Recent and continued snowfall of 1.5 to three feet, accompanied by intermittent winds out of the S and SW have increased the possibility of dangerous avalanches. Snowfall was stretched out over several days, somewhat reducing the effect of rapid loading and giving new snow more time to bond. However, intermittent storm wind velocities have had the capacity for significant snow transport and loading. This may have created dangerous wind slab on leeward aspects (N and NE) and on wind-sheltered sides of gullies and ridges. Storm winds often deposit three times more snow load on leeward slopes than on the windward sides. Note that the winds have been predominantly out of the S and SW, but have been extremely variable around the peaks, so it is important to note wind-loading on all aspects.
For at least 24 hours after snowfall subsides, natural avalanches will be possible and human-triggered wind slabs will be likely on wind and storm loaded slopes (>30°). Post-frontal winds may extend the period of hazard and/or shift wind loading patterns to other aspects.
No new natural or human triggered slab avalanches have been reported since the late November cycle until this week. Snowbowl ski patrol reported triggering a soft slab avalanche on Christmas morning from the false summit of Agassiz Peak using explosives.
The primary layer of concern appears to be the new snow’s interface with the old surface. Where icy glazed surface conditions existed, stability may be delayed by slower bonding rates and a slick bed surface.
Observers noted some reactivity in a layer of near-surface facets on the morning of 12/27 on N and NW slopes. Columns and extended column tests (ECTs) repeatedly failed on isolation at faceting observed both above and below the 12/4 rain crust. Before descending suspect terrain, testing for propagation propensity using extended column tests (ECTs) or the propagation saw tests (PSTs) is strongly recommended. The aforementioned facet layer was identified in association with our earlier rain crust and seemed most problematic on NW to NE aspects.
Near and Above Treeline:Coverage at treeline is ~2 meters or more. Wind and storm slab dangers will be highest in the first 24 hours after stormy conditions subside. New snow and storm slab appear to be unconsolidated with 33" of new snow available for wind transport and subsequent loading in the coming days. ECTs and PSTs on northerly slopes have suggested higher levels of danger due to significant faceting both above and below the 12/4 rain crust, creating a snowpack with poor structure and poor strength on those aspects. With skillful stability assessment, route-finding, and travel practices, there is plenty of safe fun to be had.
Below Treeline:Human-triggered avalanches will be possible below treeline, but will be less likely than at higher elevations where wind moves more snow and wind slab formation is most common. Regardless, avoiding hollow sounding/feeling slabs on steep slopes is advisable until these have a chance to bond to the snowpack below.
Coverage for ski touring on many slopes above 10,000’ is very good for this time of year, but early season hazards such as downed trees, stumps, and boulders may still exist. At lower elevations (<9000’) in particular, new snow may hide obstacles just below the surface.
Intermittent storm winds at and above treeline have potentially loaded leeward slopes (>30 degree) with dangerous wind slab hazards. During a recent lull between storms, winds transporting snow out of the southeast were observed, contributing to slab hazards on leeward north and northwest facing slopes. Based on some observations, wind slab and cornice development seem to be in isolated pockets rather than widespread deposition.
Moderate westerly winds are forecast for Friday night 12/27 into Saturday 12/28, increasing the possibility of wind transport.
AZ Snowbowl patrol reported the release of small, thin (6"-10”) wind slabs in the Hidden Valley area of Upper Bowl on 12/26. Although not a significant hazard at the time, the occurrence is indicative of rising instability overall.
Wind slab is most dangerous soon after deposition when the new slab has not had enough time to bond with the snowpack below.The persistence of weak layers deep in the snowpack can cause extended periods of snowpack instability.
Storm slab avalanches are possible on steep terrain, mostly at and above treeline.
If weekend snowfall reaches or exceeds maximum forecast, storm slab avalanche hazards will increase significantly, as will potential sizes of avalanches. Storm slabs usually stabilize 24-48 hours after formation.
As always, note warning signs of instability such as whoomphing or shooting cracks underfoot while traveling on untracked snow. Areas of particular concern include convexities and pockets of deep snow.
With the amount of new, unconsolidated snow, sloughing is possible on all aspects, particularly with steeper slopes. Ski with caution above terrain traps, on convexities, and on steeper slopes.
Sloughing has been observed on the morning of 12/27 from ski cuts above treeline on NW to S aspects. Unconsolidated sloughing is likely on steeper terrain.
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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 27
A series of short wave troughs have impacted northern Arizona and will continue into the weekend. These have laid down 24” of snow at higher elevations, with another 12-15” possible by Saturday morning. Snow transporting winds, mainly out of the southwest have affected snow distribution. Ridge-top wind velocities have been optimum for moving snow and loading leeward slopes.
The snow-line has fluctuated between 6500-5500 feet. One and a half to two and a half inches of snow water equivalent (SWE) were recorded at the Snowslide SNOTEL and nearby weather stations.
Snow showers will continue into the weekend, but are expected to taper off on Saturday. The total snow storm accumulation is uncertain, as this sequence of precipitation events is dynamic and ongoing. Cool temperatures, breezy conditions, and lingering unsettled weather will characterize the first half of the coming week. Looking onward from midweek, snow showers continue to be a possibility.
Arizona Snowbowl Ski Patrol reports approximately 67” (170 cm) base at 10,800 feet. Snowslide SNOTEL reports a 48” (122 cm) snow depth. So far this winter we have had 125" (317 cm) of snowfall at 10,800 feet.
Since December 20th, SNOTEL temperatures have ranged between 9° F on December 26th and 46°F on December 21st. ASBTP station (11,555 ft) reported a low of 6° F on December 26th and a high of 46.5° F on December 21st.