Snow Depth Prediction

The maximum depth of the winter snow in the Australian Alps varies from year to year. If these variations were random, then the best guess for next year's maximum snow depth would be the average depth. However, if some cyclical factors are present, then it should be possible to quantify these, and make a better guess.

From inspection of Fig. 1, it appears that an exceptionally good year is often (but not always) followed by several poor years, and that there appears to be a systematic decrease in snow depth with time. I have analyzed the year to year fluctuations assuming that, in addition to a purely random component, there are periodic components which can be derived from the historical measurements, and can be used to predict future behavior. In addition to random fluctuations, cyclic components with periods of 3.4 years and 5.4 years were found, and used to predict the maximum snow depth for next year.

As can be seen in Fig. 2, years with low snow depths occur more often than expected from purely random variations, as do exceptional years with depths exceeding 300 cm. Average and slightly above average years occur less frequently than expected.

The prediction for 2009, based on the measurements from 1954 through 2008, is for a maximum snow depth of 225 cm, 13% more than the 55 year average value of 199.8 cm, and more than the 2008 actual depth of 172.4 cm

The depths since 2005 have been over predicted. The prediction for 2009 of 225 cm is only slightly less than the 2008 prediction of 245 cm, so we may expect that 2009 will be like 2008. Over the years 1993 through 2008, the predicted minus the actual depths have averaged 41.2 cm with a root-mean-square scatter of 23.2 cm while the average minus the actual depths have averaged 42.7 cm with a scatter of 46.4 cm. It is interesting that the actual snow depths are low with respect to both the long term average and the predictions, but the predictions reduce the scatter by about a factor of two. This suggests that some improvement may be obtained by first subtracting the trend represented by the least square linear fit to the depths that is shown in Fig. 1. When this is done the predicted depth for 2009 is reduced from 225 cm to 207 cm.