Avalanche Forecast
Regions: Cascades - South West.
Easterly winds will redistribute new and recent snow onto unusual aspects Tuesday. Larger wind slabs will build near and above treeline but expect to find wind-affected snow even on open slopes below treeline. If you see signs of wind drifted snow, you can stay safe by avoiding nearby slopes greater than 35 degrees. If new snow piles up quickly, you can also trigger storm slabs on very steep open slopes in wind sheltered terrain.
Discussion
Updated at 7 AM to increase avalanche hazard to Considerable Near Tree-line
Avalanche and Snowpack Discussion
We have removed the persistent slab problem from the avalanches forecast. The lack of recent avalanche combined with data from several snow profiles suggest that the layer of buried surface hoar has gained strength. This doesn’t mean you cannot find a location where it still exists. We are still concerned that this problematic layer remains at higher elevations and in more remote locations. If you are traveling above 6000 feet in areas near the Cascade Crest, it’s worth taking the time to stop and dig. Look for any snowpack test that fails suddenly and adjust your terrain selection by steering clear of paths capable of producing large avalanches.
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. Nowhere 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 a substantial increase in 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
Every season a few backcountry travelers are fooled when a period of east winds redistribute new and recent snow onto unusual aspects in the West Slopes South zone that covers Paradise, Crystal Mt. and White Pass. All of these areas will be subject to moderate east winds Tuesday so pay attention to local loading patterns and account for the extra uncertainty.
Unusual aspects or not, look for uneven snow surfaces, snow drifts, and fresh cornices to identify where fresh wind slabs may have formed. Feeling for firm or hollow sounding snow provides clues that you may trigger a wind slab avalanche on nearby steep slopes.
If storm snow accumulates more than 6 inches Tuesday, shallow storm slab avalanches are possible on very steep open slopes and convex rollovers found in non wind-affected terrain.
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
New snow is forecast to pile up quickly on Tuesday and if it does, shallow storm slab avalanches will become increasingly likely on very steep open slopes and convex rollovers in non wind-affected terrain. Observe how fast new snow accumulates and use small test slopes to see if new storm snow instabilities are developing. When in doubt, choose lower angled terrain.
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