Last week's dangerous and reactive wind slabs now appear to have gained some stability by bonding to the snowpack below. However recently deposited windslabs may be reactive to a skier's or boarder's weight, especially with warning temperatures later in the work week. Natural and human triggered avalanches seem unlikely, however pockets of sensitive windslabs may exist. Shaded north aspects may preserve weak layers and persistent slabs.
No new natural or human triggered avalanches have been reported since Humphrey's Cirque and Snowslide Canyon avalanches, discussed in last week's snowpack summary. The Snowslide Avalanche took out 10" diameter trees. This would have been an unsurvivable avalanche.
Unseasonably warm daytime temperatures have contributed to snowpack bonding, strengthening and densification. Snow from the storm cycle that produced 93" has settled and sublimated reducing its volume and mass by 10% or much more in some places. On some north aspects, much of the new snow was stripped by winds.
Across the San Francisco Peaks, widely variable windslab and stormslab layers sit atop decomposing facets and hard rain event crusts...Southwest facing upper elevation terrain may have a reactive wind slab from last Friday- Sunday's extended northeast wind loading event.
Southwest and west winds of 15-25 mph over the last 48 hours have transported the meager remaining snow above treeline. Last weekend's strong northerly and northeast winds of sustained 40-50 mph transported large amounts of snow, loading southwest aspects with a dense surface wind slab above treeline and in exposed areas. This hard slab was reactive on a southwest aspect at 11,400' (ECTP11Q1; CT6Q2). more recent warming has left little powder snow available for transport above treeline.
Watch for relatively shallow and recently deposited wind slab on northeast, east, and southeast facing aspects. As in the case with snow falling out of the sky during a storm, wind blown snow also needs time to bond with the snowpack below.
A delayed report was received from riders who triggered a loose snow, point-release avalanche on a steep north facing cinder cone slope, below 8000 ft. This very small avalanche occurred on January 25th.
Near and Above TreelineFollowing strong NE winds last weekend, calming and warming weather prevailed throughout much of the workweek. Wind out of the west picked up on Thursday and Friday transporting snow and potentially depositing pockets of new unstable wind slabs.
Stability tests conducted in upper elevation starting zones show strong thick slabs with low to moderate reactivity on aspects besides the loaded southwest above treeline terrain. This wind slab is perched on top of a highly variable lower snowpack structure. Presumably the strong slab (four finger to one finger hardness) from the trifecta storm event occurring January 20th-23rd now have the strength to bridge old weak layers below. Propagation tests have revealed low fracture probation likelihood. The snowpack structure below the trifecta slabs is highly variable, ranging from preserved crusts and facets deep in the snowpack to homogeneous wind slab (top to bottom) and "sastrugi" (patterned snow cover where wind has eroded most of it). In areas where the snowpack is thick, the conundrum is, how strong is the slab? Although the likelihood of a skier or boarder triggering an avalanche seem low, the consequences if it were to slide might be very high, given the high volume and mass potentially entrained.
These slab layers have recently been affected by wind and warming. The weather forecast shows potential daytime temps into the 40's (°F), which can destabilize slabs. So as always, approach >35° terrain and loaded slopes with caution and assess for instability based on current conditions and actively changing conditions, to help guide your choices. Please review the snowpack and avalanche observations forum for other recent observations - find it under in the snowpack menu. You too can contribute to our BC community!!!
Below TreelineThe powder below treeline on shady and wind protected aspects remains good. Melt/freeze crusts have developed on sunny and radiation affected open southerly slopes at lower elevations. Very warm temperatures mean that it is time to look for wet snow signs such as snow rollers, and wet loose snow sluffing. Steep south facing slopes and south facing aspects of low elevation gullies will be the first to show signs of rapid thaw instability. If the snow under your feet becomes sloppy, move onto cooler or flatter terrain.
Coverage is still acceptable down to 7000 ft. but changing rapidly. The soft fun snow below 8000 ft may become melted and gone with the predicted warming trends.
The good news is that not much snow remains available to move - it was stripped by last week's strong northerly winds, or locked up in melt/freeze crusts created by solar input. The storm predicted for Monday/Tuesday (Feb. 6/8) likely will not add significant new snow. But if it does, the the windslab problem may become more likely. Watch for poorly bonded windslabs that may become more sensitive during the warming cycle beginning mid-week.
Always keep in mind, wind slabs are unpredictable, and may support the weight of a skier or rider initially, and fail suddenly with tragic consequences. Avoid snow surfaces which are recently loaded, sound hollow, and look for signs of fracturing, cracking, or whoompfing sounds.
As the temperatures start warming, watch for slushy/sloppy cohensionless snow turning into loose wet slides, especially on south aspects. If things get really warm, then loose wet slides may become more destructive wet slab avalanches. Wet slides may be possible later in the week, primarily on southerly, sunny slopes below treeline.
Avalanche and crown flank fracture on easterly slopes of Snowslide Canyon. This occurred during the January 20-23, 2017 storm cycle, and later obscured by winds and snow. Photo by Carlos Danel
Hard slab failure on southwest slope at 11,400'. ECTP11Q1.
Numerous rescues were conducted last week by the Coconino County Search and Rescue, and the Arizona Snowbowl Ski Patrol. Some of these could have been avoided by better planning and preparation. Travelers are advised to exercise caution, make slope specific evaluations and most of all, know where you are going and be prepared for the unexpected. As always, please treat this summary with appropriately guarded skepticism, make your own assessments, and contribute to our body of knowledge by reporting your observations.
Want to learn more safe backcountry habits? KPAC offers level I and II avalanche courses. They are filling up fast!!!
During winter, backcountry permits are required to access the Kachina Peaks Wilderness. More info
What difference a week can make. In the aftermath of last weekend's 40-60 mph northeasterly winds, high pressure conditions with temperatures significantly above normal characterized the workweek. High clouds and breezy westerly winds on Thursday and Friday have the effect of moderating midday surface melting, and also enhancing sublimation (the loss of snow mass back into the atmosphere). Cloud cover at night on Thursday and Friday will reduce energy loss to the atmosphere keeping temperature from dropping much below freezing. Breezy conditions and possibly a few snow flurries will occur as a modest storm passes to our north. The best chances of any precipitation will be on Monday/Tuesday, but accumulation is expected to be light.
Weather station information: On the morning of Friday February 3rd the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 76 inches (193 cm) at 9700’, and Arizona Snowbowl reported 86 inches ( 219 cm) at 10,800’. Since January 27th, SNOTEL temperatures ranged between -1° and 48° F, and Agassiz station between 12 and 42° F.
The avalanche problem/character describes part of the current avalanche danger. However because we only realease a summary once a week, the current avalanche problem will likely change.
Understanding avalanche problems is essential, because it allows you to determine your approach and strategies to risk treatment. Below are brief descriptions of avalanche problems/characters, and links to detailed information on the problem, formation, patterns, recognition, and avoidance strategies.
Avalanche Problems Explained Also see the North American Danger Scale.
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose-dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose-dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
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.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow.
Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices.
They can be triggered by light loads and weeks after the last storm.
The slabs often propagate in surprising and unpredictable ways.
This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
Deep Persistent Slab
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground.
The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs are destructive and deadly events that can take months to stabilize.
You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
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.
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.
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side.
Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridge line areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
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. The are often proceeded 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.
Snowpack Summary Disclaimer
The summaries on this site were written by Kachina Peaks Avalanche Center Board Members. They are based on a broad spectrum of data collected from weather stations, National Weather Service point forecasts and field observation by qualified individuals.
The summaries are not intended to substitute for good knowledge and decision making skills in avalanche terrain. If you have any doubt of stable conditions, please stay away from avalanche terrain. You can usually find good places to go that are not prone to avalanches, such as on low angle slopes away from avalanche run-out zones. If you have any questions about where to find such places, you should consider further avalanche educational opportunities, such as those listed on our education page.
Snowpack Summary – Format and Limitations Statement
Starting in 2012 Kachina Peaks Avalanche Center (KPAC) has publish a weekly Snowpack Summary on our website.
These summaries are currently issued on Friday afternoons. On occasion, we will give storm updates or warnings of rapidly increasing avalanche hazard at more frequent intervals.
Our objective is to reach weekend recreationist, informing this user group of prevailing conditions, but particularly warning of avalanche hazards whenever they are present.
Many people have asked us why we use the format we do, but do not include a danger rating or a hazard/stability rose as many other avalanche centers do around the west.
The National Avalanche Center (NAC) advises small operations like KPAC, who do not issue daily bulletins to not use danger ratings in our snowpack summaries due to the regular but intermittent nature of their field observations and the length of time between issuance of snowpack summaries.
A primary concern is for how conditions can change in the time between publications, potentially giving the public misleading information. At this point, we simply do not have resources to monitor the snowpack at the level necessary to accurately produce more frequent bulletins.
While we understand the benefits of a danger rating using the North American Danger Scale, we also feel that our format encourages people to dig in a little deeper, and spend some time reading what our forecasters are saying. Although the area that we forecast is relatively small, the variability has proven quite large.
Inner Basin conditions are often surprisingly different from those on the more wind-affected western side on the Peaks.
We hope the information that we provide in summaries helps give you a good overview of what is going on out there, and what avalanche problems you should be attentive to, but if there is any uncertainty, then we encourage you to ask questions via Facebook or firstname.lastname@example.org.