Avalanche Forecast
Regions: Olympics.
Dangerous avalanche conditions should develop during the morning hours as moderate snowfall and strong winds build a slightly denser slab over 8-12” of lower density snow. With sufficient loading, slabs may release at the base of this lower density snow, which sits on a crust below 5400 ft. Look for denser over less dense layers or cracking snow surfaces to indicate instability and avoid open slopes steeper than 35 degrees.
Discussion
The past week has brought fluctuating snow levels and several intense rounds of weather that have created upper snowpack variability by elevation and aspect as seen in this plot of temperatures and precipitation at Hurricane Ridge.
NWAC and NPS staff were out on Thursday (1/9) and found a supportable crust with 8†of snow on it below 4800 ft. From 4800-5400 ft, they found 8-12†of snow sitting either a single breakable crust on wind-sheltered N and E slopes or a more supportable crust where rain impacted the slopes more severely on S and W aspects. Above 5400 ft, the crust faded away. Surface snow was low-density and unconsolidated, but in isolated locations, the wind had stiffened the snow and it would break down to the recent crust.
The observers also found evidence of avalanche activity from a massive storm cycle since 12/31 which brought 7.32†of water equivalent and added 46†to the total snow depth. Avalanche paths have filled back in with wind loading.
Snowpack Discussion
January 9th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
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As we said Happy New Year and rang in 2020, snow was turning to rain at many trailheads and lower elevation Passes, not exactly the fresh start winter recreationalists had in mind. The snowpack was already looking a little thin throughout the region, especially at lower elevations. Low snow in places like Snoqualmie Pass made backcountry travel difficult and hazardous. NWAC’s snow depth climatology report was showing snow depths 25-64% of normal to kick off the start of 2020.
Things can change quickly in the Pacific Northwest and they did as we entered an extended storm cycle between January 2nd to January 8th. Strong winds, fluctuating temperatures, and heavy precipitation offered few breaks in the weather over this period limiting observations and hampering travel. Despite periods of rain at lower elevations, most areas saw several feet of new snow with big jumps in total snow depths as a westerly storm track strongly favored the West Slopes of the Cascades and the Olympics for the highest precipitation totals.
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Location
Total Snow Depth (in) 1/2/20
Total Snow Depth (in) 1/8/20
Hurricane Ridge
31
51
Heather Meadows Mt Baker
55
95
Washington Pass
49
74
Stevens Pass
41
63
Snoqualmie Pass
22
33
Alpental mid-mountain
44
63
Crystal Mt Green Valley
40
66
Paradise Mt Rainier
54
105
White Pass Upper
43
69
Timberline
36
57
Mt Hood Meadows
36
53
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We may have started with a shallow snowpack, but most locations increased their snowpack by 70% or more over this storm cycle!
During this extended and impressive storm cycle that included backcountry avalanche warnings, natural avalanches were reported in many areas Jan 6th-7th.
The Stevens Pass area was especially active over the period with over 100(!) avalanche observations made on the 6th and 7th. Professionals reported numerous avalanches in places that they hadn't previously observed avalanches and some paths avalanched multiple times in a 24 hour period. Observers reported a few very large (size D2.5-3) avalanches, originating at upper elevations with deeper crowns that likely formed from wind drifting. Topping off an active couple of days, warming temperatures lead to a widespread loose wet avalanche cycle.
The southern Washington Cascades, the Wentachee Mountains and Mt. Hood either saw less precipitation, warmer temperatures leading to more rain than snow, or some combination of the two and ended up with relatively less active avalanche conditions than areas further north.Â
A large natural avalanche on Rock Mountain near Berne along Hwy 2 east of Stevens Pass that released Jan 6th or 7th. Photo: Josh Hirshberg 1/7/20
Many small storm slabs released in the Crystal backcountry 1/6-1/7. Pinwheels in the photo suggest loose wet avalanche activity occurred when temperatures rose above freezing and snow turned to rain.
Another active and colder weather pattern is on it’s way. Enjoy yourself out there and be sure to check the forecast before heading out. Remember, NWAC is a community-supported avalanche center and when you submit an observation you make the forecast better!
-Peter Moore
It’s getting deeper! Photo: Jeremy Allyn
Avalanche Problems
Storm Slabs
The snow structure is ripe for slab development on Friday with the necessary weather ingredients arriving during the day to create a dangerous slab structure. We expect small slabs in the morning as winds begin stiffening snow surfaces and snow begins to accumulate. But as the day wears on these slabs will become larger, more widespread, and more reactive. Look for signs of denser over less dense layers, such as cracking snow surfaces, easy hand shears, or collapsing under you to indicate instability in your area. Know that wind-loaded areas will be even more reactive. Because we expect conditions to deteriorate, keep good and safe exit options in your tour plan, such as ridges or low-angle terrain. And if conditions get worse as we expect, you won’t get caught on wind-loaded, steep, or unsupported slopes where we expect the most widespread avalanche concerns.
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: Likely
Expected Size: 1 - 1