Urban Avalanche Advisory
Current Advisory as of
March 21, 2019
Expires 7:00 AM the following morning.
Issued By Tom Mattice
Primary Avalanche Problem
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.
How they form
Loose Wet avalanches form in new snow or old surface snow that is warming. Warming temperatures, strong solar radiation, or rain-on-snow make the surface snow damp or wet. As the water content increases, the snow becomes unconsolidated. They start at a point and entrain additional snow as they move down hill, and have a characteristic fan-like shape.
Where they form
Loose Wet avalanches form at or near the snowpack surface. Like Loose Dry avalanches, most Loose Wet avalanches start on very steep slopes. If a Loose Wet avalanche entrains a very large amount of snow, it can run long distances on low-angled terrain. They can form on any aspect. Because they are often associated with solar radiation, they are most common on sun-exposed slopes.
Timing and Triggering
In Colorado, Loose Wet avalanches are most likely in the late winter and early spring. Loose Wet avalanches are usually problematic for a period of hours at a time. There may be a daily cycle, as the sun warms the snow surface and makes Loose Wet avalanches more likely. The potential for Loose Wet avalanches can increase rapidly. Because they are often associated with solar radiation, the aspects where Loose Wet avalanches are likely can change through the day as the sun moves east to west.
There are many clues that Loose Wet avalanches are likely. Monitor the snowpack surface for a layer of wet, slushy snow more than several inches deep. Water may be visible between the ice grains. On small, very steep test slopes, the surface layers will slide easily. Fresh roller balls–little snowballs–falling off trees and cliff bands indicate that the snowpack surface is getting weak.
Treatment and Avoidance
Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches. Sluff management in extreme terrain is an effective technique for skilled riders. Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Monitor the snowpack surface, and move to colder, shadier slopes once the slush layers form
Learn more about Loose Wet.
Secondary Avalanche Problem
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty.
How they form
Liquid water, from snowmelt or rain-on-snow, moves through the layers of the snowpack at different rates. Wet Slab avalanches happen when a weak layer or interface becomes moist, wet, or saturated. The wet snow loses strength, and the snow above fails and avalanches. Wet Slabs fail because of a decrease in layer strength, compared to dry avalanches that often fail because of an increase in load.
Where they form
Wet Slabs often occur first at lower elevations and areas with a shallower snowpack. Facets and depth hoar layers are especially weak when wetted. After several nights of above-freezing temperatures, wet slabs can occur on many aspects.
Timing and Triggering
In most climates, Wet Slabs are spring-time events. This is when temperatures are warm and snowmelt produces sufficient free water to lubricate the snowpack. Earlier in the spring, the avalanche pattern can be cyclical, occurring in the afternoon or evening hours as melt-water accumulates, and decreasing in the morning as the snowpack cools. Later in the spring, extended periods of above-freezing weather contribute to the problem. The cyclical pattern decreases and it is possible for wet slab activity to occur at any time. In Colorado, one of the indicators of Wet Slab avalanches is two or three nights where temperatures do not drop below freezing.
Snowballing, pinwheels, and small Wet Slabs indicate the potential for larger Wet Slabs. Recent Wet Slab avalanches indicate the type of terrain another cycle will occur on, if similar conditions recur. Temperature data from remote weather stations can be useful to monitor overnight temperatures, watching for the periods of little or no overnight freeze.
Treatment and Avoidance
In many cases, snow conditions are poor when Wet Slabs are a significant problem. Most people leave the mountains or find places with firmer, less slushy snow and away from the slopes where Wet Slabs are a problem. Recognizing the weather factors that lead to Wet Slabs will help you avoid them. When solar radiation is a primary factor, carefully selecting and moving aspects through the day can be effective. Avoid terrain where and when you suspect Wet Slab avalanches. Give yourself a wide safety buffer to handle the uncertaintyLearn more about Wet Slab.
Today’s Avalanche Tip
Transitions to Spring Snow Avalanche Problems
by Alex Marienthal
Spring is here with longer days and a more predictable snowpack to facilitate objectives that are steeper and farther. These objectives are possible while maintaining a personally acceptable level of avalanche risk. However, these objectives can mean more exposure to other hazards like exposed terrain and prolonged rescue, which increases the consequences of relatively small accidents. The snowpack structure is changing from cold, dry layers to warm, wet and icy layers. This transition creates a fresh mix of avalanche problems.
Spring Avalanche Problems
Both dry and wet avalanche problems exist during spring. Dry snow avalanches result from snow and wind-loading that form slabs and add weight to exceed the strength of the snowpack. Above freezing temperatures and more direct sun affect the other half of the stability equation, causing the snowpack to melt, get wet and lose strength, and make wet avalanches more likely.
Wet loose avalanches happen when sun and above freezing temperatures warm and weaken the snow surface. Unconsolidated new snow is conducive to wet loose avalanches that will entrain more snow and become large. Wet slab avalanches occur when liquid water from snowmelt or rain percolates through a layered snowpack. Unconsolidated snow on the surface can melt quickly and introduce liquid water to the snowpack. When the snow surface is frozen in the morning, warming is necessary before wet avalanches are likely.
A snowpack that is wet from top to bottom is referred to as isothermal. Deep wet slab avalanches are most likely during and directly after the snowpack transitions to isothermal when snowmelt is susceptible to pooling along weak layers. Wet avalanche danger can be high during the transition to a fully isothermal snowpack when the snow is not frozen. Avalanches deeper than recent snowfall typically become less likely days to weeks after the snowpack has fully transitioned to isothermal.
Persistent weak layers enhance the destabilizing effect of water in the snowpack. Water can pool along weak layers and crusts and rapidly decrease snowpack strength. Persistent weak layers change by aspect, elevation, and from day to day, but they remain a danger until they are completely melted and refrozen together. In spring, the possibility of deeper, dry slab avalanches will persist longest on high elevation, shady slopes, and wet slab avalanches are possible when the snowpack is melting or during rain.
Spring Travel Advice
Snowfall will continue through April and there will continue to be new dry snow instabilities, in addition to wet snow instabilities. Long days and more confidence in stability encourage bigger objectives, more terrain, and more steep terrain. Encounters with “small and manageable” instabilities become more likely and can be deadly. Small, but unforgiving slides can catch a skier or rider off guard when a cold, bluebird morning turns to dripping sweat and t-shirts, or when corn skiing one day is followed by 6” of snow and 25 mph wind the next. Rapid changes in stability and the type of avalanche problem are normal from day to day, as well as throughout the day and with each change in aspect and elevation. Anticipate change. Be flexible with expectations.
Travel when and where the snow surface is supportable, dry, or frozen. Good skiing will be found as the frozen snow surface warms up, but timing is critical as the snow may become weak and unstable. The rate of transition from stable to unstable varies from less than an hour to multiple days, and is dependent on aspect, elevation, snow structure, and temperature change. Cold temperatures and clear skies overnight promote freezing and stability. When temperatures remain above freezing overnight the snow stays wet and unstable. Multiple days and nights with above freezing temperatures create very unstable conditions. If the snowpack did not freeze overnight, avoid avalanche terrain or go fishing. Wet avalanches can run far, so consider what is above on hot, sunny days. Does the trail, climb, or riverbank extend up to steep, sunny snowfields above?
Wet avalanche danger is low when the snow is dry or frozen in the morning, and increases if warm temperatures and sun melt the snow during the day. Evidence that wet avalanches are likely includes large rolling pinwheels of snow, point-release (wet loose) avalanches on steeper slopes, and sinking deeper than boot top into wet snow. These are signs to avoid slopes with wet snow. This article from Backcountry Magazine has some good tips for when to turn around in regards to changing wet snow conditions: http://backcountrymagazine.com/stories/mountain-skills-knowing-when-to-turn-around/
Expect wet snow stability to decrease when going from shady to sunny aspects. Examples include from north to other aspects (in the northern hemisphere), from west to east or south in the morning, or from east to west in the afternoon. Travel on east and south aspects early in the day. Sometimes they will remain cold all day, but if east aspects warm up fast, move to west, or north before those aspects warm up. High elevation and shady aspects will hold cold, dry snow the longest. Steep shady slopes can have dangerously firm snow when it has not thawed. Remember to look for weak layers, storm slabs, and wind slabs if there has been recent snowfall.
After the snowpack turns isothermal, and old weak layers have been thoroughly melted and refrozen together, the primary avalanche concerns become wet loose avalanches and relatively shallow wet or dry slab avalanches. The snowpack is not necessarily more stable, but instabilities are easier to identify and manage than the nasty persistent weak layers of this winter. The depth to which we need to worry about most avalanches is reset to that of recent snowfall, wind drifts, or relatively shallow buried thick crusts.
In spring, more daylight and confidence in stability promote success on steeper and farther backcountry objectives, but these objectives can have large consequences for small mistakes. Keep an open mind, anticipate change, and be flexible with your plan. Have a great spring and summer!