Bollettino valanghe

Mt Hood

Northwest Avalanche Center
Vedi sulla mappa

Pericolo valanghe

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

In sintesi

0700 Update:  Decreased the danger to Considerable at upper elevations due to lower snow amounts than forecasted. 

Heavy snow and intense wind redistribution will lead to very dangerous avalanche conditions developing at upper elevations. Avoid travel in wind-loaded slide paths. At lower and middle elevations, variable pre-existing surfaces include weak snow on north aspects that may result in widely breaking or surprising avalanches.

Problemi valanghivi

  • Wind Slabs

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

    Heavy snow will redistribute as it falls thanks to sustained moderate to strong winds Friday night through Saturday. Wind slabs will start smaller and more limited in distribution but grow larger and more reactive as the day progresses. By the afternoon, we expect upper elevation lee slopes to hit the tipping point for widespread natural avalanches. Slabs may bond poorly to widespread underlying slick crusts, and they may overly weak snow surfaces to produce more widely breaking avalanches in isolated sheltered northerly locations. This is a good time to seek sheltered terrain at middle or lower elevations and let the storm run its course. Avoid all lee slopes and avalanche paths in exposed terrain.

    Example: Large wind slab. These may be human or naturally triggered during the storm on Friday.

    Photo: Mt Hood Meadows Pro Patrol

  • Storm Slabs

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

    We're expecting 12-15" of new snow above the rain/snow line by the end of Friday. Storm slabs will be most reactive at middle elevations, where the snow comes in colder and bonds poorly to the underlying interfaces. At lower elevations, this snow will come in warmer and may even mix with rain, so slabs will lose reactivity quickly.  Use your favorite quick tests to assess layers forming within the storm snow and at the new/old interface which may be a crust or in some cases weaker old snow. Avoid steep open slopes and rollovers in the terrain once you have 8" or more above that interface. If you find test pits propagating at the old snow interface, expect widely propagating and more dangerous avalanches.

    Example: Storm slabs forming on steep sheltered slopes.

    Public Field Report: Pea Gravel Ridge

Avalanche Discussion

January’s unusually long dry period really changed the snowpack. Crusts and weak sugary facets top the surface in many areas. Now a series of storms will put these potential weak layers to the test.  The first and most intense round impacts the Mt Hood area Thursday night and intensifies on Friday with heavy snow, wind, and warming. While the avalanche problems are typical, the variability of the old snow/crust interface that will form with the incoming snow will be unusual. These surfaces are also highly variable, so what you find on one test slope or snow pit may not be representative of a slightly different aspect or elevation. The uncertainty that results from this variability should lead you to approach terrain more conservatively, giving steep open slopes time let us know whether we might be developing a persistent-style avalanche problem in parts of the zone.

On most aspects, you'll find firm, slick crusts. The new snow will bond poorly to those crusts and could fail easily, running quickly along that bed surface. However, pockety weak, faceting snow has developed on shaded middle and lower elevation terrain on the south side of Mt Hood, but gerally exists under a thin melt-freeze crust. 

On the north side of Mt Hood, weak snow surfaces are more widespread going into the storm and could be more problematic once loaded. NWAC forecasters visited the north side of Mt Hood on Wednesday and found that the snowpack increases significantly in the 2500-3000 ft range. Between 3000-4000 ft, they found well-developed surface hoar (up to 8 mm) over facets. A thin decomposing crust under the first facet layer separated a second layer of facets. Ultimately, challenging snow machine travel prevented them from reaching higher in the terrain. Another observer on the northwest side of Mt Hood also found 4" of sugary faceting snow above a crust on NE aspects at 3500 ft near Lolo Pass on Wednesday. 

Some lower-elevation terrain could transition from snow to rain during the day on Friday, potentially dissolving weak snow grains. However, if rain falls on more than 6" of new snow, expect wet, loose avalanche activity on slopes steeper than 35 degrees.

Your observations are especially critical to us we assess the potential for continued reactivity on a developing interface. You can submit your observation here.

Regioni coperte