The bottom line: A warm, wet storm will bring strong wind and snow to the highest elevations creating very dangerous avalanche conditions. Avoid steep slopes at upper elevations where the wind is drifting thick slabs. Stay out from under large avalanche paths that start above treeline.
Regional Synopsis
Happy New Year!
Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.
December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.
The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.
A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.
It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.
Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.
Avalanche Problems
Wind Slabs
While you can trigger wind slab avalanches at all upper elevations, the most dangerous slopes will be above treeline. Avoid avalanche terrain on leeward, wind loaded slopes. Watch for fresh drifts, blowing snow, and areas of variable height storm snow as indicators that you could trigger a wind slab avalanche. Slabs will build during the day with avalanches getting bigger and easier to trigger later in the afternoon. The most dangerous conditions will be Thursday afternoon into Thursday night.
Wind features on a ridge above a human triggered avalanche (1/1/2019)
Aspects:
All aspects.
Elevations:
Alpine, Treeline.
Likelihood: Likely
Expected Size: 2 - 2
Loose Wet
Rain may fall to 6000 feet. Wet loose avalanches will be easier to trigger as rain totals add up. Roller balls and pinwheel-like snowballs are good indicators that you may soon encounter avalanches. These avalanches are easier to predict than others, but they can push you around with heavy wet snow. Avoid steep slopes near gullies, cliffs, or other terrain traps where a small avalanche could have more serious consequences. Don't linger under steep slopes as natural wet loose avalanches could come from above.
