Avalanche Forecast
Regions: Cascades - South West.
UPDATED: Above treeline danger and avalanche problems to reflect the more significant snow accumulation and overnight winds.
Keep an eye out for areas where the wind has drifted the new snow into thicker and firmer slabs. It’s these steep wind affected slopes where you may trigger an avalanche. You are most likely to find wind transported snow just below ridgelines and on rocky, exposed features. If you see signs of wind drifted snow, you can stay safe by avoiding nearby slopes greater than 35 degrees.
Discussion
Avalanche and Snowpack Discussion
We have removed the persistent slab from the avalanches forecast. The lack of avalanches 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 most suspect of this layer in higher elevation and 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. If you see this stay off of nearby startzones.
Snowpack Discussion
January 6, 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
Snow totals from the West-South zone vary by location and elevation. In general most locations picked up 6-8 inches of new snow as of Sunday afternoon. In many areas the wind affected the surface snow forming slightly thicker and firmer slabs. It’s these wind affected areas, that you want to pay close attention to Monday. Look for uneven snow surfaces, snow drifts, or fresh cornices. Sometimes you can feel the wind slabs under you. The snow may feel firm or hollow as you travel. All of these clues will let you know you could trigger a wind slab on nearby steep slopes.
Near the volcanoes, more snow may fall Sunday night. The weather forecast models are struggling with exactly how much new snow will accumulate. If you find more than a foot of recent snow, it's time to stop and reevaluate. You may also be dealing with unstable storm slabs. Small slope test that crack and propagate can help you determine if there are storm slabs where you are traveling.
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