Avalanche Forecast
Regions: Snoqualmie Pass.
Snow and wind will drive heightened avalanche danger at the highest slopes. Watch for slopes steeper than 35 degrees with more than 6 inches of new snow or fresh drifts. Rain fell up to treeline.
Discussion
Snow and Avalanche Discussion:
With precipitation falling as rain up to treeline, the avalanche danger is minimal on all but the highest slopes. The low to mid elevation snowpack is well-adjusted to rain. A variety of old snow surface may still be found at upper elevations from weak facets on shaded slopes to very firm, slick crusts on sunny slopes. There's some uncertainty to how much snow will accumulate above treeline. Watch for slipping on firm crusts especially in terrain where a fall could have consequences or where stopping a fall may be difficult.
Check out the Regional Synopsis tab for more details.
A stout crust form near Snow Lake Divide. Recent snow has fallen on slick old surfaces at upper elevations. Photo: Chip Daly
Snowpack Discussion
January, 31, 2019
As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week with minimal new wet avalanche activity reported. This break in the weather allowed for avalanche danger to steadily decline in all regions. With stormy weather, the danger is once again elevated.
Weâve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it likely will be landing on slick surfaces. It's time to pay attention to the new old interface formed by our most recent storm.
North-South:
While a high elevation rain event, around January 23rd, formed surface crust in many regions, itâs the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.
North: On shady slopes, things havenât exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes havenât been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low to mid elevations, rain may have melted any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at the interface buried around February 1st.
South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them.
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Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara
East-West:Itâs not uncommon for our east-side forecast zones to experience lingering persistent weak layers (PWLâs). This season, weâve also seen several different PWLâs in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.
West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout  crust, and strong frozen melt-forms make up the majority of the snowpack at this time.
East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers weâve got our eyes on.
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January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .
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December 26th surface hoar. This layer can be found from 16â to 40â down and is still producing clean, planar shears with tests.
You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been âdormantâ or unreactive during the week of high pressure. The latest storm has not been enough to re-activate theses weak layers. Weâll keep tracking them to watch their progression..
The lower eastern slopes and the Columbia River. Snow exists at low elevations, but snowpack depths are shallow. Photo: Matt Primomo
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Avalanche Problems
Storm Slabs
The latest round of snow and wind will be just enough to raise the avalanche danger above treeline. Use caution in areas where you find more than 6 inches of freshly fallen snow. New snow may not stick well to the old snow surface. Watch for shallow drifts on leeward sides of ridges. While any avalanches will be small, they may slide easily on the old snow surface. Check the bond at the new/old snow interface. Use small test slopes and quick tests, like a shovel tilt test, to check how the new snow is bonding to old surfaces.
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.
Likelihood: Possible
Expected Size: 1 - 1