Avalanche Forecast for the Idaho City Mountains
Bottom Line
During the last 7 days, nature blessed the Mores Creek Summit area with an excess of 80 mm of SWE, almost 80 cm of snow at 10% density.
The first two storm periods ended on February 19th, with AWSOME POWDER snowfall totals exceeding 50 mm of SWE. The significant precipitation total and rapid precipitation rate triggered a widespread avalanche cycle. The buried surface hoar weak layer released at many northerly and easterly slopes and propagated to low-angle slopes in the 30-degree range.
The final moisture push concluded on February 24th, with 30 mm of SWE and 20 cm of wet, warm snow. Elevations below 7000 feet received varying amounts of precipitation, ranging from rain to a mix of rain/snow.
After the February 19th load event, a weather break allowed the new snow to round and sinter at the slopes that did not release during the instability cycle.
The warm period starting on February 22nd and warm snow precipitation on February 24th further accelerated the snowpack densification. Warmer temperatures facilitated rounding and sintering, speeding up with the assistance of viscous relaxation of the new WARM snow load.
In summary, in a little more than a week, over 80 mm of SWE have been added to the Idaho City Mountains. This is a significant amount of snow, and it's getting close to opening up access points at the 6000-foot level.
Avalanche Problem #1:
Most slopes with buried surface hoar released during the storm cycle. But there is potential for the release of slopes with near-surface facets now buried at 70 cm. These slopes are considered facet gardens; lightly forested, shady, and did not favor surface hoar development on February 9-10th, but developed NSF crystals. These shady (tree-shaded), steep, and northerly slopes might not have released during the last instability cycle and require a cautious approach.
Reactivity: Stubborn.
Spatial Distribution: Isolated. Shady deep north slopes above 7500 feet.
Avalanche release likelihood: Unlikely
Forecast Confidence
High confidence.
Warm temperatures and a significant load from the last three storm cycles have favored the densification, rounding, and sintering of the snowpack.
Relatively cooler temps, quiet weather, and the prospect of clear skies will allow snowpack to lose heat through infrared radiation during the night for all slopes, and most of the day for northerly slopes. These energy balance processes favor the snowpack becoming unreactive, except on isolated slopes with deeply buried NSF that did not release during the last instability cycle.
Snow and Riding Conditions
Supportable snow and much improved snow coverage below 7000 feet. The great powder from last weekend is gone. Let's do snow dances for more!
ASG Snowpack Development Chart
ASG Technical Tip
Avalanche slope angles can be confusing. It is important to realize that there are several general slope-angle attributes: starting zone angle, runout angle, connectivity angle, alpha angle, and debris-deposition angle.
Starting zone angles are necessary for an avalanche release. A runout angle above 30 degrees allows the avalanche to continue accelerating. The connectivity angle relates to the weak layer type's ability to connect to the terrain and enable remote triggering, as well as to propagate onto lower-angle slopes. The alpha angle and debris-deposition angles allow winter travellers to estimate whether they are in an avalanche path.
Terrain slope angle tracking and estimation result from integrating field angle measurements (most definitely a team skill), mapping tools, and terrain characterization (ATES 2.0).
To gain mastery of the interaction between terrain and slope angles, we strongly recommend pursuing avalanche education with knowledgeable providers.
Avalanche Science instruction mentors folks from ASG1 to ASG3 progression when Avalanche Science guides feel our mentees have mastered slope-angle estimation. Angle estimation, measurement, and management are skills that require lots of attention from mentors.