Fifteen inches of new snow was recorded on mid-mountain at Snowbowl (10,800') on Thursday morning March 23rd.. Over an inch of snow water equivalent (SWE) was recorded for the same period at Snowslide SNOTEL station (9700'). Snow showers and light accumulation continued throughout the day on Thursday adding an additional two to six inches. As a result, storm slab and wind slab avalanche hazard increased. Natural avalanches are possible and human triggered avalanches are likely during the 24-48 hours follow precipitation or wind-transported snow events.
For the short-term, winter travelers are urged to avoid avalanche terrain. Over the upcoming week, skiers and boarders should carefully examine bonding between the new snow and the old, with a critical eye towards identifying a new persistent weak layer of near surface facets that may develop between the new cold snow and old pre-storm snowpack.
The pre-storm snowpack gained a lot of heat from unseasonably warm temperatures and high pressure over the past two weeks. Cold snow meeting warm snow will create an environment for rapid facet crystal growth and therefore delayed bonding between new and old snow. This facet creating environment will be most prevalent at high elevations on south and southwest facing slopes where the temperature gradient between new and old will be greatest.
We did get a chance to examine near treeline southwestern slopes on Thursday during the late stages of the storm. The new/old snow boundary was well bonded and not reactive. No collapsing and only very minor cracking in the new snow. However there was a 1.5° C temperature gradient between the new and old snow and this could lead to a weakening of bonds over the next 24 hours. We encourage you to carefully examine the bond between new and old snow.
Near and Above TreelineNew precipitation is sufficient to cause potential instability. The main issue will be new snow avalanching on the old nearly isothermal snowpack. The magnitude of seriousness of potential slides will be proportional to amount of new snow that has deposited on top of old. Wind transported snow can create slabs much thicker than what's indicated by storm accumulation totals. Wind loading is most likely to be on north, northeast and east facing aspects. As of early Friday morning (3/24), strong northerly winds started transporting snow, potentially loading southerly aspects and cross-loading eastern and western gullies and chutes. Video of north winds by Carlos Danel.
Below TreelineBelow treeline and at lower elevation new snow temperatures are typically warmer, creating a more favorable environment for timely bonding of new snow with the snowpack below. Also, temperature gradients between new snow and old snow will be less dramatic reducing likelihood of significant near surface faceting. Despite the reduced risk of persistent weak layer development, it is still worth carefully examining the boundary between new and old just in case, and thereby avoiding the consequences of being surprised by taking of ride on new snow sliding on the old.
Southerly and westerly aspects below 10,000' are getting thin, with rocks and logs just under the surface and hidden by the recent snow. North and east aspects have fairly good coverage down to 9000'.
The development of near surface facets at the boundary of new and old snow could turn a storm slab concern into a persistent slab problem. Especially with the colder treeline temperatures in the forecast.
There is a chance for more snow next week, so storm slab issues may continue.
New snow and wind from the southwest and west during and following Wednesday's storm have created the potentially dangerous wind slab. Storm and post storm wind velocities (20-40 mph) have been idea for the movement of lot of snow. As of early Friday morning (3/24), strong northerly winds started transporting snow, potentially loading southerly aspects and cross-loading eastern and western gullies and chutes. Video of north winds by Carlos Danel.
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, have signs of fracturing, cracking, or whoompfing sounds.
Thank you for all your support. Come celebrate with us at Pay N’ Take next Wednesday! Photo by Sabrina Carlson
This season numerous rescues have been conducted by 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.
During winter, backcountry permits are required to access the Kachina Peaks Wilderness. More info
Last updated on Friday, March 24, 2017
After two weeks high pressure and spring conditions, Wednesday and Thursday brought a surprisingly potent storm with 18-23 inches of new snow to high elevations on San Francisco Peaks. The storm was accompanied by moderately sustained winds out of the west and southwest, with gusts recorded into the high 40s (mph). On Friday March 24th, northerly winds and intensified, with lots of snow observed being transported in turbulent suspension across ridge lines. Temperatures on Friday will warm briefly, before a short wave trough passes quickly through our region on Saturday bringing light to moderate precipitation to the high courtly. Throughout the rest of the week, the weather will be breezy, unsettled, and more reflective of seasonally average temperatures.
On Friday morning, March 24th the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 72 inches (183 cm) at 9700' and Arizona Snowbowl reported 103 inches (262 cm) at 10800'. Since March 17th SNOTEL temperatures ranged between 24° and 58° F and Agassiz station between 13° and 49° 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.
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 email@example.com.