Avalanche Forecast
Regions: Cascades - West.
Turn your attention to areas where the wind affected the snow and this includes fresh snow transport above treeline with moderate NW winds at these elevations Sunday night. Steer around very steep slopes and convex rollovers where you see drifted snow, wind textured surfaces, or fresh cornices. If the sun comes out, anticipate rocks and trees to shed snow and trigger small loose avalanches on very steep slopes.
Discussion
Numerous slab avalanches released on steeper terrain on Saturday in the adjacent West-North zone, with a full burial in backcountry terrain near Mt. Baker Ski Area described in a preliminary report.
The avalanche from Saturday’s burial near Mt Baker ski area. NNW aspect 5500’. Photo: Mt Baker Ski Patrol
Sunday was much quieter in adjacent zones such as the West-North zone with few reported avalanches. Some small to large loose wet avalanche activity was reported to the east of this zone on Rock Mountain east of Stevens Pass, but most slopes were stabilizing here too. Cool temperatures, a slight breeze, and thin clouds seemed to keep the snowpack cold and dry. This allowed for lingering storm layers to gain strength.Â
Below treeline, your greatest hazard may be difficult travel conditions due to stream blowouts and unexpected holes formed during the insane amounts of rain that fell at below 3000 ft this past week.
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
Keep your eye open for areas where the wind drifted the snow into slightly thicker and firmer slabs. When you find wind affected snow, steer around convex rollovers, steep slopes below cornices, and terrain above cliffs. In the adjacent West North zone, winds slabs seem to be failing on a crust formed Thursday or Friday and range from 10-15” deep in most areas. As you ascend in elevation slabs could grow thicker and produce larger avalanches.
On Sunday, NWAC staff triggered a small wind slab on a SW aspect at 6400’ in the adjacent West North zone. The avalanche was just over a foot deep and ran on a firm crust. Amazingly, numerous tracks covered the area and this was the only avalanche.
While the larger older slabs are slowly gaining strength, pay particular attention if you find evidence of fresh snow transport from moderate alpine winds Sunday night; there’s plenty of dry snow on northerly aspects available for transport and these smaller slabs may overly a crust on more southerly and easterly aspects where they could be most reactive.
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: Possible
Expected Size: 1 - 1
Loose Wet
For northern parts of the West-Central, we think the clouds kept the loose activity at bay on Sunday. On Monday, we’re expecting more sunshine and slightly warmer temperatures. You could see natural loose avalanches on steep slopes where snow falls from warming trees and rocks. If the sun comes out and/or you feel the warmth of the day, anticipates loose avalanches. New rollerballs and small fan-shaped debris can indicate loose wet avalanches are becoming more likely. Below treeline, we think this problem may be mostly played out after several days with mild temperatures and sunshine impacting these elevations.
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1