Avalanche Forecast
Regions: Cascades - East.
Very dangerous avalanche conditions are lingering on the heels of a warm, wet, and windy storm. The most dangerous areas will be at upper elevations in the western half of the zone and near the Cascade Crest. You will be able to trigger large avalanches where significant storm snow has accumulated above 4,500ft and where the wind has drifted thick slabs near and above treeline.
Discussion
Morning Update 7:00
Expect the most dangerous conditions near to the Cascade Crest in places near Holden Village and the Icicle and Teanaway drainages. Less precipitation will fall near the Highway 97 corridor and in the eastern half of the zone. Even in those locations, you may find some dangerous conditions and similar avalanche problems as listed below. Cold temperatures in the first days of the month created a thin layer of weak, sugary (faceted) snow. You can find the facets resting on a crust about 1-3 feet below the snow surface in the western half of the zone.
Snowpack Discussion
February 6, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
If we could choose one word to sum up the last week, and possibly 2020 in general, that word would be fluid. As of February 6, we are in the midst of yet another atmospheric river event, bringing large amounts of low elevation rain and high elevation snow to our region. The WSDOT-SCR avalanche team relayed that this past January has been the wettest on record at Snoqualmie Pass since precipitation records started in the 80s, and it’s their second wettest month on record. With such potent storms, it’s no surprise that our snowpack and avalanche danger has also been fluid, with fluctuations between high and low danger over short time periods.Â
February began on the tail end of our largest rain and wind event of the season, with many NWAC weather stations recording wind gusts of over 100 mph and impressive water numbers as high as almost 10 inches in a 48 hour timeframe.Â
Maximum wind gusts from weather stations in the NWAC Network Friday 1/31 into Saturday 2/1
This event had freezing levels surge to around 8000 feet across the region, introducing rain to a cold dry snowpack at upper elevations for the first time this season. This included rain above 7000 feet in the Washington Pass area, which had largely escaped the wrath of warmer storms up to this point. Dangerous avalanche conditions during this period produced widespread wet avalanches in most zones as large as D3. Along with Loose Wet and Wet Slab avalanches, Cornices failed naturally, and Glide avalanches were observed in multiple zones. Â
Natural glide avalanche (D2) that occurred on a SE aspect at 4400ft on a convex rock slab between Schriebers Meadow and the Railroad Grade. 02/02/20 Photo: Andrew Kiefer
As the rain finally subsided, cold air flooded our region, allowing the wet snow surface to begin to freeze from the top down. This sharp cooldown created what we are calling the 2/1 crust and allowed for Low danger across the region for a few days. The 2/1 crust varies in thickness but should exist in all areas of the region below ~8000 feet and may be considered as our “new groundâ€. During this period of low danger, most zones picked up modest amounts of low-density snow to overlay the 2/1 crust and even had a bit of sunshine.
Unfortunately, the period of low danger was short-lived, when two days later, another atmospheric river was on our doorstep. This time, the event started off cooler and had lower projected freezing levels. Many zones in the region were able to pick up a foot or more of new snow in all elevation bands before a switch to rain, with upper elevations receiving multiple feet of snow, creating what we are referring to as the 2/4 interface. The setup of heavy fresh snow over the low-density snow sitting on the 2/1 crust was a great recipe for avalanches on its own. Throw in the switch to rain at low and mid-elevations along with high winds and precip rates, and very dangerous avalanche conditions developed across the map on Wednesday the 5th.
This is where we find ourselves on Thursday, in the middle of a notable avalanche cycle producing wet snow avalanches below the rain line with dry snow avalanches above. The way this winter has been going so far, it’s a good guess that things will continue to be fluid going forward, let’s just hope it’s a colder type of fluid.
Loose Wet avalanche activity on Snoqualmie Pass. 02/05/20. Photo: Andy Harrington
Avalanche Problems
Storm Slabs
Above 4,000ft you may find enough recent drier snow to trigger storm slab avalanches in wind-sheltered terrain. Use caution on slopes over 35 degrees, especially on unsupported slopes and convex roll-overs, and especially where the wind has drifted fresh slabs. Put an extra margin between where you travel and avalanche terrain. Avoid traveling where you might be connected to nearby avalanche terrain, such as overhead start zones or adjacent steeper slopes. You may be able to trigger slopes from below. The most likely places for natural and triggered slab avalanches will be leeward sides of ridgelines where the wind drifted recent snow.
Feshly built slabs in the new snow may be easy to trigger as they're resting on weak snow over a crust that was buried on February 4th. We know the slabs and the weak layer exist, but there's still a lot of uncertainty about how reactive this layer could be. Dig profiles and use snowpack tests to look for signs of instability in the upper snowpack.
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: Very Likely
Expected Size: 1 - 1
Loose Wet
Rain fell to about 5,000ft, setting off a round of wet avalanches at lower elevations. These avalanches are less likely than on Thursday, but you may still be able to trigger them at low elevations. Watch for point releases and fan-shaped avalanches in the surface layers. These avalanches can be forcefull and can push you into dangerous terrain. Avoid steep, unsupported slopes and terrain traps like gully bottoms and avalanche path runout zones.
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: Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1