The Bottom Line: Large and dangerous wind slabs likely formed above treeline Tuesday night and Wednesday. Give time for conditions to stabilize on the upper mountain by avoiding steep wind loaded terrain and by limiting your exposure to large open avalanche paths that begin higher on the mountain. Near and below treeline, think about the increasing potential for loose wet avalanches if the sun pops out on steep, sunny slopes.
Snow and Avalanche Discussion
After an initial bout of snow, a strong frontal system pushed heavy rain up to at least 6500-7000 ft Tuesday night and Wednesday morning in the Mt. Hood area. Strong west winds also rocked the mountain during this period, limiting direct observations from the upper mountain on Wednesday. As we head into Thursday, we will likely be dealing with two sets of problems; hard wind slabs above the rain-line in the alpine and loose wet avalanche potential near and below treeline.
Thu 24th Jan 13:53
- Kenny Kramer
Weather Synopsis for Thursday night through Saturday
An upper-level ridge of high pressure is centered offshore and extends to the Gulf of Alaska, diverting Pacific storms well north of the area. A weak disturbance will move over the ridge and pass to the north of the area Thursday night, mainly causing increasing high clouds across the north and central part of the forecast area with no threat of precipitation. The passing disturbance will enhance cooler NW flow and temporarily lower freezing levels Thursday night.
The ridge gains strength Friday through Saturday and moves closer to the coast. This will maintain mostly fair weather with a warming trend, especially at higher elevations.
Moisture will continue to be trapped at lower elevations under the building high pressure, maintaining low clouds and fog in the lower elevations, valleys and passes with some afternoon clearing expected.
Dry conditions with mostly sunny days at higher elevations will continue under high pressure through Saturday and into the weekend with high freezing levels as the ridge remains along the coast.
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.
Winds will finally back-off on the upper mountain Thursday, but above the most recent rain-line, firm wind slabs should have built well below ridges. Given the potential for large avalanches to start up high and entrain wet or moist snow at lower elevations, Thursday is not a good day to recreate in big open terrain or to linger where exposed to much higher start zones. 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 any slope less than 35 degrees.
2 - 2
Warming temperatures and potential afternoon sunshine will increase the odds for loose wet avalanches on Thursday. These avalanches are most likely to release from steep, rocky, sunny slopes as the day progresses. Monitor changes in the upper snowpack, looking for natural pinwheels or small loose wet avalanches as clear signs of increasing danger. Avoid terrain traps like gullies where even a small avalanche can bury you.