Our second significant storm of the season arrived this week. Rapid accumulation of new snow will increase the probability of dangerous avalanches. For 24 to 48 hours following the end of the storm, natural avalanches will be possible and human triggered ones are likely. Early season conditions prevail with a thin, unconsolidated snowpack.
With significant redistribution of the snow from last week's storm, backcountry snow coverage is highly variable. Hidden obstacles beneath poorly compacted snow may still remain. Strong southerly and southwesterly winds blew Monday through Wednesday, potentially creating wind slab problems on steep (>30°) slopes. Wind slabs will be found mainly on leeward aspects (N, NE). Newly formed wind slab may be hidden below snow from the Thanksgiving Day storm, which we are still in the midst of. High elevation storm totals are predicted to exceed 24 inches near treeline creating a significant load on the snowpack below.
The snow falls beckoning the romantic, piling in sensuous pillows, softening sounds, masking the chaos of life Like all mysteries snow has intrigue, but also deception. Allure as lure, resist the bait.
As one might imagine, two feet of new snow and brisk winds will create the potential for storm snow avalanches and wind slabs. Although surface hoar was observed last week in the aftermath of our opening storm, it seems unlikely that these fragile crystals will have survived wind events earlier in the week. If surface hoar remains buried deep below new storm snow, it will reside only in very wind protected zones below treeline.
Also, it seems unlikely that sufficient time has passed between the first and second major storms to allow other persistent weak layers (depth hoar or near surface facets) to develop, though some weak remnants of October snow may remain on shaded aspects.
Near and Above TreelineA lot of high elevation snow was moved earlier in the week from windward to leeward slopes, or has been stripped altogether and returned to the atmosphere. New snow has filled in making it challenging to assess snowpack depth , coverage, and structure. The settled base as reported at Arizona Snowbowl is currently 45” (114 cm) at 10,800 feet.
Wind slabs, potentially hidden below new storm snow on steep (>30°) slopes may exist near and above treeline. These could create hazards to skier ad snowboarders and may not be detectable underfoot. At the moment, this is speculative, as observers have not had the opportunity to fully evaluate backcountry conditions. Make sure to evaluate slopes individually by digging a snow pit, evaluating bonding between new snow and old, and keeping an eye out for recently buried and reactive wind slab.
Below TreelineBelow treeline the snowpack is still relatively shallow, unconsolidated, and deceptive. Hazardous obstacles such as downed trees, stumps, and boulders may still remain hidden near the surface.
More than 27" of new snow in 24 hours with winds in the 15-40 mph range create red flag conditions for storm slab avalanches. For the near term, travel in avalanche terrain is not recommended. These fragile conditions are likely to heal relatively quickly as new snow bonds with the old. Skiing and boarding safety will be significantly enhanced by waiting 48 hours after the skies clear and winds subside before entering steep (>30°) snow loaded terrain .
Winds Monday - Wednesday may have created wind slab problems on steep slopes (>30°) near and above treeline. Wind slabs may have subsequently become buried beneath new storm snow, hiding this problem from view and the feel under your skis. The added snow load may also increase the potential size and consequence of what may have otherwise been a smaller avalanche. These conditions will be most prevalent on north, northeast and east facing slopes.
Loose snow avalanches are likely to occur on steep terrain (>35°). Caution should be taken when traveling on steep slopes, particularly above cliffs, exposed rocks, and confined gullies where being swept away would more likely result in traumatic injury rather than complete burial.
Winter arrives! November 22, 2019 photo by Derik Spice.
As of November 29, uphill travel at the Arizona Snowbowl is currently closed.
Two potent storms in less that a week, what a great way to set winter in motion. A cold front, early last week, brought 2 inches of snow to Snowbowl, and a lot of wind early in the week. Temperatures stayed mostly below freezing at high elevations throughout the week.
Thanksgiving day ushered a storm anticipated to last until Saturday morning. It will feature strong winds out of the south and southwest, and abundant snowfall at elevations above 6000 feet. This storm is very similar to our last big one, characterized by a deep digging low, a subtropical moisture tap and cold advection influenced by a strong mid latitude cold front.
Snowbowl reported 27” (53 cm) of new snow on Friday morning all falling within the previous 24 hours and an additional15” (38 cm) forecasted before the storm’s ends. Snowslide SNOTEL reported a total snow depth of 32” (81cm) early Friday morning, of which 20” was new. At least 2” of new Snow Water Equivalent (SWE) was added to the snowpack at the time of publication.
Looking on toward the weekend and beyond, expect snowfall to taper off on Friday evening or Saturday morning, leading to partly cloudy conditions for the weekend. The workweek will bring gradually warming and more settled weather.
Since November 23rd , SNOTEL temperatures have ranged between 9°F on November 26 and 36° F on November 24. ASBTP station (11,555') reported a low of -1°F on November 26 and a high of 46°F on November 24.
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.