Expect small loose wet slides to run on steep sunny slopes as the day progresses. Winds have continued to drift snow in upper elevation terrain forming large, unsupported cornices and isolated wind slab. Use visual clues to help identify these hazards, and avoid going onto, or underneath them on steep slopes.
At upper elevations recent winds have been moving snow, creating cohesive slabs. Look for visual clues to wind drifting such as rippled surface textures. Use caution especially on very steep terrain, unsupported slopes and convex roll-overs. You can test how these may react on small test slopes, or simply identify potentially problematic areas on steep slopes, and avoid them.
Warmer temperatures and more sun on Monday will create loose wet instability on slopes that face the sun. Plan your day to avoid southeast facing slopes by mid morning, south by noon, and southwest by early afternoon. Look for signs of warming such as rollerballs, gloppy snow, and snow falling off trees. If you find heavy, wet snow surfaces, you should avoid steep slopes on those aspects and/or elevations. Loose wet slides may be larger on south and west facing slopes, where cloud cover may have inhibited them to occur on Sunday.
Cornices have grown to be larger than two story houses on many ridgelines, are overhanging, and pose a definite threat on their own. Be mindful of overhead hazard as these may fail in the warm sunshine on Monday. Plan your route to avoid going underneath leeward ridges where these have formed.
Many folks got out and enjoyed a beautiful day on Sunday. By mid morning, small loose wet avalanches were occurring naturally on the southeast aspects of Rock Mountain. Winds continued to drift snow in alpine terrain, and small wind slabs were observed in isolated areas. A couple of recent observations note large, overhanging cornices with recent releases on leeward slopes. Expect more of these types of problems on Monday.
Observers targeting the interface of the 3ft deep storm snow with the 2/1 melt freeze crust on a NW aspect at 5,400ft on Arrowhead found generally favorable bonds, and only small rounded grains at the interface. This layer has gained strength over the course of the past few days, and it does not appear to be a problem any more.
Recent, deeper crowns in the Berne Camp Chutes with Glacier Peak in the background. February 9, 2020, Matt Primomo photo.
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.
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.
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.
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 ground”. 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
