The Bottom Line: Another round of snow with warming temperatures and strong winds will create slab avalanches. The additional shot of snow will continue to stress multiple deeper weak layers in the snowpack in the East Cascades, which could create very large avalanches. Avoid avalanche terrain if you see recent avalanches, shooting cracks, or hear whumphs.
Snow and Avalanche Discussion
Widespread avalanches occurred over the weekend and into the week. On Saturday, Mission Ridge Ski Patrol triggered multiple avalanches up to 3 feet deep on northwest through northeast aspects between 6-6500ft. To the north, a skier was caught and carried in an avalanche in the Cutthroat drainage near Washington Pass. Observers noted remotely triggered avalanches near Mission Ridge and a natural avalanche at Blewett Pass. Observers continue to find instabilities and on a layer of buried surface hoar from January 17th, even at elevations as low as 4,600ft. Deeper persistent weak layers still exist within the snowpack, especially further east in the range. All over the East Cascades, these layers are being stressed to their breaking point. Will the added load overnight into Wednesday be the straw that breaks the camel's back?
Be sure to check out the observations page for a number of recent, excellent entries!
Wed 23rd Jan 14:29
- Dennis D'Amico
Weather Synopsis for Wednesday night through Friday
Post-frontal showers continue to wind down this afternoon and should taper off completely in the early evening. A strong upper level ridge will rebuild offshore Wednesday night and become the dominant weather feature for the next several days.
Some low-level moisture/clouds will likely be trapped along the west slopes of the Cascades Thursday under the ridge. While cloudier conditions are expected along central-west and northwest Cascades, afternoon cloud cover should decrease for areas further south including the south Washington Cascades and Mt. Hood. The east slopes of the Cascades should enjoy mostly sunny skies on Thursday. Freezing levels will be on the rise in general, but will make for a tricky forecast with warm air struggling to make inroads for the north and central Cascades. Higher freezing levels are a better bet over the Olympics and the Mt. Hood area.
A dry shortwave trough passing through in NW flow aloft will temporarily flatten the ridge and bring an increase in mid and high level clouds Thursday night. On Friday, the upper level ridge will quickly rebuild, but we will left in a repeat scenario with the central and north Cascades struggling to take advantage of the warmer air to the west and south. Also, NW alpine winds will become moderate to strong following the trough's passage on Friday.
Sun 20th Jan 09:00
January 20, 2019
The recent weather pattern of lower accumulation storms (by NW standards) and longer stretches of calm weather should continue as we move into late January. Since January 17th, incremental snow accumulations punctuated with rising freezing levels favored the south and eastern parts of the region. Storm instabilities have risen with storms and gradually subsided.
New Snow Problems
Storms over the past week have brought a range of layers from rain crusts, to heavy moist snow, to stiff drifts, to light dry powder. Some storm days, like the 18-19th, saw reactive, but very short-lived avalanches caused by heavy precipitation and wind. Even the longer-lasting avalanche problems, wind slabs, haven't persisted for more than a few days. Where the recent snow is stressing underlying weak layers, more dangerous avalanche conditions have prevailed.
Old Snow Problems
Persistent weak layers (PWLs) have been a constant in the eastern zones of the Cascades this winter. As usual, they have been much less problematic at the Passes and west of the Cascade Crest. The latest PWL is a layer of surface hoar, buried around January 17th and found generally east of the Cascade Crest. Buried surface hoar is an active weak layer in the eastern zones and can be found to a limited extent on the eastern edge of the Stevens and Snoqualmie Pass zones. There few, if any, avalanches have been reported on the buried surface hoar. It may be most problematic in open, wind-sheltered terrain, especially well above the valley floor.
You are most likely to find other layers of old weak snow the further you move east from the Cascade crest. Here snowpacks are shallower, more variable, and generally weaker. In some locations, weak snow near the ground can still be found. These basal facets have hung around all season. Digging profiles and using snowpack tests is the best way to gain information about these old persistent weak layers. However, snowpack tests are just one piece of the puzzle. Your terrain decisions shouldn't hinge on any given test result. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.
Heavy snow, warming temperatures, and strong winds overnight into Wednesday will form thick slabs. Winds will drift the new snow into deeper slabs at upper elevations. What is the new snow sitting on? Is there strong over weak? Is the new snow cracking, or do you see recent avalanches? If you find these, avoid slopes steep enough to avalanche, including small gully like features that could increase your consequences of even a small slide. Beware of the large avalanche paths in the area as well, and don’t get up underneath large slopes.
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Put a large buffer of terrain between where you travel and any avalanche path. Slides may wrap around terrain features and surprise you and your group.
Reactivity of these persistent layers will increase as we get another load of precipitation and strong winds. To the east the main concern lies in triggering the whole snowpack to the ground. Further west, the main layer of concern is surface hoar that was buried on the 17th. You can find this most readily on sheltered but open, shaded slopes, above the valley floor. Look for shooting cracks, recent avalanches, and listen closely for whumphs. Lack of evidence in one profile or snowpack test shouldn't be a reason to travel in more consequential terrain.