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Archived

Avalanche Forecast

Feb 22nd, 2020–Feb 23rd, 2020
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Snoqualmie Pass.

Sunday sounds stormy around Snoqualmie Pass, with plenty of heavy snow and strong winds to dramatically increase the avalanche danger throughout the day. Slopes that seem safe in the morning could be quite dangerous in the afternoon. Give yourself plenty of options to easily navigate around any open slopes greater than 30 degrees where avalanches could start.

Discussion

Expect stormy conditions Sunday at the Pass. This should drive a pronounced increase in the avalanche danger during the day with the greatest hazard in the late afternoon. Choose route plans with plenty of options to adjust to the changing conditions and easily navigate around avalanche terrain. 

There’s a bit of uncertainty with this avalanche forecast due to the snow surface conditions prior to the storm. Over the past week, surface hoar, facets, and breakable curst were reported around the Snoqualmie zone. On Saturday mild and drizzly conditions seem to breakdown these weak surfaces in locations around Alpental Valley. This should help the new snow bonding to the old surface, particularly at lower elevations. However, some uncertainty still remains at elevations above 5000’ and in locations towards the eastern edge of the zone such as Keechelus Ridge and Stampede Pass. If avalanches fail on these weak snow grains, they could act in surprising ways such as triggering slopes above your head, failing well below ridgecrest, and remotely triggering. If you experience shooting cracks, feel collapses, or see avalanches occur, dial back your terrain use and steer away from any location avalanches may start, run, or stop. 

Surface hoar at Snoqualmie Pass from earlier in the week. Photo: John Stimberis

Snowpack Discussion

February 20, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

Intermittent Storms

January’s non-stop pattern of storms extended into the first week of February. Since then, we’ve transitioned to more intermittent storms with notable stretches of high pressure and dry weather. We now see more variability through the region in the snowpack layering and avalanche conditions than in January. 

Significant periods of calm weather in February have allowed avalanche danger to decrease between storms. During the clear periods, strengthening late winter sun and gradual warming has brought some loose wet avalanche cycles to sunny slopes. Even so, from February 1st-20th there were 10 days when one or more zones were forecasted at all Low danger. In comparison, the month of January had zero days with any zone forecasted at all Low avalanche danger. 

Surface hoar near Snoqualmie Pass. February 19th. Photo: Ely Gerbin

Looking Forward

As we look forward, there are a number of considerations that are pertinent to most zones. The pattern of storms separated by clear periods may form new weak layers and interfaces to monitor. Many zones hold variable surfaces that warrant consideration as a travel hazard. In the Mt Hood Meadows area, two fatalities were related to falls on slick surfaces over President’s Day weekend. Another important consideration is the cornice growth that occurred in the past month from predominantly west winds. Very large cornices loom on ridges in most zones, except for possibly the Olympic Mountains. Future warming could be the added ingredient needed for cornices to fall. A close call with a cornice-triggered avalanche near White Pass on February 12th is a reminder of the potential hazard that cornices can pose. 

 

A party of three triggered this avalanche from below on a southeast aspect at 5400ft near Skyline Lake, Stevens Pass on February 16th. Two members of the party were fully buried and one was partially buried. The party self-rescued and no injuries were sustained. Photo: Dustin Riggs 

Weak Layers

So far this season, we’ve had limited persistent weak layers to deal with. February’s periods of calm weather developed a couple of weak layers and problematic interfaces that later got buried in the snowpack. On a whole, weak layers have been short-lived and have generally resolved within a couple of days of being buried. While the following weak layers are no longer major concerns, some readers may want a more in-depth understanding of them before venturing into the mountains.

In the Stevens Pass and East Central zones, the February 4th interface consisted of a thin layer of small (0.5-1.0mm) facets sitting on the stout February 1st melt forms (crust). The crust was widespread on all aspects up to at least 7500ft and the facets were most prominent on the northern half of the compass (west through north through east aspects). A major storm and avalanche cycle February 4th-7th totaled over 60 inches of snow. The February 4th interface was responsible for widespread avalanching including slides up to size D3 at most elevations. As direct action avalanches subsided, the February 4th interface was deeply buried and didn’t produce further triggered avalanches. Interestingly, as recent as February 18th, this interface was found in a profile and produced some mixed and head-scratching test results. While this interface is not a concern for triggered avalanches, you may still be able to find it 4-6 feet below the surface, especially on sheltered, shaded slopes at upper elevations.

 

A profile from 6460ft on a northeast aspect in the Chiwaukum Range on February 18th shows the February 4th layer. Small column tests produced sudden planar results while a Propagation Saw Test did not indicate propagation.

A more elusive and problematic layer for triggered avalanches was the February 13th interface. This was also most prominent in the Stevens Pass and East Central zones in addition areas of the West Central and Snoqualmie Pass zones. At Stevens Pass a combination of large (2.0-4.0mm) surface hoar and small (0.5mm) near-surface facets developed on a variety of surfaces and were buried on the 13th. The weak layer was most problematic on east through south aspects between 4,500-6,000ft, where the grains were preserved as they rested on a thin crust. It seemed that outside of this aspect and elevation range either 1) the weak grains did not persist enough to be triggered by travelers or 2) the underlying layer, or bed surface, wasn’t hard enough for the weak layer to be reactive. A number of parties reported natural and triggered avalanches on southeast aspects during the storm cycle on Presidents Day weekend. This included a party of three that were uninjured after all being caught, with two full-burials, in a surprising avalanche near Skyline Lake on the 17th. As of February 20th, the interface is 1-3 feet below the surface and has become difficult to trigger. The February 13th interface is still visible in the snowpack and we continue to monitor it. It will likely become inactive before the end of the month.

In summary, February has been a great time to explore the mountains and enjoy the longer days for recreating in this season’s robust snowpack. Though, not without complexity and variability. Continue to monitor changing conditions and check the daily avalanche forecasts for the most up to date information. 

Avalanche Problems

Wind Slabs

The storm seems quite windy for Snoqualmie, with moderate to strong winds accelerating through the Pass and affecting all elevations. Wind slabs could develop on any exposed feature and along the sides of gullies, even well below treeline. Expect slabs to increase in size as you ascend in elevation and as the day wears on. Blowing snow may be the easiest way to see where the winds are loading snow. Other clues like drifts behind objects and cornices can also point out wind loaded slopes. Steer around any slope greater than 30 degrees when you see signs of wind affected snow.

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: All elevations.

Likelihood: Likely

Expected Size: 1 - 1

Storm Slabs

As the storm continues throughout the day, and more snow piles-up, a storm slab issue could develop in wind sheltered areas. You’ll need to monitor the accumulating snow during the day. Pay attention when you see more than 6” of new snow, small slope test produce avalanches, or you see cracking in the storm snow. If this snow falls on intact surface hoar and/or facets, remember you could trigger these avalanches in areas much lower on the slope. You can avoid storm slabs by steering around any open slope greater than 30 degrees.

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.

Elevations: Treeline, Below Treeline.

Likelihood: Likely

Expected Size: 1 - 1