Flagstaff, Arizona - Backcountry of The San Francisco Peaks and Kachina Peaks Wilderness
Format and Limitations Statement
The 12-16 inches of snow that fell last Saturday February 22nd has either bonded well to the snowpack below, or was displaced by strong winds the following week. Enough time has now passed that natural storm slab avalanches are unlikely. Areas of recently deposited wind slab may be susceptible to human triggering. The addition of new snow from an approaching storm arriving on Sunday night may increase the chances of both natural and human caused slides. No new avalanches have been reported since February 22nd.
Very strong to gale force winds blew out of the north-northeast between Sunday evening and Tuesday night. Average wind speeds exceeded 50 mph and gusts up to 84 mph were recorded at ASBTP (11,555’). The overall effect of the wind might have created dangerous wind slab, or reduced the hazard by blowing all of the snow away.
Where new snow has become scoured away, ice axe and crampons are strongly recommended as surface conditions can be very firm depending on the time of day, temperature, and coverage. Always be cautious of wind slab formation on steep, leeward slopes. Observe local signs of wind stripping and hollow drum sounds whenever you travel above treeline. Where wind has not scoured snow, depths remain at 3-5 feet. New snow falling on these firm surfaces may be reactive, potentially sluffing with little provocation. Approach steep terrain with caution.
With a decrease in elevation comes a decrease in snow depth. Obstacles become the main hazard below 9500 feet, but are present on all aspects at all elevations. Sunny and southerly slopes below 9500 feet are getting difficult to navigate on skis. Always expect the unexpected. With recent new snowfall, obstacles have become covered and hidden just below the surface. Ski and snowboard with care.
Lower elevation terrain has received enough warm temperatures so that the snowpack is nearing an isothermal state. This means the entire snowpack is near the freezing point. Under this condition, the snowpack is generally strong, and has a well-bonded structure when frozen together at night. Rapid warming and air temperatures in the high 40°s F can weaken the structure resulting in wet avalanches. Avoid steep terrain when the snowpack becomes too sloppy for enjoyable travel. In an isothermal snowpack, conditions can degrade rapidly.
Wind transported snow was evident throughout the past week. During several days ideal saltation (wind transporting snow) velocities were recorded at 11,555 feet. On Tuesday February 25th, gale force ridge-top winds out of the NNE stripped many windward slopes by the process of turbulent suspension, most likely returning much of our above treeline snow back to the clouds.
Still, pockets and even slopes of unstable wind slab (most likely hard slab) cannot be ruled out. Use caution on potentially wind loaded terrain, especially near ridgelines.
Although the forecast for the upcoming week looks cool and breezy, if mid-day temperatures exceed expectations, loose wet avalanches will be possible. These could occur primarily on lower elevations southerly slopes and near radiation absorbing terrain features, like rock outcrops above treeline. Lower elevation snowpack is becoming isothermal, so rapid melting can occur when temperatures rise into the upper 40°s F.