The storm that impacted our region on Wednesday and Thursday, December 4 and 5, delivered at the low end of what was forecasted. Eight inches of new dense snow was recorded at the study site at 10,800 feet. The snow was heavy and wet, measuring at least 10% water. Winds shifted to the northwest as cooling turned rain to snow later in the storm. At lower elevations the rain effectively put an end to much of the powder skiing that was enjoyed over the last week. Mash potatoes and crusts were the results. No wet avalanches were reported.
Except above treeline (>12,000’), rain crusts are pervasive. Near treeline, crusts are buried beneath new snow or exposed where new snow has been wind scoured. Below the snow line of the recent storm (<9000’) water saturated snow exists, much of which has now refrozen with cooler temperatures.
Looking forward, deep slabs from the "White Friday Storm" seem to be bonding well with the snowpack below. Some localized instabilities may exist from the new snow, but natural avalanches seem unlikely. Pockets of instability in both new storm slab and newly formed wind slab may be sensitive to human triggers. It is probably prudent to avoid pillowed or hollow sounding slopes (>35 degrees) at and above treeline. Conditions should improve over the next 24 hours.
Several field observers have noted that the snowpack is gaining strength and showing little propensity to fracture. Most of these observations were made prior to the most recent storm. Concerns of avalanche hazard will be primarily confined to new snow and new wind transported snow.
Near and Above Treeline:The forecast for 9-15" of new snow near treeline on Wednesday/Thursday was not met. West winds were observed, but generally lighter than anticipated. The ASBTP weather station anemometer has not been reporting since Saturday morning November 30, presumably due to wind riming. As a result, wind velocity thresholds for transporting snow along ridge tops is somewhat speculative.
Below Treeline:Rain fell below 9,000 feet. In some locations nearly one inch was recorded. Overall, some snowpack ablation occurred and rain crusts formation was widespread. No wet avalanches were reported.
It should be noted that rain crusts provide fertile environments for near surface facet growth, so keep an eye pealed for this weak layer phenomena in the near future.
Below the snow line, rain from the recent storm has saturated the upper snowpack structure. The free water has refrozen, locking grains together and strengthening the snowpack. In some zones, the snowpack is behaving like isothermal spring-like conditions, rock hard in the morning, but softening during the day.
Coverage for ski touring is still good for early December, but early season hazards such as downed trees, stumps, and boulders may still remain hidden near the surface at lower elevations. Coverage above 9,000’ is still generally three feet of snow. Lower elevation snowpack (7500') has been adversely impacted by recent rain.
Pockets of wind slab may exist from snow movement during the most recent storm. Although both snowfall and wind speeds appear to have failed to meet forecast thresholds, pockets of wind slab are still possible. Watch for new wind slabs above treeline, particularly on easterly and southeasterly slopes. Pillowed snow has also been observed near ridge tops on northerly 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.
As of December 2nd, uphill travel at the Arizona Snowbowl is closed.
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.
Updated December 6, 2019
Normally, eight inches of new snow would seem significant, but in the aftermath of last week's "White Friday Storm" and our season's opening storm the week before, precipitation on December 4 and 5, hardly raises an eyebrow. Still, it is important to avoid complacency. The ASBTP station anemometer has not been reporting for some time. As a consequence, we have only observations of wind velocities, but little empirical data. Also of note, visibility has been restricted due to low cloud cover and fog. Rain and mist were reported at 11,500’ in the early stages of the storm. Winds were moderate out of the northwest as snow fell. Precipitation was mainly rain below 9,000’ with some locations reporting up to an inch.
On December 6 and 7, the sky will clear somewhat and temperatures will warm to seasonal normals as a broad high pressure ridge moves through our region. At treeline high temperatures are expected to rise slightly above freezing and lows in the 20s. Another A Pacific low is expected Saturday evening through Monday, but at the moment, precipitation amounts seem modest with Two to seven inches of snowfall forecasted.
Arizona Snowbowl Ski patrol reports a 52" base at 10,800 ft. Snowslide SNOTEL reports a 36” snow depth, down from a November 30th high of 52".
Since November 29, SNOTEL temperatures have ranged between 6°F on November 30th, and 44°F on December 2nd. ASBTP station (11,555') reported a low of 0°F on November 30th and a high of 41°F on December 2nd.