The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable.
Known problems include Wind Slabs and Persistent Slabs.
The Bottom Line: A blustery storm is creating dangerous avalanche conditions and could reawaken old buried weak layers. You can trigger large avalanches that could break above and around you near and above treeline on thickly-pillowed, wind loaded slopes. If you see fresh avalanches or experience collapses or cracks shooting through the snow, avoid all slopes 35 degrees and steeper.
Summary
Weather Forecast
Wed 9th Jan 14:43
- Robert Hahn
Weather Synopsis for Wednesday night through Friday
A rainband associated with a low tracking northward offshore will bring rain at generally elevated snow levels west of the Cascade Crest. Cold air east of the Cascade Crest will gradually erode from south to north, but may still provide a messy mix of rain, sleet and snow east from the lower Cascade passes eastward in the evening and overnight hours. Mt. Baker continues to be favored by the southerly flow pattern (2" of water overnight), but other areas will start receiving moderate precipitation overnight.
The passage of the rainband is likely to decrease easterly flow enough to change both Stevens and Snoqualmie Passes to rain by Thursday early morning and scour out the and scour out the cold air pool east of the Cascade Crest as the rainband doesn't provide much cooling aloft. Precipitation type has been very difficult to forecast during this event.
A weak warm front will brush the region on Thursday, bringing generally light rain and snow on a southerly flow.
Thursday night and Friday should see some clearing as most of the weather action stays just offshore to our west with a ridge building over eastern Washington, keeping us dry.
Regional Synopsis
Mon 7th Jan 09:00
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.
Problems
Wind Slabs
While you can trigger wind slab avalanches at all elevations, they 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 on the leeward (down wind) side of ridges. The combination of cold temperatures, light snow, and gusty wind has drifted snow very efficiently and into unique areas. Expect wind drifts in open areas below treeline, and cross-loading on slopes well below ridges. Take note of how the distribution of wind slabs will change as you move through terrain. Wind slab avalanches could step down into buried weak layers and result in a very dangerous avalanche.
Watch for signs that you can trigger a wind slab avalanche, such as; Actively 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 being caught in 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
Stop and re-group in safer areas, well out from under and away from wind-loaded avalanche terrain.
Aspects:
All aspects.
Elevations:
All elevations.
Likelihood
Likely
Expected Size
2 - 2
Persistent Slabs
The current snow and wind-loading is adding stress to weak layers of snow well below the surface. This storm may be enough to re-awaken these old buried weak layers. If you see obvious signs of avalanche danger, like recent avalanches, collapsing, or cracks breaking through the snow, avoid all slopes 35 degrees and steeper. Persistent slab avalanches are dangerous and difficult to predict. In 2017/18 these deadly avalanches were the cause of every avalanche fatality that occurred in the Cascades.
There are two weak layers to watch for:
1) A layer of surface hoar that formed around Christmas is buried 1 to 2 feet below the surface. This layer is mainly a concern above 6,000ft. Around the New Year, observers reported avalanches on this layer on Northerly through Southeasterly aspects. These avalanches were surprising in how widely they broke across the slopes.
2) Weak facets near the ground are a concern in the east end of the zone. Observer at Mission Ridge and above Mazama have reported this snowpack structure. If you trigger an avalanche on this layer it could be over 4 feet deep. Use caution where stiff slabs sit over weak snowpack less than 5 feet deep and on steep, unsupported slopes on the eastern edge of the range.
