Avalanche Forecast
Issued: Dec 29th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
Another day of quiet weather will maintain minimal avalanche danger. While avalanches are unlikely, you may encounter shallow drifts near the Cascade Crest and old weak layers of snow throughout the zone. Expect a storm and major shift in the weather for Tuesday.
Summary
Discussion
Expect another day of benign weather before a storm hits early Tuesday. An inch or two of recent snow may be found in the mountains with slightly more accumulation closer to the Cascade Crest. On Saturday, ridgetop winds may have been strong enough to form very small and isolated wind slabs above treeline. A near-surface crust can be found up to around 5,500ft west of Hwy 97, making travel difficult. Without much else going on in the snowpack, this is a good time to track the current snow surface conditions, any shallowly buried interfaces, or even deeper layers of concern. Snow coverage is still thin, especially at low elevation and slopes further east of the Cascade Crest.
Wind textured surface snow is a visual clue that can alert you to recent wind drifting. Mission Ridge, 12/28, 2019. Photo: Josh Hirshberg
Snowpack Discussion
December 26, 2019 (The regional synopsis is updated every Thursday @ 6 pm)
No matter where you are in the northwest, the snowpack looks a world different than it did a week ago. A major storm, widespread avalanches, developing weak layers, and an initially thin snowpack made for a hectic week. Let’s take a look at a few large scale trends for our holiday season.
A very large natural avalanche (D3) on the Shuksan Arm near Mt Baker that occurred during the storm cycle. Photo: Adam U.
Wet and Wild
“Some of the highest precipitation rates I’ve ever seen.” “Absolutely puking!” “Rivers running in the streets.” “Snowing snow hard I can’t see my hand in front of my face.” These were all statements we heard describing the wet and wild storm that impacted the Northwest from about Thursday (12/19) through Sunday night (12/22). Water totals were staggering in many locations (Table 1). For Hurricane Ridge, Mt Baker, Washington Pass, and high elevation terrain this deluge translated into significant snowfall. However, warm air in the central and southern parts of the region brought rain well into the near treeline bands. Whether your favorite spot saw rain or snow, four things are clear. 1. This was a huge loading event. 2. A widespread natural and triggered avalanche cycle (up to D3) occurred. 3. Avalanche danger spiked during and just after the storm before trending down through the week. 4. The snowpack changed dramatically.
Location
Precipitation 12/19-22
Hurricane Ridge
4.97”
Mt Baker Ski Area
7.35”
Washington Pass
3.53”
Stevens Pass
6.28”
Leavenworth
2.94”
Snoqualmie Pass
7.89”
Crystal Mt Ski Area
7.45”
Paradise, Mt Rainier
6.57”
Mt Hood Meadows
2.18”
Table 1: Precipitation totals for select weather station locations December 19-22.
The Emergence of Persistent Slab Avalanche Problems
Coming out of this huge loading event, there was hope that many of the early season weak layers had been destroyed. Unfortunately, this was not the case in some locations. Old weak snow layers reared their heads in areas east of the Cascade Crest and near Crystal Mountain. While the exact extent and character of these layers can differ slightly, most locations are finding a layer of weak sugary facets associated with a crust about a foot above the ground. Persistent slabs are tricky to assess. Do not solely rely on snowpits and snowpack tests to help you choose terrain. How will these layers change going forward? Only time and observations will tell.
You may find weak sugary facets near a crust about 12 inches above the ground similar to what you see here. Photo: Jesse Charles
Low Tide Snowpacks During the Holidays
The storm this past week definitely helped the meager early-season snowpacks in all locations. Areas like Hurricane Ridge, Mt Baker, and Washington Pass experienced a jump of 20 or more inches in their snow depths over the past week. However, even with these increases, an early season snowpack still describes most locations. Expect numerous obstacles as you travel including open creeks, rocks, and trees. With this thin snowpack, limited access, and difficult travel we still have limited observations in some regions.
Open, deeply incised creeks in the Alpental Valley. Photo: Dallas Glass
Lack of information leads to a higher than normal degree of uncertainty. If you travel to higher elevations or more remote trailheads, recognize you could experience different conditions than the forecast suggested. You can help us fill in the gaps by submitting your observation here.
Happy Holidays!
-Dallas Glass
Problems
Persistent Slabs
While triggering avalanches is unlikely, there is a lingering concern in areas of shallow snow where a layers of weak facet persists. This is the most probable on slopes facing toward the north half of the compass, in the Wenatchee Mountains, and where the snowpack is shallower than 3 feet deep. Persistent slab avalanches can be surprising and can break widely across terrain. The best way to deal with this low-likelihood, high consequence scenario is to pick conservative terrain options. Specific features to avoid are where stiff drifted slabs overlie the facets, steep convex rollovers, areas of variable height of snow, shallow and rocky features, and unsupported slopes.
A layer of facets, formed in late November, can be found near the bottom of the snowpack, often resting on a stout crust and sometimes capped with a thin crust. The layer is typically about 1-3 feet below the surface. It was responsible for numerous large avalanches and signs of instability during the storm and avalanche cycle around December 20th. Other weak layers, such as facets in the lower third of the snowpack and recently buried facets and surface hoar, (found in this observation) could become the source of avalanches with the coming storm.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 30th, 2019 10:00AM
