Avalanche Forecast
Regions: Cascades - North East.
Expect dangerous conditions as Friday progresses. Low density snowfall and wind strong enough to transport it will create slab avalanche problems near and above treeline. Exercise cautious route finding to stay off of, and out from underneath large slopes near and above treeline, where very large avalanches are possible on a deeply buried weak layer.
Discussion
On Thursday, temperatures were cold, the sun poked out, and folks enjoyed a fine winters day before the next round of snowfall arrives. On Wednesday, observers found a fairly widespread cycle had occurred on Monday or Tuesday, with a few larger slides that buried Hwy 20 with several feet of debris. In the days leading up to January 6th, a string of instability took place, as folks remotely triggered large avalanches on buried weak layers 2 to 4ft down.Â
A deep, recent crown was observed on Wednesday, January 8th on a north facing slope in Cutthroat Drainage. Photo: Matt Primomo
Snowpack Discussion
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
Wind Slabs
At upper elevations and on open slopes near treelline, the wind may drift the new and previous snow into thick slabs, creating strong over weak layering. These may become touchy and more dangerous as one increases elevation and as the day progresses. Be observant, especially towards the latter half of the day, and look for shooting cracks as you travel. If you begin to see signs of instability, it is time to step it back and avoid slopes 35 degrees and steeper. Be aware that a small wind slab in the upper layers of the pack may step down to a more deeply buried weak layer.
With plenty of low density snowfall, expect loose dry avalanches to become easy to initiate as the day progresses. These may occur naturally, running down steep slopes during times of heavy snowfall. Be aware of where you are in the terrain. Avoid stopping underneath steep slopes, terrain traps, or places where getting caught or pushed may be consequential.
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
As a wise local recently put it: "The alpine is very much becoming a wilderness again." As the stormy pattern kicks back into gear, we have a growing amount of uncertainty in the snowpack at upper elevations. What we do know is that we are experiencing a period of prolonged instability near and above treeline due to a layer of buried surface hoar, and/or weak snow near a crust. This layer can be found about 4ft below the surface at 6,000ft but is likely quite a bit deeper at higher elevations and in wind deposited areas. It is becoming deep enough that you may not observe feedback as you travel such as collapses or shooting cracks. New snow and wind may increase the load on these layers on Friday.
Put a large buffer of terrain between you and any slope that is steep enough to slide. It may be possible to trigger a slab from far away or on adjacent slopes. Be diligent to not stray too close to roll-overs and changes in slope angle. A healthy amount of respect for steep slopes is still warranted.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 2