Avalanche Forecast
Regions: Snoqualmie Pass.
Choose terrain with plenty of options to easily avoid steep open slopes and adapt to changing conditions. Obvious signs of unstable snow, such as new avalanches, should trump any opinions you have about the snowpack and cause you to seek out lower angled terrain away from avalanche slopes.
Discussion
Poor visibility limited observations around the Snoqualmie zone Thursday. NWAC staff and the WA DOT reported several natural avalanches along I90. The exact character of these slides were difficult to determine, however debris piles up to D2.5 were observed. At lower elevations, numerous loose wet avalanches originated from very steep and rocky slopes.Â
Natural loose wet avalanches below treeline Thursday. Photo: Andy Harrington
Cooling temperatures and decreasing precipitation should allow the avalanche danger to trend down Friday. At the same time, new snow could build fresh slabs at upper elevations. Consequently, we have a high degree of uncertainty in the forecast. You could experience very different dangers throughout the day with lower elevations being more dangerous in the morning and upper elevations peaking in the afternoon.Â
Snowpack Discussion
January 23rd, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
January to Remember
A parade of storms since the New Year began brought relentless precipitation, and impressive snowpack growth throughout the region. Areas along the west slopes of the Cascades have rarely gone more than 24-48hrs without precipitation. In the past three weeks, the Volcanoes and Passes received 20-28in of water equivalent, translating into incredible snowfall totals - Mt. Baker Ski Area reported 240in of new snow since January 1. The majority of NWAC weather station sites are reporting snow depths well over 120% of normal for this time of year. Ongoing precipitation and fluctuating snow levels have maintained elevated avalanche danger throughout the month, with avalanche warnings issued for nearly all forecast zones on 4 separate days (January 6, 7, 12, 23). Â
Snow depth imagery for the Northwest Region on January 23, 2020. Many areas in the Cascades and Olympics show well over 100 inches of snow on the ground. Image courtesy of NOHRSC Regional Snow Analysis.Â
Location
January 1-23 Precipitation (Water Equivalent)
Hurricane Ridge
14.79â€
Mt Baker Ski Area
28.65â€
Harts Pass
11.2â€
Stevens Pass
20.26â€
Leavenworth
4.01â€
Snoqualmie Pass
25.43â€
Paradise, Mt Rainier
24.13â€
Mt Hood Meadows
22.34â€
Table 1: Precipitation totals for select weather stations January 1-23, 2020. Huge numbers at Baker, Rainier, Hood, and the Passes and there is still another week left this month.
MLK Weekend and a Pattern of Rain on Dry Snow
After a prolonged cold period with arctic air and lowland snow, MLK weekend brought unseasonably warm temperatures and a rapid thaw. High snow levels and rain caused wet avalanche activity throughout the region January 18-19. A few very large natural avalanches occurred at upper elevations where all precipitation fell as snow. A cooling trend followed, creating a pronounced crust that is now buried in almost all forecast zones. Cold, dry snow January 21-22 was followed by a rapid warm-up and the most significant rain on snow event yet, causing avalanche warnings on January 23 for 5 of our 10 forecast zones.
The crown of a very large avalanche around 10,000ft on the Newton Headwall of Mt Hood which likely occurred during the storm late last week. 01/19/20 Photo: Kevin Kayl
The buried MLK crust is widespread throughout the region and a prominent marker in the snowpack. Crystal Backcountry 01/20/20 Photo Jeremy Allyn
One More Week To Go
January isn’t over yet, and the long term forecast continues to show an active and wet weather pattern for the Northwest. The low-snow, drought-like conditions of the early season seem like a distant memory at this point. We’ve certainly made up for lost time in 2020, and this already impressive month looks to end with a bang.
Avalanche Problems
Loose Wet
This problem could be better described as “wet snow.” It’s hard to think about wet avalanches when you see snowflakes falling out of the sky, but it will take more than a few hours of cold to heal this unstable wet snow. Above 4500’ you could encounter lingering wet slab avalanches where 1.5-2.5’ of snow rest on the MLK crust layer. Below the rain-snow line, you could find an unconsolidated wet surface. Wet snow avalanches can be inherently difficult to assess and predict. Give wet snow problems a wide berth by steering around any open slope greater than 35 degrees.
We have not received reports of any glide avalanches from the Pass, however, large rain events like this often produce them. Be leery of traveling on or under any slope where you see crevasse like cracks in the snow.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
At higher elevations, new storm slabs should grow during the day Friday. If you travel to higher terrain, steer around steep open slopes when you encounter more than 6” of new snow or see areas where the wind drifted the snow into thicker slabs. We expect the new snow to bond well to the old surface as temperatures cool. However, the recent swings in the weather and uncertainty with the forecast should invoke caution as you travel.
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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1