Avalanche Forecast
Regions: Cascades - East.
Impressive storm totals and strong wind have created cohesive slabs on upper elevation slopes. Large avalanches remain possible, especially in the western half of the zone where much more snow has fallen. Expect small loose slides on slopes that face the sun by mid day.
Discussion
Sunday will be the first day of full sunshine for the western half of the zone since February 4, when a 4 day storm began. Heavy snow and rain fell continuously for 4 days, amounting to 3ft to 5ft of snow. All of this new snow now sits above a layer of weak snow over a crust from February 1. On Stevens and Snoqualmie Passes, recent large and very large avalanches have slid down to this crust, with crowns from 4ft to 9ft deep. While we don't have evidence of this layer being as problematic as it has been at the Cascade passes, we have very little information to work off of. The nature of the thick slab and uncertainty regarding the weak layer warrant careful evaluation before venturing onto steep slopes on Sunday. The sun is bound to shine, and poses as a wildcard for how this hefty new slab may react.
On Saturday, an observer in Icicle Creek found a cycle of small (D1.5) slab avalanches about a foot deep on north and east aspects above 6,200ft. Observers have found major differences in snow totals by elevation. In Icicle Creek, settled snow amount since February 1st ranges from 1.5 feet at 4500ft to over 3 feet at 6,800ft. Where tested, February 1 crust has not been overly reactive in this area. Tests did show one sudden result on an east aspect at 6,800ft in the Bob Creek drainage. Below 5,200ft the snowpack is wet and saturated underneath the recent storm snow.
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
Wind Slabs
At upper elevations, recent strong westerly winds have created cohesive slabs within the new snow. Look for visual clues to wind drifting such as surface texture and recent cornice development. Identify potentially problematic areas on steep slopes, and avoid them.
In the south and west part of the zone such as the Salmon la Sac and Teanaway, deep storm snow overlying a crust from early February may still create large avalanches. Slopes may not be easy to trigger, but if you hit the right spot, or with a big enough trigger, could initiate deep and surprising slab avalanches. Use caution especially on unsupported slopes and convex roll-overs. Look for signs of instability such as recent avalanches and shooting cracks. Dig profiles and perform tests to help evaluate how all the new snow is bonding within itself and on the early February crust.
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: Possible
Expected Size: 1 - 1
Loose Wet
The sun will quickly begin to change snow surfaces on southeast to south, to southwest aspects as the day progresses. Plan your day to avoid slopes that face the sun by late morning. 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 are more likely in the western half of the zone, where more new snow has accumulated.
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: South East, South, South West.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1