Avalanche hazard has increased over the last 24 hours with the addition of at least 2.7 inches of snow water equivalent (SWE). St. Valentine delivered in the form of rain at and below 11,000', and high density snow above. This is an exceptional load on the snowpack and may overburden the weak faceted layer buried since December. This is the same layer that failed last week naturally in Snowslide Canyon due to wind loading, resulting in a significant avalanche.
Fortunately the snowline fell to below 9500' as the cold front passed through Thursday evening.
The storm cycle is forecast to continue. The potential for natural and human triggered avalanches will be likely on slopes exceeding 30 degrees. Problems will be new wind slabs, new storm slabs and persistent slabs. The addition of new snow may transition our persistent slab problem into more of a deep persistent slab problem as slab thickness exceeds the threshold of one meter.
For the near future, travel in avalanche terrain near and above treeline may be hazardous. Give the new high density snow a chance to bond with the snowpack below, by waiting it at least 48 hours, and moderating slope angles to near or below 30 degrees. Avoid wind loaded areas and test each slope for signs of instability.
Read more about rain on snow and the possibility of near surface facets.
Rain on snow may saturate the snowpack, percolating down to a weak layer and decrease cohesion between snow layers. Also, rain will increase the temperature of the surface, creating a temperature gradient suitable to produce near surface facets (NSF) when new snow does accumulate. NSF may prevent new storm/wind slabs from bonding well to the existing snow.
There was a crown line about 1/2 way down Jay's Slide - a NNW aspect on Fremont. Picture from Sat. Feb 9, 2019. This avalanche likely occurred on Tuesday or Wednesday, February 5 or 6. Estimated classification is U N R2D2 U.
Near and Above TreelineWatch for new wind slab development on northeasterly and easterly aspects. Winds have blown from the southwest and are forecasted to blow southwest and west over the weekend. Cornices are starting to form on the lee side of ridgelines. Avoid cornices, as they may collapse unpredictably and send you downslope.
Persistent strong winds have characterized the last two weeks. Most recently, intense north wind up to 65 mph scoured windward terrain above treeline on Monday, February 11. Many southwest, west and northwest slopes are scoured above treeline.
Watch for reactive persistent weak layers. These may be buried under 2 to 5 feet of slab on east, northeast, north, and northwest aspects. Southeast aspects may be suspect as well, especially on slopes connected to easterly aspects.
Prior to this week's precipitation, wind scoured zones reduced the snowpack down to bedrocks in some areas. Crampons and ice axe will be helpful where new snow has been packed into hard wind board. You may find this on southwest and west slopes. Perhaps others.
Below TreelinePersistent slab instabilities exist in our snowpack. A reactive, faceted weak layer exists about 30 to 50cm from the ground, primarily on east, northeast, north and northwest slopes.
Recent avalanche activity indicated that the buried December facets can be reactive with enough load. This weak layer will be re-energized by the addition of new dense snow and rain from the St. Valentine storm. Carefully assess run out zones and terrain traps.
You will find great skiing and riding on low avalanche hazard slopes if you stay off of and out from under slopes that are greater than 30° in steepness.
Watch for new wind slab development on northeasterly and easterly aspects. Winds are forecasted to blow southwest and west over the weekend. Cornices are starting to form on the lee side of ridgelines. Avoid cornices, as they may collapse unpredictably and send you downslope.
Post weekend winds may create slabs on other aspects. As always watch for cross loading in gullies. Slopes just below ridges and on the flanks of shoulders should be considered suspect.
New storm slabs and wind slabs will be your primary problem over the next week.
Keep an eye on the ASBTP weather station. Readings between 15 and 35 mph indicate the potential for snow transport and formation of wind slabs. Look for various links under the weather menu above. Note that this station may get rimed during storms and report erroneously.
Give new snow time to bond with the old snow. Waiting at least 24 hours after new snow, and 48 hours after wind events will decrease your chances of finding unstable storm/wind slabs.
Weak faceted snow-layers under old and new wind/storm slabs exist at elevations above 10,000'. Column test in Beard Canyon on January 11th and near Alison Clay Bowl on February 4th indicating persistent slab problems. The Snowslide Canyon avalanche was triggered by wind loaded snow falilng on a persistent slab.
This problem has been found below, near and above treeline on northwest, north, northeast and east aspects. Reactive stability-test failures have occurred near and below treeline. The Snowslide Canyon avalanche crownline was above treeline approximately 12,000'
This problem is not widespread and predicting its location has been problematic.
The addition of new snow may transition our persistent slab problem into more of a deep persistent slab problem as slab thickness exceeds the threshold of one meter. Hopefully we will find evidence that this problem is healing and we can remove from future updates.
Crown from Snowslide Canyon natural avalanche. Failure occured on December facets, likely Tuesday or Wednesday, February 5 or 6.
Weather updated February 15th. Northern Arizona continues to be in a favorable flow pattern, with short wave troughs bringing modest amounts of precipitation followed by our customary snow stripping winds out of the north. On February 11th, Arizona Snowbowl (10,800') reported 9" of new snow. This was followed by nuking winds out of the north northwest 30-40 mph and gusting to above 60 mph. On windward aspects above treeline, the snowpack was scoured down to boulders.
At the time publication, we are in a pause between a series of disturbances. The formidable and warm St. Valentine storm has passed and the wind kicked up on Friday, which was evident from ridge top snow plumes. Thursday’s warm and wet Valentine storm resulted from a cold front tapping into significant subtropical moisture, resulting in high elevation rain and some very wet snow above 10 ,500 feet.
Our current storm cycle is expected to relax in intensity over the weekend, as colder post-frontal temperatures arrive. Lingering snowfall will persist on President's Day across much of northern Arizona. The snowline is expected to drop to as low as 3500 feet, with snow showers continuing into the workweek, but with little additional accumulation anticipated. On the distant horizon, prediction models are still too far out for precise determinations, but storm patterns appear to be setting up for more vigorous storm activity later in the upcoming week.
On Friday morning, Feb. 15th, the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 53” (135 cm) at 9,730'. Arizona Snowbowl reported a settled base of 72” (183 cm) at 10,800'. So far this winter, 188" (477 cm) of snow have fallen at the mid-mountain study site. Since February 8th, SNOTEL temperatures have ranged between 5° F on February 12th, and 40° F on Feb. 12th. For the same period, ASBTP (11,555') reported temperatures between -4° F on Feb. 11th, and 34° F on Feb. 12th.
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.