Bollettino valanghe

Swan Range

Flathead Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
Open full map
Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Choose to ride in wind-sheltered terrain without evidence of drifts, dunes, or pillows. Paying attention to changes in snow density and surface texture is an excellent way to reduce your risk of being surprised by triggering a wind slab avalanche. Triggering a 2-3 foot thick avalanche is possible in areas where buried weak layers exist. These weak layers recently produced avalanches on shady aspects that face north and east. 

Problemi valanghivi

  • Wind Slabs

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

    Light and moderate winds have and will continue to transport new low-density snow into thin slabs. These are found in the terrain below ridges, in cross-loaded gullies, and downwind of saddles. As you travel in elevation, poke your ski pole or step into the snow to feel for changes in density. Identify wind-affected terrain by dune-shaped drifts or pillows of snow. Cracking around your machine or board is a great clue that you are in wind-drifted snow. Rippled or scalloped surface snow can also be a sign that wind has removed snow from the area to a leeward side. Manage your risk by avoiding wind-drifted slabs and seeking out sheltered terrain.

    (1/16/2023)

    Noisy Basin's sagging cornices along Aeneas Ridgeline.

    Photo: sw

  • Persistent Slabs

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

    Observers continue finding buried weak layers of facets or surface hoar about a foot below the surface (or 2-3 feet in the Swan).  This layer is responsible for a rider-triggered slide over the weekend. Generally, this concerning layer is found on shady aspects at mid and upper elevations. You may recognize the presence of this layer with hand pits or hasty pits. The easiest way to manage this problem is by choosing lower-angle planar slopes that are free of convexities. Runouts free of surface obstacles are a good buffer in case something goes wrong.

    (1/15/2023)

    The grey stripe is your clue that this problem exists below the surface. Right now it is found below 4 to 24 inches of snow depending on wind drifting. 

Avalanche Discussion

If you take advantage of January’s extra daylight and get into upper elevations, you’ll find both of today’s problems above 6500 feet. For our Wind Slab problem, all three forecast zones have weather stations that reported overnight light to moderate winds with strong gusts.

  • The Swan’s Range- Aeneas Ridge weather station clocked an impressive gust of 51 mph, with average wind speeds around 20 mph since 7 pm yesterday (Monday). 
  • Marias Pass weather station (Flathead Range & Southern GNP) recorded average wind speeds of 14 mph with gusts reaching 40 mph.
  • Hornet Weather Station in the Whitefish Range reported gusts of 36 mph and an average wind speed of 19 mph.

Although the new snow in all three forecast zones has been limited to 4 inches or less, these recorded winds have redistributed low-density snow and will continue to build thin slabs. Even though yesterday(Monday) had less wind recorded at our working weather stations, my partner and I noted how fast the 1 to 2 inches of recent snow was actively being transported around as we gained elevation in Rescue Creek basin in the Flathead Range.

Given the variation in the wind-drifted snow distribution, we were also amused by how the surface crust changed vastly in short distances under this recent soft snow. The stout crust was slick, supportable at low elevations, and warranted ski crampons. By 5000 feet, we had made several transitions of traveling on wind-board sastrugi, various crusts,  pockets of 4-inch wind slabs, frozen chunder, and finally, at about 5600 feet, we got into 3 to 4 inches of new soft snow resting above the supportable MLK crust. 

The diversity of new and old surface snow we encountered is a good reminder of how spatial variation within a given snowpack is a variable that we struggle to fully understand without x-ray vision, but nevertheless, must account for whenever we slide or ride on the snow. This is the same reason why forecasting for buried surface hoar (Persistent Slab problem) is challenging. Before surface hoar gets buried by new snow, winds can break apart the crystals or sun can melt the crystals, leaving irregular patches of surface hoar within a given slope. This is why spatial variation of surface hoar can be difficult to map. So far our data suggests that the sun and wind have destroyed this tricky weak layer on South and West aspects, prior to it getting buried. Given the uncertainty, the safest strategy to manage this problem is with a wide margin of error and conservative terrain choices.

The good news is that there is fun skiing and riding to be had in all three forecast zones. Keep both problems in mind if you head out this week and please let us know what you see.

Regioni coperte