Avalanche Forecast
Regions: Stevens Pass.
You can trigger avalanches in the recently fallen snow. Conditions are most dangerous at upper elevations where more snow fell and westerly winds have formed drifts. With cold temperatures and significant snow accumulations in the past 4 days, the snowpack needs time to adjust to the heavy load of new snow.
Discussion
At Stevens Pass, the Brooks weather station has recorded 92" of accumulated snow in the past 7 days and 48" since January 10th. Heavy snowfall over the weekend brought reactive storm slab conditions with rounds of natural and triggered avalanches to size D2. Avalanches occurred in all elevation bands, with a majority running in the top 1ft and involving different storm interfaces each day. Over the weekend evidence of unstable snow was easy to find, but it may be less obvious now. Snowpack tests have pointed at a lingering instability at the old/new snow interface, buried on the 10th and now 2.5 feet below the surface- a layer to monitor moving forward.Â
With all the new snow, travel conditions are deep and challenging. Remember that even away from avalanche slopes, hazards exist like tree well falls and snow immersion suffocation. Make sure you’re informed and travel with a partner (www.deepsnowsafety.org). Expect the coldest temperatures of the season to date.
Snowpack Discussion
January 12th update: In the past week, there have been two fatal avalanche accidents to the east of NWAC's forecast area. One occurred near Kellog, ID and another outside of Baker City, OR. Local avalanche centers will perform accident investigations including final reports. You can find preliminary accident information at avalanche.org.
January 9th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
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.
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
It will be easiest to trigger avalanches at upper elevations. Use caution on all slopes steeper than 35 degrees. Use extra caution on leeward, wind-loaded slopes where you see drifted pillows of snow and thick wind features. Avoid aspects that have been obviously loaded by westerly winds from the past couple days and steer around wind pillows. Use small test slopes to determine if the snow can slide. Watch for clear signs of unstable snow like recent avalanches, shooting cracks, and collapses. Loose snow avalanches could occur on steep, wind-sheltered slopes.
Cold temperatures are preventing recent storm layers from bonding as quickly as they normally would. Continue to give the new snow time to stabilize before traveling through large steep avalanche terrain. Avoid terrain traps like gullies where even a small avalanche could stack up debris and become dangerous. Seek out low-angle, supported terrain, and smaller pieces of terrain.
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