Avalanche danger is expected to rise quickly on all aspects near and above treeline by Friday morning, January 20th. Natural avalanches will be possible and skier triggered avalanches likely. Sizes of avalanches are expected to range from MEDIUM to LARGE during and for at least 24 hours following the storms.
Backcountry travel in avalanche terrain is NOT recommended for the period starting on Friday morning, extending through the entire weekend and potentially into midweek.
Due to avalanche mitigation work Arizona Snowbowl ski area will close uphill access from Friday evening January 20th to Sunday morning January 22, 2017.
Locations above 7000’ are expected to receive 12 and 24 inches of new snow, with snow water equivalent (SWE) between 2 and 3 inches. Heavy snow loads will accumulate on several observed potential weak layers, including old wind slab, buried surface hoar, near surface facets and depth hoar. The good news is that our base is quickly getting deeper. Human triggered and natural avalanches were reported over the weekend.
Our first reports for the season of natural and skier trigger avalanches started last weekend. These were small to medium sized soft slab avalanches on Telemark Path, Alison Clay Path, and in Heck No Chute in the Flagstaff Spring Cirque. Widespread loose snow avalanches (sluffs) were also reported on steeper slopes. Several weak layers have been identified in the snowpack, some showing fracture propagation potential during snow stability tests.
The National Weather Service in Flagstaff has issued a WINTER STORM WARNING - IN EFFECT FROM 6 AM THURSDAY JANUARY 19 TO NOON MST SATURDAY JANUARY 21 FOR NORTHERN ARIZONA LOCATIONS ABOVE 5500 FEET.
Avalanche problems will be primarily STORM SNOW, during and soon after precipitation events, however we anticipate lingering WIND SLAB and PERSISTENT SLAB problems, potentially lasting through next week and beyond. Steep slopes on all aspects above and near treeline will be vulnerable to avalanches. Untrained or inexperienced users are particularly vulnerable to the lure of powder without recognition of the potentially deadly consequences.
Near and Above Treeline:On Saturday January 14th, backcountry skiers triggered a soft slab avalanche in Alison Clay path. This SS-AS-R2-D2 slide ran for approximately 300 meters leaving a debris pile 10 to 15 meters wide and a crown depth of 6 to 12". A natural soft slab avalanche was observed in the Telemark Path in the Inner Basin (SS-N-R2-D2). This was reported by the Level 2 avalanche class on Sunday January 15th, probably releasing on Saturday afternoon or evening. On Wednesday January 18th, a group of skiers in the Inner Basin observed a crown line in Heck No Chute, showing evident previous slab avalanche activity (size and character unknown). Reporting your observations, stories, and mistakes can save lives!!!
These events are not surprising as numerous stability tests performed last weekend revealed persistent slab problems, including MLK weekend storm-snow poorly bonding to the rain crusts from the previous week. Careful examination reveals tiny faceted crystals both above and below the crusts at many locations (S, SW, and W) between 10000 and 11500 ft elevations. Both Extended Column (ECT) and Propagation Saw Tests (PST) occasionally indicated significant fracture propagation propensity, however variability was high, with some test not propagating at all.
Last weekend, there were numerous reports of surface hoar development on a range of aspects and elevations.
Midweek observation indicated that moderate to strong northwest wind and sun was destroying surface hoar in the Hard Core Ridge area. However, large surface hoar was reported at lower elevations (<11000 ft) near Fremont and Doyle Peaks on January 18th. Surface hoar buried and preserved under newly arriving snow would make a deadly mix. Look for slab over surface hoar on the slopes of cirques, bowls and chutes.
Below Treeline:Even in the forest below treeline, instabilities have been noted, however spatial variability is very high. Even within the same test pit, results of stability tests have shown contradictory results. Some loss of propagation energy has been noted since last weekend. Overall snowpack structure is weak, strength is moderate to strong and propagation propensity (energy) highly varied, but improving. As discussed in above, watch for surface hoar buried and preserved under new snow.
Backcountry skiers and boarder should be particularly aware of (and avoid) terrain traps, such as the steep sided narrow gullies that characterize sub-treeline conditions of the Peaks. Stay out of these when possible and cross them one at a time with spotter if they cannot be avoided. Low angle (<30°) tree skiing, with no steeper slopes overhead will be your safe bet.
New storm snow will need time to bond with the old snowpack. This generally happens within 24-48 hours after the end of precipitation.
Southwest, west, then northwest winds are forecasted through the weekend. Windslab and wind loading are expected, and these problems could linger through the week.
Our persistent slab conditions (discussed above) may be reenergized by the added load of new snow. As the name implies, persistent weak layers can remain reactive for days after storms. This problem is difficult to predict and manage, and requires a wide safety buffer to handle the uncertainty.
Persistent weak layers include: buried surface hoar, depth hoar, near-surface facets, or faceted snow. 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.
Travelers are advised to exercise caution and make slope specific evaluations. 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.
Want to learn more safe backcountry habits? KPAC offers level I and II avalanche courses. They are filling up fast!!!
During winter, backcountry permits are required to access the Kachina Peaks Wilderness. More info
As KNAU’s Gillian Ferris says, "a winter storm trifecta" is currently impacting our region, as well as, large portions of western United States. These storms will bring the most intense winter precipitation we have had this season. Snowfall total could potentially exceed 24 inches and load the old snowpack with 2-3 inches of snow water equivalent (SWE).
The first pulse was a short wave trough, starting on Thursday and was forecasted to lay down less than a foot at higher elevation. Snow level started at 6000 feet and dropped to 5000 feet by Thursday night. Snowfall will taper off on Friday morning.
A second pulse arrives on Friday evening and lasts through Saturday morning. This slightly longer wave trough will have the greatest precipitation potential. We anticipate heavy snowfall with accumulations in the 1 to 1.5 foot range above 10000 ft.
A fast moving short wave ridge will create another brief break until a third colder pulse arrives on Sunday night. Atmospheric instability is expected to last into midweek with lingering snow showers. Snowline will drop to 4500 feet, with breezy winds out of the southwest lingering. Subfreezing maximum temperatures and moderate winds will prevail for the first half of the workweek above 7000 ft.
On the morning of Friday January 20th the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 68 inches (173 cm) at 9700 ft, and Arizona Snowbowl reported 79 inches (201 cm) at 10800 ft. These values are expected to increase dramatically as heavy precipitation continues periodically for the next several days. Since January 14 the SNOTEL temperatures ranged between 11 and 35°F and Agassiz station between 12 and 25°F.