Waiting on a mule deer crossing…
Snowfall, temperature and wind are three factors that quickly change avalanche conditions. Ridge-top winds have been used to assess avalanche conditions with mixed success due to high variability. Few analyses have tested the effect upper atmospheric winds have on avalanche conditions. This study attempts to quantify the effect upper atmospheric wind direction and speed have on the spatial pattern of natural avalanching in the mountains near Gothic, Colorado. The Gothic dataset consists of over 3,300 natural avalanches spanning a time period of 33 years. These data are associated with daily new snow water equivalence (SWE) collected at Gothic, as well as 500mb wind direction and speed from the National Centers for Environmental Protection/National Center for Atmospheric Research (NCEP/NCAR) gridded reanalysis project. I hypothesize that prevailing 500mb winds are more likely to be associated with natural avalanches in avalanche paths with a starting zone aspect lee to the prevailing free air winds. In addition, I hypothesize that the odds of a natural avalanche occurring increases with increasing new SWE, increasing 500mb wind speeds, and 500mb wind direction. These hypotheses are tested using simple probability analysis as well as a two-component hurdle model. As expected, avalanche paths lee to the 500mb wind direction have an increased probability of avalanching in relation to paths facing in other directions. However, exceptions do occur, some of which can be explained by cross-loading. The hurdle model results indicate that after accounting for new SWE, 500mb wind speed is significant in determining whether or not a day will be considered an avalanche day. Once a day is determined to be an avalanche day, 500mb wind direction is an important determinant for the daily avalanche hazard after accounting for new SWE. These results have practical significance. They give avalanche forecasters confidence that upper air wind direction is a useful predictor of the pattern of avalanche activity at the valley or mountain range scale, and the exceptions observed show that such predictions cannot be applied at the path scale. These scale issues demonstrate the general nature of backcountry advisories and why they cannot be applied at the scale of individual slopes.
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