augustys

A frequent topic of discussion in this forum pertains to the skill of the most common "dynamic" snow-to-liquid ratio (SLR) methods, or those SLRs that *change* throughout the course of a snow event. Which approach is best? Are the "dynamic" approaches really better than using a constant SLR during a given event (e.g., 12:1)? In March, 2017 I compiled nearly 200 distinct cases of liquid precip commensurate with associated snowfall, which permitted me to calculate the SLR for each case. I calculated three dynamic SLR methods for statistical comparison with the observed SLRs: Kuchera SLR method, NCEP SREF SLR method, and the AWIPS SLR method, as well as three constant SLRs 10:1 SLR, 11:1 SLR, and 12:1 SLR. The results are attached in the Excel screen capture. As you can see, the three dynamic SLRs are far superior to the three chosen constant SLRs, and of the three dynamic SLRs, the Kuchera method and the NCEP SREF method have very similar error statistics. I probably couldn't claim that either was superior with statistical significance. The Kuchera SLR method, developed by a good friend and colleague of mine as a function of snowfall density research in Roebber et al. (2003), is probably the most well-known of the dynamic SLR methods because both pivotalweather and WeatherBELL have it as an option in their snowfall products. What I find interesting is that many people in this forum accuse the Kuchera SLR method of being too "aggressive" or "over-doing it." One former Omaha TV meteorologist (who shall remain unnamed) even called it the worst SLR method he's ever seen. That claim is obviously patently false and based on no real, objective evidence. To the contrary, while my sample size is somewhat small, on average the Kuchera SLR method has a slight *negative* bias when compared to observed SLRs! In other words, at least for this sample, its snowfall output was slightly *lower* than reality, not too high (or the progging excessive snowfall)! Hopefully this clarifies any misconceptions about the actual skill of the most common SLR approaches, and gives users confidence that the snowfall output using especially the Kuchera SLR or NCEP SREF SLR is reliable.