Avalanche Forecast
Issued: Jan 2nd, 2019 6:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Loose Wet.
Josh Hirshberg,
The Bottom Line: A warm, wet storm will bring strong wind and snow to the highest elevations. Expect very dangerous avalanche conditions where snow and wind built slabs over a layer of buried surface hoar. Only venture into the mountains if you are confident in your ability to stay far away from avalanche terrain.
Summary
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.
Weather Forecast
Thu 3rd Jan 14:11 - Dennis D'Amico
Weather Synopsis for Thursday night through Saturday
After raining cats and dogs at most sites along the west slopes of the Cascades this morning, a plume of warm frontal moisture has lifted north this afternoon, providing a break for the south Washington Cascades. However, the heavier precipitation and stronger winds will stay focused on the Olympics and Mt. Baker area through this evening.
The associated frontal boundary will pass from north to south through the Cascades tonight and precipitation rates will pick up once again. Snow levels will be slow to cool, but a brief period of easterly flow may allow snow levels to briefly drop to Pass level early Friday morning. Once the front passes through, snow levels will settle rather uniformly across the region in the 4000-4500 ft range.
On Friday, the front will split apart as the parent low pressure system stalls off the central B.C. coast and a new upper low digs off the California coast. Post frontal SW flow should mainly benefit the Mt. Baker area in the way of significant accumulating snow showers above 4000 ft. Other areas will see light showers Friday.
Showers should end Friday night with dry conditions forecast through Saturday morning. An approaching longwave trough will begin to shift showers into the Olympics and south Washington Cascades and Mt. Hood on Saturday afternoon.
Problems
Persistent Slabs
Snow, rain, and wind are putting a new load on an already weak snowpack. You can trigger dangerously large and surprising avalanches on slopes over 30 degrees. Avalanches may be more reactive and break wider than you can expect. You can trigger persistent slabs from a significant distance. You don’t even need to be on the slope to trigger them. Put plenty of space between you and any suspect avalanche terrain. Stay far away from slopes that roll to over 30 degrees, and make sure you are well out from under any areas where avalanches could start.
While you can trigger avalanches at all elevations, the most dangerous slopes will be near and above treeline. Avoid avalanche terrain on leeward, wind loaded slopes. Watch for fresh drifts and blowing snow as indicators of where avalanches could be triggered most easily. 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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Loose Wet
Rain may fall to 5500 feet. Wet loose avalanches will be easier to trigger as rain totals add up. Wet loose avalanche could break deeper and start a larger and more dangerous slab avalanche. 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.
Aspects: All aspects.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Jan 3rd, 2019 5:00PM
