The bottom line: Avalanche conditions will deteriorate during the afternoon as the winds, snow and temperature increase throughout the day, making conditions more dangerous than on Monday. Lower density snow sits on a crust. Denser snowfall or winds might form a slab. If snow comes in sooner or more heavily than anticipated, be prepared to make conservative terrain choices.
Snow and Avalanche Discussion
Timing will be a major factor on Tuesday. The storm is currently forecast to impact the Mt. Hood area much more significantly Monday night, but if it comes in early, expect higher hazard than forecast.
Regional Synopsis
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.
A storm slab at Mt Baker.
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.
Surface hoar in the East Central zone
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.
Avalanche Problems
Wind Slabs
You might trigger a fresh wind slab wind slab near or above treeline as winds, increasing precipitation, and warming temperatures favor their development. Old wind slabs may linger in unusual locations as NW winds deposited snow on unusual aspects Sunday and Monday in the Mt. Hood area. Approach steep unsupported slopes with wind-drifted snow cautiously, feeling for firm or hollow sounding snow as a sign that wind slabs may be present. You can stay safe by traveling on ridges, wind-scoured areas and supported slope less than 35 degrees. Deteriorating weather is your sign that avalanche conditions are worsening and if you find heavier snowfall, winds transporting significant snowfall, or strong warming, move to safer terrain.
