Recent and continued snowfall of 1.5 to three feet, accompanied by intermittent winds out of the S and SW have increased the possibility of dangerous avalanches. Snowfall was stretched out over several days, somewhat reducing the effect of rapid loading and giving new snow more time to bond. However, intermittent storm wind velocities have had the capacity for significant snow transport and loading. This may have created dangerous wind slab on leeward aspects (N and NE) and on wind-sheltered sides of gullies and ridges. Storm winds often deposit three times more snow load on leeward slopes than on the windward sides. Note that the winds have been predominantly out of the S and SW, but have been extremely variable around the peaks, so it is important to note wind-loading on all aspects.
For at least 24 hours after snowfall subsides, natural avalanches will be possible and human-triggered wind slabs will be likely on wind and storm loaded slopes (>30°). Post-frontal winds may extend the period of hazard and/or shift wind loading patterns to other aspects.
No new natural or human triggered slab avalanches have been reported since the late November cycle until this week. Snowbowl ski patrol reported triggering a soft slab avalanche on Christmas morning from the false summit of Agassiz Peak using explosives.
The primary layer of concern appears to be the new snow’s interface with the old surface. Where icy glazed surface conditions existed, stability may be delayed by slower bonding rates and a slick bed surface.
Observers noted some reactivity in a layer of near-surface facets on the morning of 12/27 on N and NW slopes. Columns and extended column tests (ECTs) repeatedly failed on isolation at faceting observed both above and below the 12/4 rain crust. Before descending suspect terrain, testing for propagation propensity using extended column tests (ECTs) or the propagation saw tests (PSTs) is strongly recommended. The aforementioned facet layer was identified in association with our earlier rain crust and seemed most problematic on NW to NE aspects.
Near and Above TreelineCoverage at treeline is ~2 meters or more. Wind and storm slab dangers will be highest in the first 24 hours after stormy conditions subside. New snow and storm slab appear to be unconsolidated with 33" of new snow available for wind transport and subsequent loading in the coming days. ECTs and PSTs on northerly slopes have suggested higher levels of danger due to significant faceting both above and below the 12/4 rain crust, creating a snowpack with poor structure and poor strength on those aspects. With skillful stability assessment, route-finding, and travel practices, there is plenty of safe fun to be had.
Below TreelineHuman-triggered avalanches will be possible below treeline, but will be less likely than at higher elevations where wind moves more snow and wind slab formation is most common. Regardless, avoiding hollow sounding/feeling slabs on steep slopes is advisable until these have a chance to bond to the snowpack below.
Coverage for ski touring on many slopes above 10,000’ is very good for this time of year, but early season hazards such as downed trees, stumps, and boulders may still exist. At lower elevations (<9000’) in particular, new snow may hide obstacles just below the surface.
Intermittent storm winds at and above treeline have potentially loaded leeward slopes (>30 degree) with dangerous wind slab hazards. During a recent lull between storms, winds transporting snow out of the southeast were observed, contributing to slab hazards on leeward north and northwest facing slopes. Based on some observations, wind slab and cornice development seem to be in isolated pockets rather than widespread deposition.
Moderate westerly winds are forecast for Friday night 12/27 into Saturday 12/28, increasing the possibility of wind transport.
AZ Snowbowl patrol reported the release of small, thin (6"-10”) wind slabs in the Hidden Valley area of Upper Bowl on 12/26. Although not a significant hazard at the time, the occurrence is indicative of rising instability overall.
Wind slab is most dangerous soon after deposition when the new slab has not had enough time to bond with the snowpack below.The persistence of weak layers deep in the snowpack can cause extended periods of snowpack instability.
Storm slab avalanches are possible on steep terrain, mostly at and above treeline.
If weekend snowfall reaches or exceeds maximum forecast, storm slab avalanche hazards will increase significantly, as will potential sizes of avalanches. Storm slabs usually stabilize 24-48 hours after formation.
As always, note warning signs of instability such as whoomphing or shooting cracks underfoot while traveling on untracked snow. Areas of particular concern include convexities and pockets of deep snow.
A series of short wave troughs have impacted northern Arizona and will continue into the weekend. These have laid down 24” of snow at higher elevations, with another 12-15” possible by Saturday morning. Snow transporting winds, mainly out of the southwest have affected snow distribution. Ridge-top wind velocities have been optimum for moving snow and loading leeward slopes.
The snow-line has fluctuated between 6500-5500 feet. One and a half to two and a half inches of snow water equivalent (SWE) were recorded at the Snowslide SNOTEL and nearby weather stations.
Snow showers will continue into the weekend, but are expected to taper off on Saturday. The total snow storm accumulation is uncertain, as this sequence of precipitation events is dynamic and ongoing. Cool temperatures, breezy conditions, and lingering unsettled weather will characterize the first half of the coming week. Looking onward from midweek, snow showers continue to be a possibility.
Arizona Snowbowl Ski Patrol reports approximately 67” (170 cm) base at 10,800 feet. Snowslide SNOTEL reports a 48” (122 cm) snow depth. So far this winter we have had 125" (317 cm) of snowfall at 10,800 feet.
Since December 20th, SNOTEL temperatures have ranged between 9° F on December 26th and 46°F on December 21st. ASBTP station (11,555 ft) reported a low of 6° F on December 26th and a high of 46.5° F on December 21st.
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.
Large cornices are generally rare in Arizona, but they have been observed during very snowy winters.
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.
Glide avalanches are very uncommon in Arizona.
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.