Cascades - North East Avalanche Forecast
Jan 15th, 2020 10:00AM
A new blanket of snow, along with wind may create unstable conditions on Thursday. Look for slabs on open slopes, and expect them to be larger and more dangerous the higher you go. Avoid trusting slopes steeper than 35 degrees that have thick drifts.
A cold, snowy regime continues. With the very cold temperatures, the precipitation may accumulate to a healthy blanket of new snow. On Wednesday an observer triggered a small slab on a test slope at 6,500ft with a crown approximately 20" deep. He found a number of larger slides with crowns ranging from 3 to 6ft on northeast and east aspects of Big Kangaroo. On Tuesday, observers reported drifts and wind slabbing in the past 48 hours at upper elevations. On Monday, an observer near Vasiliki Ridge reported 2 feet of recent storm snow since January 10th and tests showing lingering instability on the Dec 27th surface hoar, buried over 3 feet below the surface. On Saturday, observers reported triggering small avalanches in recent storm snow above 6,000ft on southerly aspects of Delancy Ridge.
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.
Total Snow Depth (in) 1/2/20
Total Snow Depth (in) 1/8/20
Heather Meadows Mt Baker
Crystal Mt Green Valley
Paradise Mt Rainier
White Pass Upper
Mt Hood Meadows
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.
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!
It’s getting deeper! Photo: Jeremy Allyn
By the time morning rolls around the heavier snowfall and active wind loading may diminish, but firm slabs may be prime for human triggering. Southeast and east winds on Wednesday night may form these slabs in somewhat unusual places. In addition, there are previously formed slabs from west winds that may hide underneath this new snow. You are most likely to trigger a slide on steep slopes with recently formed drifts in open terrain that is exposed to easterly winds. Look for hollow, drum like snow as you transition onto exposed slopes. If you see cracking and feel slabby snow, avoid slopes steeper than 35 degrees.
Suspect storm slabs if you find more than 8" of new snow along with signs of instability such as cracking or recent soft avalanches. Use small inconsequential test slopes to check how the new snow is bonding. With the cold temperatures and very low density new snow, loose dry avalanches may be more of an issue than slabs. In any case, if snow moves downhill easily avoid steep, consequential terrain.
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.
Deep Persistent Slabs
It is becoming unlikely, though not impossible to trigger a deep slab near and above treeline. It has been about a week since the last slide is thought to have occurred on this deeper layer of buried surface hoar and/or weak snow near a crust. This layer was responsible for at least two large, remotely triggered slabs earlier in January. The recent observations of deep and wide crowns from Big Kangaroo point to this layer as well. Snowpack tests have continued to show sudden results in past days. Look for this layer about 5ft below the surface at 6,000ft near Washington pass, though it may be quite a bit deeper in wind deposited areas.
Give yourself plenty of distance from big steep slopes above 6,000ft. A small slab or cornice failure on the surface may step down to this deeper layer, creating a much more destructive avalanche.
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.
Valid until: Jan 16th, 2020 10:00AM