10" of dense snow from Sunday, January 6, has improved touring conditions, but has also negatively affected snowpack stability. Available snow for transport is currently being sublimated and transported by 30-40 mph north and northwest wind, potentially creating wind slabs on leeward slopes and cross loading terrain features.
Observers reported localized, reactive wind slabs near and above treeline in the Inner Basin, as well as cracking and whumpfing below treeline. This indicates the new load of dense snow combined with the weight of a skier is triggering the persistent facet layers sandwiched in the snowpack. Consider the consequences of an avalanche on the slope you are evaluating: size, runout, and destructive potential of a release.
Human triggered avalanches are possible, and likely on slopes with recent wind transport. Note that weak layer failure is also occurring on low angle terrain and below treeline, thus evaluating run out zones and terrain traps are advised.
Snowpack depth has increased to a depth of 70 to 100 cm on shaded and wind loaded aspects at and above treeline. Snow profiles and stability tests from west and north aspects above treeline indicate poor structure, moderate strength, and increasing energy and reactivity on the weak facets beneath wind slabs and new snow. See the profile below.
Column test in Beard Canyon today indicating reactive snowpack.
Small and localized skier triggered wind slab releases above treeline have been observed this week.
Near and Above Treeline:Localized wind slabs are sensitive and fracturing under the weight of a skier/boarder. The 10" of new snow is much denser than previous storms and is overloading the weak faceted layers within the snowpack. Increased fracture propagation and energy has been observed in stability testing at and above treeline, indicating an increasing trend in the probability that new precipitation forecast this week will increase the avalanche hazard.
Additional precipitation warrants an assessment of wind loading in terms of aspect and elevation. Above treeline slopes are much more susceptible to wind slab formation.
Remember most slab avalanches occur during a storm or within 48 hours of accumulation.
Below Treeline:Cracking and whumpfing have been observed below treeline, an indicator of a reactive weak layer within the snowpack. The near surface facets from the cold spell in late December are collapsing under the weight of the new snow and skier traffic. Carefully assess run out zones and terrain traps.
The recent storm was not sufficient, however, to cover the many obstacles at lower elevations as the snowpack depth tapers to 15-20" below 10,000'.
Wind scouring and loading is occurring above treeline with north winds at 30-40 mph. The next few days are forecast for milder conditions, however, currently, localized wind slabs are fracturing above treeline on loaded aspects.
Wind slabs can form in extremely localized areas. Often only a few inches separates safe snow from dangerous snow. We often hear people say, “I was just walking along and suddenly the snow changed. It started cracking under my feet, and then the whole slope let loose.” - from avalanche.org
Keep an eye on the ASBTP weather station. Speeds between 17 and 30 mph are ideal for transporting snow and creating wind slabs. Look for various links under the weather menu above.
Weak faceted snow-layers under old wind slabs exist near and above treeline. New warmer high density snow that fell on January 6th has increased this problem by adding heat to the surface and aiding in facet development at the boundary with the cold snow below.
Wind slabs may also overload persistent weak layers in the upper snowpack. Extended column tests and collapsing snow have revealed this problem to exist below and above tree-line on northerly, northeasterly and northwesterly aspects.
Column test in Beard Canyon today indicating persistent slabs.
The snowpack is exhibiting increased energy and propagation during recent stability tests. Carefully evaluate slopes on all aspects and elevations.
Precipitation is in the forecast for the next few days with varying levels of confidence in actual snow totals. The next significant storm seems most likely for Wednesday - Thursday, January 16-17. New snow load will increase the reactivity of weak layers within the snowpack and avalanche potential.
The bullseye slope angle for avalanche activity is 38 degrees. Moderating slope angles to 30 degrees and less drastically reduces the likelihood of triggering an avalanche.
See our courses page to register for a class and get avy savvy.
Backcountry permits are required for travel in the Kachina Peaks Wilderness and available at local USFS locations, as well as, at the Agassiz Lodge on Saturday and Sunday 8:30 -11:30 a.m.
For information on uphill travel within the Arizona Snowbowl ski area, please refer to www.flagstaffuphill.com and https://www.snowbowl.ski/the-mountain/uphill-access/ for details. Access to the Kachina Peaks Wilderness is available from the lower lots at Snowbowl via the Humphreys Trail and Kachina Trail.
The storm that hit last Saturday night and Sunday, January 6th, dropped 10 inches of wet snow at 10,800 feet on San Francisco Peaks. Were it not for its high density, there would be little concern, however; Snowslide SNOTEL station at 9,730 feet recorded 5 inches, and over one inch of snow water equivalent (SWE) making the snow about 20% water. Ten inches of snow at this density on our structurally questionable snowpack has resulted in increased reactivity and propagation in stability tests.
Since then, gradual warming and average temperatures characterized the week. Wind has been light to moderate out of the south and southeast, until today when 30-50 mph north winds transported and sublimated much of the snow available for transport.
A short wave trough moves through this weekend with cloudy skies and snow flurries on Saturday and Sunday, January 12th and 13th. The following week will bring increasingly unsettled weather Monday through Wednesday with good chances of measurable precipitation, as a short wave passes over us on the leading edge of a more significant storm system.
At the time of publication, it was still too early to forecast the precise timing or impact of this major storm system, but it appears that at least the southern edge will hit us. The snow level is expected to be around 6,000 feet along the I-40 corridor and 7,000 feet to the south.
On Friday morning, January 11th, the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 20 inches (51 cm) at 9,730 feet, and Arizona Snowbowl reported a settled base of 40 inches (102 cm) at 10,800 feet. So far this winter, 79 inches (201 cm) of snow have fallen at the mid-mountain study site. Since January 4th, SNOTEL temperatures have ranged between 19°F on January 8th, and 44°F on January 4th and 9th. For the same period, the AZ Snowbowl Top Patrol Station’s (ASBTP 11,555 feet) temperatures ranged between 12°F on January 6th, and 41°F on January 4th.