in the Swan Range. Identify and steer around small, isolated slabs of drifted snow. 

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