Avalanche Forecast
Issued: Jan 31st, 2020 10:06AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
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.
Summary
Discussion
Over 3 inches of rain has fallen as high as 8000 feet at Paradise since Thursday afternoon, driving wet snow related avalanche danger. Although considerably less rain has fallen in areas such as White Pass and Crystal, the danger has still been elevated. 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, due to 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.
Snowpack Discussion
January 30th, 2020 (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.
Snotel weather stations report well above average precipitation amounts for the month of January. Source: www.nrcs.usda.gov
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 42”, 33”, 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!
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.
A small skier triggered avalanche (D1) within new snow on a NE aspect at 5600ft on Mt. Herman. 01/28/20 Photo: Zack McGill
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.
Problems
Wind Slabs
We have low confidence in the amount of precipitation we may receive but 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
Expected Size
Loose Wet
Even with decreased likelihood, the snowpack may still be capable of producing surprising wet avalanches, especially earlier in the day Saturday. Although listed as Loose Wet, other wet snow related avalanches such as Wet Slab and Glide are still on our minds and these problems are often intermingled. Loose Wet avalanches often start at a point and are capable of entraining lots of snow, triggering slabs, and traveling far distances as they descend. Rollerballs, pinwheels, and fan-shaped avalanche debris are signs of Loose Wet avalanches. Steer around slopes greater than 35 degrees and avoid terrain traps such as creeks, gullies, or cliffs if you notice these signs or find unconsolidated wet snow.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 1st, 2020 10:06AM
