Avalanche Forecast
Issued: Jan 31st, 2020 11:57PM
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
Forecast updated at 8 AM: Very dangerous conditions will develop above treeline as a strong cold front brings heavy snow and intense winds that will rapidly building large to very large and increasingly dangerous wind slabs. Avoid all avalanche terrain exposed to large avalanche slopes where avalanches may run from upper elevations on Mt. Hood. At lower elevations, choose wind-sheltered terrain and non-wind-loaded slopes less than 35 degrees to stay safe once the rain changes to snow and starts accumulating.
Summary
Discussion
Mt. Hood picked up between 0.5-1.0" of liquid water with snow levels rising to around 9000 ft within the last 24 hours. Mt. Hood should be generally dry overnight with heavy rain and snow picking up in the morning hours on Saturday as a cold front brings rapidly lowering snow levels down to about 5000 ft by 10 AM and continuing to drop further as the day continues. This transition from rain to snow segregates the avalanche problems you will deal with on Saturday, but the developing wind slab problem will be far more severe and dominates from 10 AM onwards.
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
Forecast updated 7:55 AM due to more snowfall than originally forecast: Heavy snowfall combining with strong to extreme alpine winds should create rapidly deteriorating avalanche conditions which will quickly become very dangerous above treeline. Avoid all avalanche terrain at these elevations or anywhere avalanches may start at these upper elevations and run to lower elevations. These avalanches may build to large or very large size and they will be larger late in the day.
As snow levels lower and winds keep cranking, wind slabs will build near treeline and into the upper below treeline elevation band by the afternoon. Choose wind-sheltered terrain where possible. Observe the new snow above a refreezing crust to determine the nature of the wind slab hazard in terrain. Recognize that harder slabs may be likely given the stregth of the wind and hard slabs may break more broadly across terrain features, so even a shallow slab can have major consequences. At upper elevations, slabs will be thick, large, and potentially unmanageable. Yellow flags on Saturday include firm or cracking snow surfaces under your feet, anywhere you find 6" of snow above the crust, or find evidence of active transport (moving snow) or recent transport (fresh cornices, scoured features, or textured snow surfaces). If you identify a yellow flag, you've confirmed a wind slab problem in the terrain and should navigate slopes less than 35 degrees to stay safe.
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: All elevations.
Likelihood
Expected Size
Loose Wet
The snowpack is saturated and loose wet avalanche danger should be trending downward, but heavy rainfall arriving early Saturday morning keeps this as a problem early in the day. Remember that small loose wet avalanches can be dangerous when combined with terrain traps like gullies, cliff bands, or open creeks.
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: Below Treeline.
Likelihood
Expected Size
Valid until: Feb 1st, 2020 11:57PM
