Bollettino valanghe

Stevens Pass

Dec 28 – 29, 2018Northwest Avalanche Center
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Archiviato · scaduto 8 years ago

Pericolo valanghe

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

In sintesi

Heavy rain to treeline and blowing snow at upper elevations will create dangerous avalanche conditions. If you venture into the mountains during this wet and stormy weather, expect a dramatic change in conditions from earlier in the week. Avoid being on or under steep slopes where the snow is wet, and stay off of slopes at upper elevations with freshly wind drifted features.

Problemi valanghivi

  • Wind Slabs

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

    Snow and wind will build fresh wind slabs. You can trigger these avalanches at the highest elevations on leeward slopes. Steer around recently drifted features and where the snow is thickly pillowed. While these avalanches are the second problem on the list, they could have serious consequences. Wind slabs could break above you and could be big enough to kill you. At mid elevations moist snow will be adding an extra element to the snowpack structure. Here you may see both wet avalanches and wind drifting.

  • Loose Wet

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

    Heavy rain will fall on the previously dry layers of snow. This is a perfect combination for wet avalanches. Much of the rain and avalanches will come overnight, but activity will continue through Saturday. While loose wet avalanches are likely, they are also easy to predict and avoid. They typically break where you impact the snow. Roller balls and pinwheeling snow are good indicators that you may soon see wet avalanches. If you see avalanches, there's a really good chance you can trigger more in similar terrain. Stay off of steep slopes (over 35 degrees, 40 degrees is perfect) where the upper snowpack is wet. Don't linger below these slopes, as avalanches may occur naturally and come down from above. 

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2

    This is a low likelihood, high consequence problem. The chances of triggering are very low, but the resulting avalanche could be unsurvivable. Few backcountry travelers have experience with avalanches of this magnitude. Use caution in the eastern end of the zone (east of Smith Brook) and anywhere with less than 5 feet of snow on the ground. There, the snowpack is similar to what you will find in the East Central forecast zone. You can trigger deep persistent slab avalanches more easily in these areas. 

    The best way to reduce the risk of the high consequence of deep persistent slab avalanches is to:

    -Limit the amount of time you spend on or near large slopes 35 degrees and steeper.

    -Put an extra buffer of terrain between where you travel and where avalanches could start, run, or stop.

    -Stay away from features where avalanches are commonly triggered like: rocks, steep roll-overs and convexities, unsupported slopes ending in cliffs or steep drops, and areas of shallow, variable snow.

    -When in doubt, avoid avalanche terrain.

Snowpack Discussion

Regional Synopsis December 26, 2018

In most parts of the state, a stout melt freeze crust was formed when it rained to high elevations around Thanksgiving. The one exception to this event was in the East North Zone, where the precipitation from the Thanksgiving storm was all snow. A quick storm at the end of November put a small amount of snow above the melt-freeze crust, and preserved the older basal facets in the northeastern areas.

Cold and clear weather dominated the first week in December, with valley fog and very cold temperatures east of the crest. The surface snow sat around and decomposed. Surface hoar grew large on top of this.

The jet stream took aim at the Pacific Northwest in the 2nd week of December.  Most notably, light storms buried and preserved a widespread layer of surface hoar and/or near surface facets on december 9th. From December 9th to December 23rd, storms kept coming. Freezing levels fluctuated, but never moved much above 5000ft throughout the Cascades (although the southernmost volcanoes and Mt. Hood saw rain well above 6000).

Initially, the storm track favored the northern zones. The accompanying avalanche cycle began on December 11th. Most of these slides were soft slabs, but some propagated widely on the December 9th layer. Higher snowfall totals in the West North resulted in very large (D3+) avalanches in the mountains along Hwy 542.

A second, and larger avalanche cycle occurred during heavy snowfall and strong wind events between December 18th and 20th. Although these cycles were once again most prevalent in the northern and eastern zones, big storm totals around Mt. Rainier tipped the balance down south as well. This 2nd cycle was impressive, with very large and destructive avalanches (some D4) reported. The culprit was once again the December 9th surface hoar/facets (and/or the basal facets in the northern and eastern zones).

Today we have a large difference in snowpack depths between the Pacific Crest and the Eastern Slope. This is nothing unusual, as more often than not the west side of the Cascades and the passes get more snow than areas further east. Moving forward, places with a deep snowpack (say greater than 5ft) and warmer temperatures may continue to gain strength. Areas with a shallow snowpack (say less than 3.5ft) may take much longer. In a general and applied sense, this means the avalanche danger/conditions may begin to diverge between the western and eastern zones. As always, please share your photos and experiences with us!

Happy Holidays

Regioni coperte