Just one inch of new snow was recorded at 10,800' over the past week. Otherwise, dry breezy and warm conditions dominated. No natural or skier triggered slab avalanches have been reported so far this winter. Midweek winds were replaced by building high pressure and above average temperatures. The snowpack is maturing, gaining strength, and in sun exposed locations, disappearing. Although residual pockets on unstable wind slab may remain, natural and skier triggered slab avalanches are unlikely until we receive more snowfall.
High temperatures (10-15 degrees F above normal) have created surface crust on most aspects where snow cover remains. This is true even at high elevations. In some areas, near surface faceted crystals have been observed growing at the base of the crusts. Although, this condition isn't an immediate concern, these poorly bonded facets may provide the weak layer ingredient once new snow accumulates above. This could bring about potentially hazardous avalanche conditions once a foot or more of snow falls from the sky or fills in from the wind.
On lower elevation slopes, poor coverage continues and has gotten worse. Obstacles abound and coverage is spotty. Backcountry skiers and boarders will need at least another foot of snow for favorable travel conditions.
Near and Above Treeline:Our shallow snowpack is becoming the persistent and degenerating problem. Sunny slopes have lost significant snow, making for challenging route-finding and lots of booting. The lack of new snow can give rise to the scarcity heuristic trap. With precipitation on the Christmas horizon, let's not spoil the holiday with errors in judgment.
At high elevations on east facing aspects, facet crystals were found in association with a surface crust. These formed near the top of the snowpack, where dramatic temperature differences (gradients) create small poorly-bonded angular crystals called near surface facets. These are not problems right now, however, a load furnished by a slab of new snow on top can cause failure (collapse) of this weak layer triggering an avalanche.
Below Treeline:At lower elevations snow is spotty at best. In wind protected areas in the Inner Basin, surface hoar was observed. These beautiful jewel-like crystals of frozen dew form on the surface when skies are clear, warm days are followed by cool nights and relative humidity is high. Inversion conditions are particularly favorable to its formation, so surface hoar tends to form in basins.
Surface hoar looks like fragile sparkly feathers on the snow surface. These crystals, like other persistent weak layers, only become a problem when new snow accumulates on top. Keeping track of locations and environments where this weak layer forms or lingers can be helpful in remembering where hidden instability resides.
Wind has been variable over the past week. Although wind speeds have been ideal for blowing snow, most of what is left on slopes is packed and resistant to movement. Still, some pockets of unstable hard slab may remain, or may have formed recently. These are likely to be smaller in size, but susceptible to triggering by skiers.
See our courses page for upcoming free courses, including a FREE Introduction to Avalanches Seminar at Aspen Sports on January 3.
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.
Uphill travel on terrain within the Arizona Snowbowl ski area opens December 21st. 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.
Over the week we experienced normal to above normal temperatures. Winds have been variable. On Tuesday night and Wednesday, winds out of the north and northwest had ideal velocities for transporting snow. The weather station at the top of Snowbowl (ASBTP) recorded sustained winds of over 20-40 mph between 11:00 pm Wednesday and 4:20 am Thursday. A maximum gust of 70 mph was recorded at 11:50 pm on Wednesday.
This coming week, dry conditions will continue until Christmas, when we are hoping for a pattern change and gift from Santa.
On Friday morning December 21st, the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 12 inches (30.5 cm) at 9,730 feet, and Arizona Snowbowl reported a settled base of 27 inches (68 cm) at 10,800 feet. So far this winter, 42 inches (107 cm) of snow have fallen at the mid-mountain study site. Since December 14th, SNOTEL temperatures have ranged between 15°F on December 18th, and 47°F on December 20th. For the same period, at AZ Snowbowl Top Patrol Station (ASBTP 11,555 feet) temperatures ranged between 17°F on December 18th, and 39°F on December 20th.