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ABSTRACT
A 30-yr climatology of the snow-to-liquid-equivalent ratio (SLR) using the National Weather Service (NWS) Cooperative Summary of the Day (COOP) data is presented. Descriptive statistics are presented for 96 NWS county warning areas (CWAs), along with a discussion of selected histograms of interest. The results of the climatology indicate that a mean SLR value of 13 appears more appropriate for much of the country rather than the often-assumed value of 10, although considerable spatial variation in the mean exists. The distribution for the entire dataset exhibits positive skewness. Histograms for individual CWAs are both positively and negatively skewed, depending upon the variability of the in-cloud, subcloud, and ground conditions.
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1. Introduction
To forecast snowfall amounts for a winter extratropical cyclone (ETC), the forecaster employs a two-step process. First, current dynamic and thermodynamic fields must be analyzed in conjunction with numerical model forecasts to determine a quantitative precipitation forecast (QPF). This QPF represents the liquid equivalent expected to precipitate from the system. To convert this liquid equivalent to a snowfall amount, a snow-to-liquid-equivalent ratio (SLR) must be determined. An SLR value of 10 is often assumed as a mean value; however, this value may not be accurate for many locations and meteorological situations. Even if the forecaster has correctly forecasted the QPF, an error in the predicted SLR value may cause significant errors in forecasted snowfall amount-the forecast variable that is disseminated to the public. As an example, a QPF value of 0.25 in. may produce 2 in. of snowfall for an SLR value of 8, or 5.5 in. of snowfall for an SLR value of 22, a sizeable difference in terms of societal impacts.
As early as 1875, the U.S. Weather Bureau provided a typical SLR value of 10:1 to its observers, later instructing observers in 1894 that the 10:1 ratio was only a rough approximation (Henry 1917). In 1878 a 10:1 mean SLR value was determined for Toronto when an observer came to this conclusion after a long series of experiments (Potter 1965). A number of studies have shown that there is considerable variation from this estimate depending on location and various environmental parameters (e.g., Henry 1917; LaChapelle 1962; Grant and Rhea 1974; Doesken...