You can trigger a large wind slab due to continued transport of fresh snowfall with moderate, gusty winds. Wind slab hazard will increase as you ascend. Before crossing steeper slopes at lower elevation, check for a layer of snow near the surface that wants to release as a cohesive unit.
Wind slabs were not particularly reactive on Sunday due to shallow new snowfall and the thin ice layer formed on Saturday. However, winds continue to transport lower-density fresh snow. Wind slabs should be growing and more reactive than Sunday near and above treeline.
The problem can be avoided by sticking to wind-sheltered locations or choosing wind-scoured aspects and ridgelines.
A subtle snow density change in the shallow snow that accumulated Sunday will create a layer for storm slabs to run on. Very shallow 1-2” storm layers were reactive on Sunday at Mt. Hood Meadows. If the depth of the new snow is greater than 8”, use small slope tests and hand shears to look for a layer that wants to release as a cohesive slab. If present, avoid unsupported slopes and slopes steeper than 35 degrees.
Snowpack and Avalanche Discussion
Mt. Hood Pro Patrol reported that a freezing fog layer formed Saturday produced a breakable crust into the above-treeline terrain, which mitigated avalanche hazard on Sunday. The wind event Saturday night had blown snow out of start zones.
Recent professional reports indicate the bottom 50” of the snowpack to comprised of well-bonded dense snow and crust layers. The most recent crust was buried January 4th and should extend to the top of the near treeline terrain. New snow of varying depth sits on top of the crust.
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.
