The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low.
Known problems include Wind Slabs and Persistent Slabs.
The Bottom Line: If you travel near ridgelines or on exposed slopes near and above treeline, you will probably see areas where the wind has drifted the new snow into firm slabs. You are most likely to trigger an avalanche where these rest on steep slopes.
Summary
Weather Forecast
Mon 7th Jan 17:09
- Dennis D'Amico
Weather Synopsis for Monday night through Wednesday
5 pm Update: Corrected precipitation type in Weather Forecast for Tuesday.
A transitory upper level ridge will quickly pass through the region tonight, opening the door for mid and high level clouds to spread up from the south. A mature low pressure system well offshore will spin light warm frontal moisture over the area on Tuesday also from south to north during the daylight hours, with the highest snowfall amounts likely in the Mt. Hood area and to a lesser degree the Crystal Mt./Paradise area.
Offshore easterly flow will ramp up quickly overnight through Tuesday and E-SE winds should increase quickly near and west of the Cascade crest from Snoqualmie Pass and southward. The easterly flow will also mute the warming trend during the day for all areas. The associated weak cold front should move through Tuesday night, producing generally light precipitation.
A trailing longwave trough will begin to quick the whole mess through late Wednesday afternoon and Wednesday night. With each round of precipitation in southerly flow, the cold air trapped in and around the Cascades including Mt. Hood will erode with gradually rising snow levels and increased chances for mixed precipitation.
Regional Synopsis
Sun 6th Jan 09:00
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.
Problems
Wind Slabs
With colder temperatures, moderate winds, and another shot of snow, wind slabs will be a concern at upper elevations. In locations where wind has drifted the new snow into firmer and thicker slabs, you may be able to trigger an avalanche. The new snow may be poorly bonded to a firm, refrozen crust. Use small, inconsequential test slopes to check how these slabs are bonding. You are most likely to find wind slabs just below ridgelines, and on the sides of cross loaded gullies on open slopes. Look for wind driven snow, fresh cornices, and snow drifts to indicate wind slabs may be on nearby slopes. You can use wind stripped areas, ridgelines, and lower angled slopes to avoid triggering wind slabs as you travel.
Aspects:
All aspects.
Elevations:
Alpine, Treeline.
Likelihood
Possible
Expected Size
1 - 2
Persistent Slabs
Folks have been consistently finding a layer of buried surface hoar from Christmas. From Mission Ridge, to Icicle Creek, to Dirty Face, its out there. You are likely to find this layer from 1 to 2 feet down from the surface. This layer is mainly a concern above 6,000ft, where the absence of a stout melt freeze crust above it may be keeping it in play. Recorded activity on this layer has been on Northerly through Southeasterly aspects. These avalanches were surprising in how widely they broke across the slopes. Be suspect of steep, open slopes at the uppermost elevations. Be observant of shooting cracks or collapsing, or the absence of a thick rain crust near the surface. At lower elevations, where water has percolated through much of the upper snowpack and has formed a stout melt freeze crust, this layer is much less of a concern.
The further east you get, the less snow there is on the ground. Recent observations from Mission Ridge and above Mazama speak to a shallow snowpack with weak snow near the ground. This poor snowpack structure warrants respect, because if the layer near the ground failed, the avalanche could be well over 4 feet deep. Places to be cautious of in particular are where stiff slabs sit over weak snow, slopes where grass and rocks poke through with deep drifts nearby, and steep, unsupported slopes on the eastern edge of the range.
