Avalanche Forecast
Regions: Snoqualmie Pass.
With a dynamic weather pattern like we’ll see Friday night into Saturday, we are not without uncertainty. That said, avalanche danger should increase throughout the day and as you ascend in elevation due to strong winds and a transition from rain to snow. Keep an eye out for wind-affected slopes and red-flag indicators of instability such as recent avalanches, whumpfing, and shooting cracks.
Discussion
Close to 3 inches of rain has fallen as high as 8000 feet at Snoqualmie Pass since Thursday afternoon, driving wet snow related avalanche danger. Observations on Friday were limited due to the potency of the storm, but the information we do have suggests that a wet avalanche cycle occurred. Moving forward, freezing levels should drop to 6500 feet Friday night as we begin to transition between storms. Heavy rain will continue to fall in all elevations bands during the early morning hours on Saturday. By mid-morning Saturday, freezing levels should drop quickly from 6500 feet to 3500 feet, with a frontal passage around 9:00-10:00 am. Expect rain to transition to snow with this frontal passage, changing the primary avalanche danger from wet snow problems to new snow problems.Â
At lower elevations, rain may have significantly affected travel with open creeks, collapsed snow bridges, and heavy wet snow.
Snowpack Discussion
January 30th, 2020Â (The regional synopsis is updated every Thursday @ 6 pm)
Looking Back at January
As we turn the corner from January to February, this is a good time to look back at the last month’s snow, weather, and avalanche conditions. January was notable for continuous storms and elevated, yet quickly resolving avalanche danger. In most parts of the region, the snowpack grew significantly. Snow depths went from far below average at the New Year to near normal by the end of the month. While numbers are still being compiled, January 2020 could rank high for total precipitation amounts in some locations. As we move forward, there are currently few layers of concern in the existing snowpack.Â
Snotel weather stations report well above average precipitation amounts for the month of January. Source: www.nrcs.usda.gov
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Weather
January was marked by an onslaught of storms with very few breaks in precipitation lasting 24, or even 12 hours. Most notable was the sheer amount of precipitation that these storms delivered. Most NRCS Snotel weather stations are reporting at least 125% of average precipitation amounts for January, with many over 200% of average. Not all of this fell as snow. For the month of January, the Mt. Baker, Snoqualmie Pass, Paradise, and Mt. Hood Meadows weather stations measured an astounding 42â€, 33â€, 32†and 28†of water (both rain and snow) with 1 day still left in the month. If you do some quick math, those 4 stations average near or above an inch of water a day! Â
Snow levels and temperatures were less consistent than the overall stormy weather. On the heels of a warm December, the first week of the New Year followed suit. From about January 8th through the 19th a cold period had its grasp on the region, bringing snow to the lowlands and copious amounts of light, champagne powder to the mountains. The last third of the month saw continued storms. Temperatures moderated and warm air brought occasional rain to middle and even upper elevations.Â
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A small skier triggered avalanche (D1) within new snow on a NE aspect at 5600ft on Mt. Herman. 01/28/20 Photo: Zack McGill
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Snow and Avalanche Danger
After a meager start to the winter, the snowpack grew exponentially in January. In fact, the storms were so intense in the middle of the month that they challenged the accuracy of remote weather stations. Snow blocked precipitation gages and knocked out cellular towers and phone lines used to transmit data. Currently, snow depths from reporting sites are near average.
While the storms often brought rapid spikes in avalanche danger, the hazard was quick to taper off. Throughout the month, there were 12 days when NWAC issued High Danger in one or more zones and 11 days when there was at least some Low Danger in forecasts. It’s notable that there were no days with all Low Danger in any forecast zone. In addition to the snowpack growth, a positive side to the storms was that weak layers had little time to form. In general, any facets or surface hoar that formed were very short-lived weak layers. Most avalanche problems involved new snow, wind or wet snow. There are still some areas of shallow, weak snowpack in typical places like the Wenatchee Mountains and the most eastern extent of the Cascade Mountains. Aside from that, the snowpack layering in most zones is marked by a few lingering storm interfaces in the upper snowpack and intermittent crusts from rain events. If you dig in the snowpack around the region, the most prominent crusts you’ll find formed from rain events roughly around January 1-3rd, 7th, MLK weekend, 23-25th, and the 31st.
Looking Forward
The winter snowpack is now well established for recreating and traveling through the mountains. Much more so than it was a month ago. Looking ahead, there’s a good chance this stormy pattern will continue at least into early February. We’ll keep monitoring how the snowpack continues to change near the surface with each weather event. You can check the daily Avalanche and Mountain Weather forecasts for the most up to date information.Â
Avalanche Problems
Wind Slabs
We have low confidence in the amount of precipitation coming our way, especially with the possibility of a convergence zone setting up, but we do have good confidence that high winds will be present, particularly at higher elevations on Saturday. These winds will be capable of transporting any new snow onto leeward slopes or the sides of gullies, creating wind slabs near and above treeline. Throughout the day and as you ascend in elevation, pay attention for signs that wind slabs may be present, such as blowing snow, textured snow surfaces, and freshly built cornices. If you are traveling in locations more sheltered from the wind, storm slabs may develop as new snow accumulates. In either case, if you see shooting cracks in the snow or find heavy snow over soft snow, you should avoid any open slopes greater than 35 degrees.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Glide Cracks
Even with decreased likelihood, the snowpack may still be capable of producing surprising wet avalanches, especially earlier in the day Saturday. Although listed as Glide, other wet snow related avalanches such as Wet Slab and Wet Loose are still on our minds, as these problems are often intermingled. Even with the cooling trend, the snowpack needs time to adjust to all the recent rain. It remains possible for large glide avalanches to occur naturally in isolated areas. Avoid travel underneath steep rocky slopes and cliff bands, where snow could shed from above. Loose wet avalanches may still pose a concern at lower elevations as well and are capable of entraining lots of snow, triggering slabs, and traveling far distances as they descend. Conditions like these bring quite a bit of uncertainty and are hard to assess. Travel cautiously and build in a wide margin for error.
A release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often preceded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
Predicting the release of Glide avalanches is very challenging. Because Glide avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide avalanches.
This Glide avalanche broke to the ground on a smooth, grassy slope. From all the mud on the bed surface, water pooling at the base of the snowpack likely caused the failure.
Glide avalanches occur when water lubricates the interface between the snowpack and the ground. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1