Avalanche Forecast
Regions: Stevens Pass.
Light, dry snow and gusty winds have increased avalanche danger at all elevations. The most dangerous slopes are near and above treeline where you can trigger avalanches that could break above and around you. You can avoid triggering these avalanches by staying off thickly-pillowed, wind loaded terrain and steering well-around wind sculpted features on slopes 35 degrees and steeper.
Discussion
Snow and Avalanche Discussion
Light, dry snow snow fell Sunday night into Monday with gusty westerly winds. On Monday at Stevens Pass, observers reported small, skier triggered avalanches on easterly aspects near treeline. I measured up to 12.5†inches of new snow in the past 24 hours and saw blowing snow and significant wind drifts forming in exposed areas and leeward slopes. Expect light snow and more wind to maintain similar conditions for Tuesday.
Snowpack Discussion
January 7, 2019
This latest round of storms in early January continues to highlight some of this season’s region-wide themes in our snowpack. I’d describe them as Bottom to Top, North to South, and East to West. Let’s take a look at each of these.
Bottom to Top:
In general this winter has been on the warm side in the Cascades. No where is that more evident than in the lack of low elevation snow. Heavy rain events continue to wash-away and thoroughly wet the lower elevation snowpack. In most regions you will see substantial increase is the height of snow from low elevations (3000 ft), to mid elevations (4500 ft), and again at upper elevations  (5500/6000 ft). Above the typical rain lines (5000-6000 ft) a deep and cold mid-winter snowpack exists.
Jan 5, 2019: The dark timber in the Coast Range highlights the thin snowpack at lower elevations and the deeper winter-like snowpacks near and above treeline: Photo: Simon Trautman
North to South:
With few exceptions, this season’s storm tracks have favored the North Cascades. Sunday, Mt Baker’s Heather Meadows weather station passed 100 inches of snow on the ground. A quick look around the state’s weather stations demonstrates just how deep this snowpack is. Most other locations are reporting between 50 and 75 inches. The massive amounts of snow for the northern forecast zones are reflected in several impressive avalanche cycles spread out over the last month.
NWAC weather station graph of the height of snow on the ground. Mt Baker passed the 100 inch mark Sunday.
East to West:
Several stacked persistent weak layers have been buried in the eastern forecast zones. This isn’t unusual, but it is noteworthy. The thickness of the slabs over these weak layers can vary greatly. Closer to the Cascade Crest, the deep snowpack may more closely resemble Stevens or Snoqualmie Passes. Further east, shallower snowpacks, and significant variability are keeping snowpack assessment tricky.
Canadian sized surface hoar found buried by 18†of snow in Icicle Creek above Leavenworth on December 31st: Photo by Matt Primomo.
So what now? Well, on January 3 and 4 a high elevation rain event impacted areas from Mt Baker to Mt Hood. In many locations this has created a firm and thickening crust. In these locations, this new crust is limiting the impacts of any deeper weak layers. In locations that didn’t receive as much rain like WA Pass, Mission Ridge, Crystal Mt, and White Pass, how the small amount of rain impacted the mid and lower snowpacks remain to be seen.
Rain and rime crust at 6,500ft above Gallagher Head Lake in the East Central Zone, Jan 5th. This layer was 3 inches thick in this location. Matt Primomo photo.
One thing is for sure, we’re not even a month into the winter season, there’s a lot more winter to come.
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Avalanche Problems
Wind Slabs
You can trigger wind slab avalanches that could be big enough to bury or kill you near and above treeline. You’re most likely to trigger avalanches on slopes that are steeper than 35 degrees and situated on the leeward (down wind) side of ridges. The combination of light snow and gusty wind has drifted snow in unique way. Take note of how the distribution of wind slabs changes as you move through the mountains. Here are indicators that you can trigger a wind slab avalanche on a given slope:
-Active wind transport or blowing snow
-Cornices hanging over a slope at the top of a ridge
-Wind sculpted features or thick lens-like pillows
-Especially smooth snow cover and deeper snow compared to the opposite side of the ridge
The best way to manage the risk of triggering a wind slab avalanches is to:
-Avoid leeward, wind-loaded start zones and cross-loaded features low on slopes
-Steer around wind features as well as convexities, where slopes roll steeper
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: Likely
Expected Size: 1 - 1
Loose Dry
Loose dry avalanches are relatively easy to predict and avoid. If you don't pay attention to them, they could push you into hazardous terrain. You can trigger point releases on slopes 40 degrees and steeper. This is exactly the kind of terrain where an avalanche could push you off a cliff or into trees or rocks. These terrain traps can heighten the consequences of being caught in a small loose dry avalanche.
Up to 3 interfaces can be found within the snow that fell in the past week. The top 1-2 feet of dry snow is sitting on a firm crust that formed on January 3rd. Slope tests on small, inconsequential slopes or snowpack test like the "shovel tilt test" can help you determine whether you can trigger a loose dry avalanche.
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Elevations: Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1