By Saturday, new snow and strong wind is expected, increasing overall avalanche hazard. The forecast predicts strong wind and moderate amounts of snow through Sunday, Feb. 19th. Near surface facet growth in the top 20-30 cm of the pre storm snowpack may dramatically increase the hazard with enough snow and/or wind loading.
On Feb. 13th, an avalanche crown was observed on ~NW slopes of Core Ridge below treeline. This looks like a storm slab that likely released sometime during the Feb. 11/12 storm cycle (see below photo). Cracking and collapsing wind slabs were observed on February 14/15 on westerly and southwesterly slopes of Temptations Chutes and Agassiz Peak. Some minor roller ball activity was observed on steep (>45°) ~southerly slopes in the Humphreys Cirque area on Wednesday, February 15th. Wind loaded southwest aspects may be reactive, especially above treeline.
Surface hoar was observed on northerly slopes, below treeline in the Inner Basin. Wind associated with the next storm may destroy these beautiful weak layer crystals. However, in wind sheltered locations, buried surface hoar may act as a persistent weak layer.
Last weeks warm spell has densified most of the snowpack, more so on southerly and sun exposed aspects. New weak layers will most likely be established in the Feb. 11/12 snow due to a strong temperature gradient in the upper 20 - 30 cm of the existing snowpack.
Spring-like on Wednesday and Thursday, February 15-16, have helped settle instabilities associated with this past weekends storm snow and initiate a melt freeze cycle. Instabilities may still exist in steep terrain loaded by the northeasterly winds of Feb. 14/15th, with wind slabs and cross loading developing on west and south aspects near and above treeline.
Near and Above Treeline:The storm forecast for Saturday and Sunday, February 18-19, may create new storm and wind slabs. Time is needed for the new snow to bond with the snowpack below. Most avalanche accidents occur within 24 hours of new snow.
The wind slabs that developed on west and southerly aspects on Feb. 14/15 may become more sensitive with additional snow and wind loading. In some places, these slabs may be resting on weak near surface facets that started developing when last weekend's cold snow settled onto a warmer snowpack. The warm/cold imbalance created a temperature gradient that allowed for facet growth. Recent pit analysis reveals a lingering strong temperature gradient in the top 20 -30 cm of the snowpack. Facets will continue to develop 20 - 30 cm below the old snow surface, potentially creating a persistent weak layer upon which new storm and wind slab will rest. New snow may be too much for this newly forming weak layer, possibly resulting in natural and human triggered avalanches.
Below Treeline:On Wednesday and Thursday, February 15-16, soft recrystallized powder snow could be found on shady north aspects below treeline. Sun and radiation affected slopes developed melt/freeze crusts, along with reports of good corn snow and spring skiing.
Coverage is great above 9000'. The warm temps have melted much of our snow below 8000', especially on sunny slopes.
New snow may create storm and wind slabs, especially above treeline. Even with light precipitation amounts, wind can deposit significant amounts of snow on leeward slopes, as well as cross load gullies and chutes. Forecasted warm temperatures for February 20-22 may increase slab failure potential as the temperature rises.
Wind slabs usually stabilize in less than a week. Reduce your exposure to avalanche hazard by waiting several days after a big loading event.
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.
Cold new snow falling on a warm snowpack may set up an environment for rapid production of tiny near surface facets at the interface. The snowpack has gained some heat the past two days, especially on sun exposed slopes at higher elevations, and on many aspects below 10500'. The addition of cold snow on top will create a favorable environment for rapid near surface facet growth. This will delay bonding between new snow and the snowpack below.
Winter travelers are urged to pay particular attention to how well new snow accumulating February 18-19, bonds with the old snow below.
If new snowfall reaches or exceeds maximum depths forecasted, storm slab avalanches may become likely. The development of near surface facets at the boundary of new and old snow could turn a storm slab concern into a persistent slab problem.
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 Thursday, February 16, 2017.
The precipitation of Feb. 11/12 delivered a nice refresh to the peaks. It appeared that the inner basin received more snow than the western side of the peaks, with the Inner Basin SNOTEL reporting about 12 inches of new snow, and about 1.5 inches of new snow water equivalent. The system also brought much needed colder temperatures, refreezing our wet and warm snowpack.
Feb.14/15 delivered sustained winds of 10-30 mph from the NW through NE, and by Thursday, Feb. 16th temperatures rebounded with about 10° (F.) above seasonal average.
Starting Friday and continuing through the weekend, a period of cooler and unsettled weather will affect northern Arizona with occasional rain and high elevation snow showers. Near treeline, 10 to 20 inches of new snow is predicted, along with strong westerly, southwesterly and southerly winds.
Fair and mild weather is expected by Monday/Tuesday, and by Wednesday increasing clouds and slight chances of precipitation return.
Weather station information:
On the Thursday, February 16th, the Inner Basin SNOTEL site (Snowslide Canyon) reported a snow depth of 76 inches (193 cm) at 9700’, and Arizona Snowbowl reported 83 inches (211 cm) at 10800’. Since February 11th, SNOTEL temperatures ranged between 14° and 49° F, and Agassiz station between 19° and 34° F.