The Bottom Line: The recent snow still needs time to stabilize. It remains possible to trigger an avalanche at mid and upper elevations on slopes 35 degrees and steeper. Asses upper snowpack layers carefully before venturing into steeper terrain.
Snow and Avalanche Discussion
Since Thursday, neighboring zones received up to 2.5in of water with snow levels near 4000ft. This translated into 12-18in of snow at mid and upper elevations. At lower elevations, a few inches of wet, heavy snow accumulated, while all rain fell below 3500ft. No avalanches were reported on Saturday. Natural and triggered avalanches were reported Friday. These avalanches broke 1-2ft deep at the old/new interface or just above within new snow layers. Avalanches occurred between 4500-5500ft primarily on northerly aspects. Some avalanches were large enough to bury a person (200ft wide and running 400ft feet downslope) but most avalanches were small. The old snow surface is a firm crust that acted as a productive bed surface. New snow instabilities are of most concern above 4500ft where this old snow surface stayed dry. Below this elevation, the new snow appears to have bonded well to old snow surfaces that remained moist/wet. Cooling temperatures over the next 24 hours will solidify the snowpack at lower elevations.
Sun 20th Jan 14:33 - Dennis D'Amico
Weather Synopsis for Sunday night through Tuesday
A weak low pressure system traversing across Oregon this afternoon has produced several inches of light snow over the south Washington Cascades and Mt. Hood area. Light snow and rain will come to an end for these same areas this evening with dry and fair weather forecast on Monday.
High pressure building offshore around 135W will flop into southern B.C Monday. Increasing subsidence aloft (sinking motion) combined with weak onshore flow at the surface should trap some moisture and low clouds below Cascade crest level along the west slopes of the Cascades. Sunnier skies are a much better bet along the east slopes of the Cascades tomorrow with freezing levels relatively cooler. Look for a stiff NW wind in the alpine Monday as well.
The ridge will stay anchored offshore but will flatten Monday night and Tuesday allowing a fast-moving weather system to move across the area in NW flow aloft. A light to a moderate shot of precipitation should mainly be in store for the west slopes of the Cascades and Mt. Hood as snow levels rise quickly later in the day.
Wed 16th Jan 09:00
January 16, 2019
Since Friday January 11, we’ve enjoyed a spell of generally nice weather in the mountains. This allowed for some great views, enjoyable outings, and lots of snow observations. A more active weather pattern beginning Thursday January 17th will bring this nice weather to a close.
During this time period, the snow surfaces around the area have changed dramatically. This forms the foundation for a few current trends we are seeing in the mountains.
New Snow Problems
Reports from around the area indicate a wide variety of snow surface conditions prior to new snow on January 17th. We’ve heard about breakable crust, very icy surfaces, sugar facets, surface hoar, and rime. What snow surface you encounter can depend on aspect, elevations, and general location.
As a series of winter storms impact the area, how will the new snow bond to the old snow surface. This can be tricky to predict. As the snow starts to pile up make lots of shallow snow observations. Shovel tilt test, hand shears, and small slope test can all help you track how the new snow is bonding to the old snow at different aspects and elevations. Stop and take a look at where the snow is failing. Do you see large grains of snow? Do they look like feathers? Do the act like sugar? How far below the current snow surface are they?
Old Snow Problems
There are still lingering persistent weak layers in the eastern zones of the cascades. You are most likely to find weak older snow in areas further east from the Cascade crest where 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. The only way to gain information about these old persistent weak layers is to get out your shovel and dig. 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.
Although avalanche activity is trending down, continue to give the new snow time to stabilize. The most likely place to trigger an avalanche is on northerly aspects above 5000ft on unsupported or convex slopes 35 degrees and steeper. Large avalanches may still be possible to trigger. Storm slabs may become more reactive the higher in elevation you travel. Be careful in upper elevation areas where more snow accumulated, and on leeward slopes where deeper drifts of snow exist. Watch for recent avalanches, shooting cracks and collapses. Seek out small, inconsequential slopes to test the snow before venturing into steeper terrain. Dig down to the January 17th crust layer (buried about 1 foot deep in most locations) and investigate how the new snow layers are bonded. If you find sudden results in hand pits or column tests at the new/old interface or within new snow, seek out lower angled or supported terrain to reduce your chance of triggering an avalanche.
Aspects: All aspects.
Elevations: Alpine, Treeline.