Avalanche Forecast
Issued: Dec 27th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
Triggering an avalanche in wind-drifted snow remains possible. Recent observations indicate deeper weak layers in the snowpack are becoming unreactive. Evaluate snow and terrain carefully and avoid steep, wind-loaded rollovers at upper elevations.
Summary
Snowpack Discussion
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 the skies clear and we move into high pressure, take note as to which avalanche paths have run large on deep, weak layers, and those which haven’t. Be sure to track surface conditions, as this next period of cold, clear weather may create the next weak layer when the storm track does turn back toward us. As always, please share your photos and experiences with us!
Happy Holidays
Problems
Wind Slabs
Light snowfall on Friday paired with variable winds will continue to redistribute snow and build fresh wind slabs. You are most likely to encounter the problem above treeline on the lee side of ridges or on cross-loaded terrain features. Work to identify wind-drifted snow by observing sudden changes in snow surface texture and hardness. Stick to sheltered slopes or wind-scoured areas to avoid the problem. Steer clear of unsupported wind-loaded slopes 35 degrees and steeper to reduce your chances of triggering an avalanche.
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine.
Likelihood
Expected Size
Deep Persistent Slabs
We are closely tracking the deep persistent slab problem in neighboring areas, but have limited observations directly from the West Slope Central zone. Be conservative with your terrain selection if you travel in this area given the uncertainty.
You are most likely to encounter this problem on north and east aspects above 5000ft where a layer of surface hoar and facets is buried 6-8ft deep. Carefully evaluate the snowpack before entering larger avalanche terrain and venturing into higher elevations.
Deep persistent slabs are difficult to assess and predict. Our current situation still requires patience. Crowns of very large avalanches are stark reminders of the dangers that may lurk deep within our upper elevation snowpack. It has been several days since one of these avalanches occurred, and uncertainty remains regarding the sensitivity of this problem. Although triggering a deep persistent slab avalanche is unlikely, the consequences of being caught in one remain very high.
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 or near the ground. 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. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 28th, 2018 10:00AM
