The Bottom Line: New and thick storm slabs should form overnight and into the morning as a moisture-laden winter storm impacts the West - Central region. You will be most likely to trigger avalanches in locations above the overnight rain/snow line where more than 8 inches of snow accumulated, or the wind drifted the snow to greater depths. You can avoid triggering an avalanche by staying away from open slopes greater than 35 degrees.
Snow and Avalanche Discussion
A winter storm is impacting the West-North area Tuesday night into Wednesday morning. We don’t know whether the impressive forecasted ~1.5” of overnight snow water equivalent will materialize. We expect avalanche danger to peak overnight during the heaviest precipitation and warmest temperatures which are expected to occur Tuesday night. Avalanche danger should then slowly decrease throughout the day as precipitation ends and temperatures cool. However, avalanche danger may increase locally during periods of heavier snowfall under an anticipated convergence zone which may develop near or north of Stevens Pass.
Two small skier triggered avalanches were reported to the north of this zone in the Mt Baker backcountry Tuesday. These small slides highlight the presence of a firm crust below the recent snow. Avalanches Wednesday could slide on this old crust allowing them to run farther and faster.
Wed 23rd Jan 14:29
- Dennis D'Amico
Weather Synopsis for Wednesday night through Friday
Post-frontal showers continue to wind down this afternoon and should taper off completely in the early evening. A strong upper level ridge will rebuild offshore Wednesday night and become the dominant weather feature for the next several days.
Some low-level moisture/clouds will likely be trapped along the west slopes of the Cascades Thursday under the ridge. While cloudier conditions are expected along central-west and northwest Cascades, afternoon cloud cover should decrease for areas further south including the south Washington Cascades and Mt. Hood. The east slopes of the Cascades should enjoy mostly sunny skies on Thursday. 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 are a better bet over the Olympics and the Mt. Hood area.
A dry shortwave trough passing through in NW flow aloft will temporarily flatten the ridge and bring an increase in mid and high level clouds Thursday night. On Friday, the upper level ridge will quickly rebuild, but we will left in a repeat scenario with the central and north Cascades struggling to take advantage of the warmer air to the west and south. Also, NW alpine winds will become moderate to strong following the trough's passage on Friday.
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.
This 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. Cooling temperatures should limit this problem to below treeline during the daytime hours.
1 - 1
We expect new snow to fall with warming temperatures and moderate winds. This should create upside-down storm snow. If this occurs, you will be to trigger avalanches on open slopes greater than 35 degrees. Avalanches will grow larger and be easier to trigger as you go up in elevation, or venture into areas where the wind drifted the new snow. You can use small slopes to test the storm snow. Has the area received more than 8 inches of new snow? Do you see cracking? Can you feel stronger snow over weaker snow? Do you see signs of wind drifted snow? When you answer yes, storm slabs are nearby.