Bollettino valanghe

Mt Hood

Feb 16 – 17, 2019Northwest Avalanche Center
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Archiviato · scaduto 8 years ago

Pericolo valanghe

Mon
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
Apri la mappa completa

In sintesi

New snow and wind loading maintains concern about a weak layer 2-3 feet deep near and below treeline that can produce very large slabs that break widely across terrain features. Recently formed wind slabs or surface snow avalanches may be easier to trigger on Sunday and can step down to the dangerous weak layer. Choose simple terrain to safely navigate this tricky snowpack.

Problemi valanghivi

  • Wind Slabs

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

    Saturday afternoon and evening, moderate to strong winds are anticipated to have created fresh wind slabs while older slabs linger from moderate winds Thursday and Friday on steep, leeward slopes. Recent cornice formation and textured snow surfaces indicate that wind slabs are present in your area.

    Where wind slabs linger near treeline and overlap with the persistent slab potential, don't thread the needle between these avalanche problems and stick to slopes 30 degrees or lower. A triggered avalanche may step down into deeper weak layers to create deadly avalanches, so think seriously before testing larger slopes.

  • Persistent Slabs

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

    New snow and loading will stress the identified weak facet layer 2-3’ deep and maintains a very real threat of triggering a slab that can come down on top of you and can be triggered from a long distance away. These avalanches are difficult to predict and can break wide and in surprising ways by wrapping around terrain features. Be especially suspicious near and below treeline where weak snow over a firm crust was most likely buried intact and recent avalanches have occurred.

    If you hear large whumpfs, experience sudden collapses in the snowpack, see shooting cracks in relatively flat terrain or remotely trigger an avalanche, nature is trying to tell you deadly persistent slab avalanches are possible.

    Even in lower elevation terrain below treeline, choose simple terrain, avoiding steep chutes or open slopes capable of producing a large avalanche.

Discussion

Snow and Avalanche Discussion

By Saturday evening, 6.6” of storm snow had accumulated during the day and was being blown around by moderate to strong winds with more snow and transport anticipated.

The epic storm cycle that wrapped up Wednesday produced several deep avalanches averaging around 4 feet that were triggered Wednesday and Thursday by both skiers and explosives near and below treeline.

On Thursday and Friday, Mt. Hood Meadows Patrol identified a layer of weak snow (1-2 mm facets) above a hard crust as the weak layer/bed surface combo on northerly aspects below treeline.

This weak layer/crust combination is likely most reactive near treeline and in the upper treeline elevations, but information is still limited. Stay safe by keeping your terrain selection simple.

The risk of tree well and snow immersion suffocation continues in areas with deep snow. Don't travel alone and always keep in contact with your partner. You can learn more about deep snow safety here. 

If the sun comes out on Sunday, expect roller balls and loose wet activity that will entrain the surface snow and shed off rocks and steep slopes.

Snowpack Discussion

February 15, 2019

Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experience cold and very storm weather. Significant snowfall has added up in all forecast zones. Records from Snoqualmie Pass DOT avalanche workers back to 1973 show that February 11-12th set a record for the most snow recorded in a 24hr period at that location. The table below shows storm totals starting February 8th through the morning of the 13th

5 day totals ending morning of Feb 13th

Water Equivilent (inches)

24hr storm totals

(inches)

Difference in Height of Snow (inches)

Hurricane Ridge

1.97

N/A

+ 30

Mt. Baker

1.94

44

 

Washington Pass

1.66

NA

+ 16

Stevens Pass

2.71

49

 

Snoqualmie Pass

3.91

80

 

Mission Ridge

1.86

38

 

Crystal

2.91

59

 

Paradise

4.55

N/A

 

White Pass

N/A

57 (4400ft)

+ 26 (5800ft)

Mt. Hood Meadows

4.70

43

 

Heavy precipitation brought many mountain regions to their tipping point. Avalanches ran readily with a peak of snowfall intensity. For Stevens Pass, Snoqualmie Pass, East Central, West South, Mt Hood, and possibly West Central zones we have good confirmation that this cycle happened from the night of February 11th through the 12th. In other zones, snow totals haven’t been significant enough for widespread avalanche cycles, or we lack data (like in the East South zone).

A natural persistent slab avalanche (D2.5) on a southeast aspect at 6,600ft. Grindstone Mtn in Icicle Canyon. Likely ran 2/12. Photo: Matt Primomo

The high rates of precipitation drove avalanches in the storm snow. Notably, a persistent weak layer of facets and surface hoar was buried in most zones on February 8th. Storms produced a widespread and prolonged cycle of avalanches on the February 8th interface, involving a variety of aspects and elevations. Local ski patrols, highway workers, and backcountry travelers reported extensive avalanching with widely propagating crowns and very sensitive conditions. With less stormy weather, observers have just begun to get a sense of the extent of the avalanche activity. Triggering persistent slab avalanches will be a concern for backcountry travelers in zones where the February 8th weak layer is active for at least the near, if not distant future. Stay tuned for more updates.

Large surface hoar near Snow Lake Divide on February 7, 2019 just before it was buried on the 8th. Photo: Jeremy Allyn

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