The bottom line: You can trigger a large and dangerous slab avalanche on leeward slopes where strong winds have been transporting fresh snow near and above treeline. Conservative decision-making will be essential, particularly above the rain/snow line from Tuesday night. Where rain has occurred, you may trigger small loose avalanches on slopes greater than 35 degrees.
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. The avalanche danger is likely to peak overnight or early in the morning and will decrease with gradual cooling and as rain and snow showers decrease.
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.
Wed 23rd Jan 09:33
- Dennis D'Amico
Weather Synopsis for Wednesday & Thursday
A strong frontal system passed through the region Tuesday night, leaving post-frontal showers mainly for the west slopes of the Cascades and Mt. Hood area this morning. Areas along the west slopes of the Cascades/Mt. Hood picked up 6-12+ inches before a switch to rain at 4000 ft in the north, 5000-5500 ft in the central and south Washington Cascades and 6000 ft at Mt. Hood. Strong westerlies seen from Stevens Pass and south will come down later this morning, except at Mt. Hood where strong westerlies will linger through much of the day.
Showers will diminish through this afternoon, becoming isolated in the evening. A strong upper level ridge will rebuild offshore Wednesday night and Thursday with dry conditions forecast tomorrow. Some low-level moisture/clouds will likely to be trapped along the west slopes of the Cascades Thursday with sunnier skies forecast along the east slopes of the Cascades. 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 look like a better bet over the Olymipcs and Mt. Hood area.
A warm system will bring rain up into the near treeline band. The Loose Wet avalanche concern will peak overnight or early in the day as rain and warmer temperatures continue to add liquid to the snowpack. These can ruin your day particularly if they push you into a trap or obstacle or over a cliff.
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Moderate to heavy snow combined with rising freezing levels and strong to extreme winds Tuesday night will create large wind slabs which you are likely to trigger anywhere above the rain-snow line. Moderate winds will continue transporting snow above the anticipated 6500 ft snow line on Wednesday morning. This avalanche problem will extend below treeline if the snow level is lower than anticipated. You might find hard, punchy and supportable (hard) slabs or slabs that you can trigger in softer snow. Due to the strength of the winds, these slabs are likely to be larger and extend further away from ridge crests than usual. Steer well clear of wind-loaded slopes steeper than 35 degrees. When unsure whether a slope was wind-loaded, use ridges or low-angle slope to travel through the terrain. In very wind-protected areas, the upside down snow may present as a storm slab. You can use very small steep rolls to test the snow for this problem.