Avalanche Forecast

Issued: Dec 24th, 2018 10:06AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Storm Slabs and Deep Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Santa has sunshine and soft snow in store for you on Christmas. Despite the winter wonderland, you may still trigger avalanches in the recent storm snow. You are most likely to trigger an avalanche at higher elevations, in areas where the wind affected the snow, or on convex rollovers. Very large avalanches are becoming less likely but should leave you wary of traveling in and around terrain capable of producing large avalanches. 

Summary

Snowpack Discussion

Regional Synopsis: December 24, 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 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

Storm Slabs

An icon showing Storm Slabs

1-2 feet of new snow fell above 3500 feet within the past 48 hours in this zone. You can find weaknesses in the snow that are still capable of producing avalanches. If you are planning to travel near and above treeline or in areas where the wind affected the snow, you will still be able to trigger these soft slabs. At lower elevations and in wind protected areas, you are most likely to trigger a storm slab on convex rollovers, in terrain steeper than 40 degrees or on an unsupported slope. You can use smaller inconsequential test slopes to observe how the storm snow is bonding to the old snow.

When the sun comes out, expect small loose avalanches to occur on steep, rocky, sunny slopes. Don’t let them catch you off guard, especially if you are on slopes that could have dangerous consequences.

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

We've been tracking the deep persistent slab potential in surrounding areas. We have no information about reactive weak snow layers in this zone, but make terrain choices with this in mind given the gravity of the avalanche problem.  

In the Mt. Baker area, we know this layer is generally found 6-8 feet below the snow surface and above 5000’. This layer is likely not quite as deep in the much of the Central West zone. You would be most likely to trigger a deep avalanche from thin spots in the snowpack, such as around rocks, or if a smaller, new snow avalanche step down to the old weak snow. Hopefully this layer is healing, but until we can confirm that, we want you to keep thinking about this low likelihood, but high consequence avalanche problem.

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

Possible

Expected Size

2 - 2

Valid until: Dec 25th, 2018 10:06AM