Natural avalanches unlikely; human triggered avalanches possible.
NW, N , NE, and E aspects avalanched during the February 27/28 storm. Similar slopes that did not avalanche during the February 27/28 storm should be considered suspect. Some of the avalanches filled back in with windblown snow, potentially reburying weak layers with a new load. Ski and skin tracks may be found on these suspect slopes, but this does not prove that the slopes are safe. There may be sensitive trigger spots lurking in steep isolated terrain.
Strong southwesterly winds may create new windslab problems over the weekend.
A warming trend late next week may introduce water into the snowpack, leading to destabilization, primarily on S, SW and W aspects - where a "facet sandwich" combination of layers may exist about 20 to 30cm below the surface.
Strong winds blew and 20-30 inches of snow, with 2-3+ inches of snow water equivalent (SWE) accumulated during the February 27/28 storm. This resulted in numerous storm and wind slab avalanches in Humphreys Cirque and Hardcore Ridge area. Crownlines of 1 to 2 ft on NW, N , NE, and E aspects have been observed. These likely released early Tuesday morning before sunrise.
On Tuesday morning, cracking, whoompfing and collapsing was observed below treeline on southerly aspects of Gully Two (Agassiz Peak) and westerly aspects of Humphreys Peak. The likely cause is a layer of facets between two melt freeze crusts about 20-30cm down in the old snowpack. These developed when the cold Presidents weekend snow accumulated on a warmer melt/freeze crust. Subsequent warming created the upper crust of a facet sandwich, then the February 27/28 storm loaded these layer, leading to collapsing and whoompfing.
During avalanche mitigation work on Wednesday morning, March 1st, Arizona Snowbowl Ski Patrol triggered a slab avalanche in upper bowl, which resulted in a portion of spur catwalk becoming buried with debris. These areas were closed to the public during the mitigation work.
The avalanche was triggered near the false summit of Agassiz peak with a hand thrown explosive. 110cm slab collapsed on a rain crust bookended by two facet layers. There were 1cm facet layers on either side of the crust and the lower faceted layer was failing onto the older snow. The avalanche ran down approx. 650 vertical feet and buried part of spur catwalk. This is the second time this year this area has run and the last one was bigger. SSW winds have loaded over 5m of snow over the course of the winter on this aspect (N). Just a friendly reminder to cautiously watch for loading in the backcountry and pay attention to the weather.
Near and Above Treeline:Over the weekend, wind loading below ridgelines may occur, as well as cross-loading in chutes below the ridge lines. Hard wind slab and rime ice may be found above treeline. Ice axes and crampons may help prevent a slide for life on steep icy slopes. Much of the snow above treeline has been wind affected.
Uncertainty remains on whether the previously discussed facet sandwich remains sensitive. Spatially variable pockets of instability should be suspected on southern, southwestern and westerly aspects near treeline - above and below. Near surface facets, like all facets, tend to persist as weak layers upon which avalanches are potentially triggered.
The facet sandwich on southern, southwestern and westerly slopes may become an issue on warm, windless sunny days and a traveler may trigger a wet slab release.
Near surface facets may be found just below the new February 27/28 storm snow. As discussed above, these may be the culprit for many avalanches that released during the storm. Suspect slopes are those that did not release or slopes where new windblown snow filled in slopes that did release. These slopes may be found on NW, N , NE, and E aspects.
Dangerous overhung cornices with large cracks have been observed on Humphreys Peak Ridge and Hardcore Ridge. These are fairly unusual locally, at least in terms of the extent of overhanging structure. Don't be caught off guard!
Cornices can be extremely dangerous as they can collapse under the weight of a person, or with warming temperatures, and potentially trigger avalanches on slopes below. When traveling on ridges, give cornices wide berth, by route finding well to the windward side of the ridge.
Mid to late in the week unstable wet snow may develop as treeline temperatures push 40° F. These condition will be found primarily on southerly and westerly aspects, first at lower elevations, and then progressing towards treeline.
Below Treeline:Good powder snow exists on sheltered northerly slopes. Watch for unstable snow mid to late in the week as below treeline temperatures push 40+°F, primarily on southerly and westerly aspects.
Watch for weekend winds depositing snow on leeward slopes, as well as cross loading gullies and chutes. Wind slabs usually stabilize in less than a week. Reduce your exposure to avalanche hazard by waiting several days after a big loading event.
Wind slabs have been found on every aspect this season: be alert for loaded areas near ridgelines and cross-loaded terrain features; hollow sounding slabs, and fracturing or whumpfing in the snowpack.
Always keep in mind, wind slabs are unpredictable, and may support the weight of a skier or rider initially, and fail suddenly with tragic consequences. Avoid snow surfaces which are recently loaded, sound hollow, have signs of fracturing, cracking, or whoompfing sounds.
As treeline temperatures climb above freezing next week, watch for snowballing, pinwheels, and small wet slabs indicating the potential for larger wet slabs. Southerly and westerly aspects will be your primary culprits and wetting conditions will will be initially evident at lower elevations and progress higher later in the day.
Suspect slopes are those that did not release (during the Feb. 27/28 cycle) or slopes where new windblown snow filled in slopes that did release. These slopes may be found on NW, N , NE, and E aspects. These suspect slopes may get loaded by windblown snow over the weekend.
Ski and skin tracks may be found on these suspect slopes, but this does not prove that the slopes are safe. There may be sensitive trigger spots lurking in steep isolated terrain.
We are unsure of just how persistently weak these slopes may be.
Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This season numerous rescues have been conducted by Coconino County Search and Rescue, and the Arizona Snowbowl Ski Patrol. Some of these could have been avoided by better planning and preparation.
Travelers are advised to exercise caution, make slope specific evaluations and most of all, know where you are going and be prepared for the unexpected.
As always, please treat this summary with appropriately guarded skepticism, make your own assessments, and contribute to our body of knowledge by reporting your observations.
Arizona Snowbowl uphill policy.
Want to learn more safe backcountry habits? KPAC offers level I and II avalanche courses. There is one class left this season.
During winter, backcountry permits are required to access the Kachina Peaks Wilderness. More info
Last updated on Friday, March 3, 2017
Strong winds blew and 20-30 inches of snow, with 2-3+ inches of snow water equivalent (SWE) accumulated during the February 27/28 storm.
A short lived warming trend develops on Friday and Saturday. Windy conditions will develop Sunday ahead of a cold front. The cold front will move through northern Arizona from Sunday night into Monday, with light precipitation amounts possible. Monday will be on the cool side, about 5 or so degrees below average. From then on, expect warming temperatures the rest of the week as a strong ridge builds off the west coast. By Friday of next week, temperatures will be in the neighborhood of 10 degrees above average.
On Thursday evening, March 2nd the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 92 inches (234 cm) at 9700’, and Arizona Snowbowl reported 109 inches (277 cm) at 10800'. Since February 17th SNOTEL temperatures ranged between 2° and 40° F and Agassiz station between 0° and 38° F.