Last weekend's storm produced low clouds, drizzle and wind rime, but very little new snow. Arizona Snowbowl reported a meager three inches of snow at 10,800' on the 9th of December, with up to six inches of rime. One quarter to half an inch of rain fell at elevations below 12,000' creating new crusts in many locations. Thankfully the storm ended cold, covering high elevation crusts with a thin layer of snow. The new snow was dense and moist, bonding quickly with the snowpack below.
Since then, cool temperatures and dry conditions have allowed new snow to bond, stabilizing the upper snowpack. Winds have been light to moderate out of the southwest and west. No new avalanches have been reported since the late November cycle.
Wind exposed terrain has been stripped of new snow cover, resulting in icy “slide for life“ conditions. Crampons and ice axe or self-arrest ski poles may be necessary to prevent dangerous sliding falls.
Worthy of note and of growing concern is the development of faceted near surface crystals observed by several observation teams. These are associated with the rain crust from the December 4-5 storm. Observers have noted this new weak layer of near surface facets (NSF), has demonstrated reactivity by propagating a fracture when using Extended Column Tests (ECT). Facets were noted on both top and bottom of this crust.
Where this layer has been found, the snowpack can be described as demonstrating high strength, poor structure and some propagation propensity. At the moment, high strength of the slab above makes triggering a slide on this layer seem unlikely, however, this is speculative since slab strength is extremely variable across the landscape. In the future, this layer may become very troublesome when loaded with new storm snow or wind slab. Read more about near surface facets in the Avalanche Problem I section.
Until the next storm or wind event, the likelihood of natural avalanches seems low. However, human triggered avalanches where the new weak layer is found, may be possible. Do not let your guard down. Localized pockets of instability can linger after generalized conditions stabilize, especially on shaded aspects and colder elevations.
Wind slabs formed on top of near surface facets could be particularly reactive. Even a few hours of sustained winds of 15-40 mph can create dangerous wind slabs on leeward facing slopes and on cross loaded gullies at and above treeline. It is always prudent to avoid pillowed or hollow sounding slopes (>35 degrees) until stability tests indicate good bonding with the snow below the slab.
Snowpack stability could become threatened by a change in our current fair weather pattern. Colder unstable conditions are in the forecast, as current high pressure is replaced by a cold front. The forecast is for light snow. If current predicted snowfall is exceeded, an increase in avalanche hazard may be the result.
Near and Above Treeline:With only a few inches of new snow covering a rain crust from last weekend’s storm, snow surface conditions and ski-ability are highly variable. A layer of near surface facets has been observed on colder aspects (N, NE) at 10,800' and below. The ASBTP weather station anemometer was not reporting regularly due to wind riming. As a result, wind velocity thresholds for transporting snow along ridge tops is speculative. However, considering the moisture in the last snowfall and cold temperatures since, not much snow is available to move.
Below Treeline:Last weekend rainfall and wet mist were observed up to 12,000 feet. New surface crusts were formed in most locations. Some crusts were covered by a thin layer of new snow, which fell as the storm cooled at its culmination.
Coverage for ski touring is still good for this time of year, but early season hazards such as downed trees, stumps, and boulders may still remain hidden near the surface at lower elevations.
Persistent slabs develop when a cohesive slab of snow overlays a persistent weak layer. Persistent weak layers develop within the snowpack as a result of imbalances in temperature/vapor pressure that force molecules of water vapor to migrate and form faceted crystals. The resulting angular crystals create poorly bonded weak layers upon which avalanches can be triggered. Persistent slab avalanches make up the majority of what are called “delayed action avalanches” which occur days after a storm has ended, often in response to skier or boarder weight.
Read more for hints on assessing persistent slab problems
This type of avalanche problem is challenging to forecast, since the avalanche occurrences are commonly a result of collapse of the overlaying slab. Since slab strength is a function of thickness and hardness, which can be highly variable even on a particular isolated slope, slab strength assessment can be misleading. Persistent slab problems can last for weeks, and are often re-energized by the addition of new snow.
Digging pits and using the Extended Column Test (ECT) or the Propagation Saw Test (PST) are your most reliable tools. Remember, if the test produces fracture propagation, no matter how strong the slab, the result is always a bad sign and should influence your critical assessment and decision making.
Pockets of wind slab may exist from snow transport during the most recent storm. Although both snowfall and wind speeds failed to meet forecast thresholds, pockets of wind slab are still possible. At near treeline and below, these may be on top of a buried weak layer of near surface facets, making the situation even worse. Watch for new wind slabs above treeline, particularly on northeasterly aspects. Pillowed snow was also observed near ridge tops on north aspects.
This problem should be confined to near and above treeline terrain. Wind slabs are most reactive during formation, and the first day or two after, except where they sit above persistent weak layers where instability may linger.
As of December 13th, uphill travel at the Arizona Snowbowl is OPEN. Also, the Kachina Peaks Wilderness is accessible by the Humphreys trail and the Kachina trail from the lower parking lots at Arizona Snowbowl.
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.
Weather updated Friday December 13
Following mid-high elevation rain and a few inches of snow last weekend, conditions have been dry, cool and occasionally breezy. Mountain temperature were moderate (20- 30s deg F) having a generally stabilizing influence on the snowpack.
Looking forward, brisk winds out of the west and southwest (15-25 mph) will precede an approaching cold front. The storm will arrive on Saturday bringing cooler temperatures and wind, but not much snow. Accumulations are expected to be 1-2 inches.
The aftermath of this front will issue in cold weather through the middle of the upcoming week. High elevation temperatures will sink to the low teens and single digits. Some warming will follow, as high pressure briefly prevails, but another storm could materialize by next weekend. Overall, zonal flow seems to be establishing itself, with most of the mid-latitude cyclonic energy passing to our north.
Arizona Snowbowl Ski Patrol reports a 49” (124 cm) base at 10,800 ft. Snowslide SNOTEL reports a 32” (81cm) snow depth. So far this winter we have had 91” (231 cm) of snowfall at 10,800 feet.
Since December 6, SNOTEL temperatures have ranged between 12° F on December 9th and 44°F on December 12th. ASBTP station (11,555') reported a low of 14°F on December 9th and a high of 41°F on December 6th.