Dashboard Regions Weather Stations Radar Alerts Glossary
Contact About
Log In

Register for an account and never miss a forecast again!

Register

Avalanche Forecast

Archived

Dec 21st, 2018–Dec 22nd, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

.

We aren’t out of the woods yet, but it’s becoming harder to trigger a very large deep slab avalanche. Large loads like a cornice failure or finding shallow spots in the snowpack are exceptions to think about. Any avalanche failing on a deeply buried weak layer is likely to be very large, destructive and deadly. Limit your exposure in terrain where avalanches start, run and stop.

Discussion

Discussion

Several large or very large natural, skier, and explosive triggered avalanches occurred in the Crystal area Monday through Wednesday. Many of these avalanches acted in surprising ways including wrapping around terrain features, running long distances, and responding to relatively small triggers.

Thursday’s storm brought rain and high winds to West Slopes South zone. On Friday, professional observations from the Crystal backcountry and the Tatoosh Range (south of Paradise) revealed no new significant avalanche activity following Thursday’s storm. We don’t have any information about this layer in the White Pass area.

A supportable rain crust was found at higher elevations while the snowpack was still refreezing below treeline. While the 12/9 persistent weak layer was identifiable in snowpits in both areas, it was rounding and gaining strength. Several intermittent crusts could be found above the 12/9 layer in the upper snowpack. Near treeline, only shallow wind slabs were found on isolated lee slopes. However, recent winds have caused wide variations in snow depths - something worth noting when thinking about deep persistent slab avoidance.

Snowpack Discussion

Regional Synopsis: December 19, 2018

Why has the avalanche danger been so high for so long? Two reasons: A very active and wet weather pattern combined with a widespread persistent weak layer.

For perspective, the approximate snow totals from 12/9-12/19 are:

  • Mt Baker: 102”

  • Washington Pass: 55”

  • Stevens Pass: 76” mid-mountain

  • Snoqualmie Pass: 68” mid-mountain

  • Crystal Mountain 70” Green Valley

  • Paradise: 78”

  • Mt Hood Meadows: 44” mid-mountain

  • Olympics: 48”

In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause of numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess. In short, the scenario is tricky and getting more dangerous by the day.

Higher snowfall totals along the Hwy 542 corridor/Mt Baker area have driven several avalanche cycles during this period. Loading from recent storms has been more incremental to the east and south of the Mt. Baker area, so we have not yet experienced a widespread cycle in other areas.

When will we reach the breaking point? It’s hard to say. What we do know is we have a deep weak layer, reports of very large explosives triggered slides at Mission Ridge and Crystal Mountain, and more storms on the way.

Be patient and continue to stick to lower angle slopes with nothing above you.  This is a good time to avoid areas where avalanches can start, run, and stop.

We’d like to thank all of you who have sent NWAC your observations. If you are out in the mountains, let us know what you see.

Problems

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.