The Bottom Line: Large avalanches may be easier to trigger on persistent weak layers after the recent storm. Winds may create reactive wind slabs in the alpine. Choose your routes with these in mind. Step back if you find signs of instability such as shooting cracks, hear whumphs, or find slabby snow over weaker snow.
Snow and Avalanche Discussion
The East Central Cascades picked up from 2.0" to .3" of snow water equivalent, depending on location. Snow accumulated rapidly as goose feathers fell with cold temperatures initially, then the temperatures warmed up and the winds blew. This caused a widespread natural avalanche cycle throughout the Cascades. Many large paths ran in the area, entraining wet snow as the debris ran through the water saturated lower elevation snowpack. Most of these appear to have ran within the storm snow, but some may have taken out persistent weak layers. It appears that it rained up to around 5,000ft, but uncertainty remains regarding how far down the water percolated into the snowpack. A number of folks have talked about how the sun is becoming stronger already. This may have caused a wet loose cycle on Wednesday and Thursday.
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.
2 - 3
Large avalanches will be easier to trigger after the recent storm added to the slab over the weak layer.
To the east of Hwy 97, the main concern lies in triggering the whole snowpack to the ground, and a funky layer of crusts, facets, and surface hoar in the mid-pack down a foot or two. As recently as the 24th, an observer noted an "earth shattering WHUMPH!" at 5,000ft near Blewett Pass. Here, the snowpack is shallow and largely variable. Avoid areas with thick slabs adjacent to thin, rocky slopes.
Further west, the main layer of concern is a layer of surface hoar that was buried on the 17th. You can find this layer down a foot or two west of Hwy 97, and will be most readily preserved on sheltered, shaded open slopes from 4,000ft to 6,000ft. Look for shooting cracks, and listen closely for whumphs. Snowpack tests can also help identify these weak layers. Lack of evidence in one profile or test often isn't enough information to make sound decisions from. Avoid getting up underneath steep slopes, travel conservatively with these layers in mind. Put a large buffer of terrain between where you travel and any steep slopes. Slides may wrap around terrain features and surprise you and your group.
A bump in winds from the northwest may transport snow from cold, north facing slopes up in the alpine. Where snow is redistributed, dense slabs may form. Freshly drifted slabs are likely to be more reactive. When you find more than 6” of snow has drifted in or accumulated, it’s time to evaluate it. You can check how the new snow is bonding by using small, inconsequential test slopes and quick hand pits. Is there strong over weak? Are the slabs cohesive enough to crack? If so, it may be best to steer clear of firm, slabby snow on steep slopes.