The Bottom Line: Avalanche conditions continue to gradually improve, however, new fresh wind slabs may form in open terrain above treeline Friday. Avoid open slopes greater than 35 degrees, and any slopes receiving wind transported snow, where it will still be possible to trigger avalanches. During extended sun breaks, watch for the potential for wet avalanches on sunny slopes.
Snow and Avalanche Discussion
Unstable slabs continue to improve and stabilize following a natural avalanche cycle reported in the Mt. Baker zone during peak warming and precipitation Tuesday night. In addition, large loose wet avalanches released naturally during sun breaks Wednesday. Both of these avalanches types (Loose and Slab) entrained significant amounts of snow and ran long distances. The slab avalanche problem will continue to improve Friday but is more likely to persist above treeline.
Fri 25th Jan 16:19
- Robert Hahn
Weather Synopsis for Friday night through Sunday
The high-amplitude upper-level ridge offshore pushes slightly further into the region with temperatures aloft warming further tonight into tomorrow, giving us the warmest temperatures we've experienced since November over our mountains by late Saturday into Sunday. Weak systems are spilling over the top of the ridge into SE Alaska and are dropping south through the Rocky Mountains, leaving our region largely unimpacted. Low-level moisture is generally decreasing, so clouds should be mostly clearing out on Saturday. Low-level inversions and fog or freezing fog are likely in some locations. Saturday will be the warmest day for our mountains since November 19th, with freezing levels pushing 11000 ft. On Sunday, a trough moving east along the Rockies will knock back temperatures slightly in our region, but it will remain unseasonably mild.
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.
Wind slabs continue to be the dominant problem at higher elevations. Older, deeper wind slabs are gaining strength following Wednesday's storm. While they are gaining strength, you are most likely to trigger these larger slabs on very steep slopes, near convex rollovers, or just below cornices. New, smaller wind slabs will also form Friday. These shallow slabs will be easier to trigger and could cause older wind slabs to fail. Either way, you want to pay attention to signs of wind transported snow. Do you see blowing snow? Is there a smooth pillow like feature? Does the snow surface feel firm and hollow? When you answer, “yes,” avoid nearby slopes greater than 35 degrees.
2 - 2
Freezing levels will push to near 10,000 ft Friday. With significant recent storm snow, wet avalanches will be possible on steep, southerly facing slopes. Do not linger below steep, southerly slopes during periods of sunshine. Take time to consider your travel route. Will you be crossing on or under these slopes later in the day? Are there alternative routes, if the sun comes out? Are there any high consequence features, such as gullies or cliffs, that might make loose avalanches more dangerous?