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ABSTRACT
An objective technique to determine forecast snowfall ranges consistent with the risk tolerance of users is demonstrated. The forecast snowfall ranges are based on percentiles from probability distribution functions that are assumed to be perfectly calibrated. A key feature of the technique is that the snowfall range varies dynamically, with the resultant ranges varying based on the spread of ensemble forecasts at a given forecast projection, for a particular case, for a particular location. Furthermore, this technique allows users to choose their risk tolerance, quantified in terms of the expected false alarm ratio for forecasts of snowfall range. The technique is applied to the 4-7 March 2013 snowstorm at two different locations (Chicago, Illinois, and Washington, D.C.) to illustrate its use in different locations with different forecast uncertainties. The snowfall range derived from the Weather Prediction Center Probabilistic Winter Precipitation Forecast suite is found to be statistically reliable for the day 1 forecast during the 2013/14 season, providing confidence in the practical applicability of the technique.
1. Introduction
Quantifying and communicating uncertainty has challenged the weather enterprise (Hirschberg et al. 2011). This is particularly true for snowfall, where uncertainties in precipitation amount, precipitation type (e.g., Kain et al. 2000; Lackmann et al. 2002), and snow-to-liquid ratio (SLR; e.g., Roebber et al. 2003; Alcott and Steenburgh 2010) are simultaneous factors affecting snowfall predictability. Further, regional features such as topography and proximity to moisture sources may affect predictability (e.g., Maglaras et al. 1995; Niziol et al. 1995; Schultz et al. 2002). A series of northeast U.S. snowstorms in the early 2000s (24-25 January 2000, 30 December 2000, and 4-5 March 2001) of varying forecasting success (Kocin and Uccellini 2004, 4-6) have highlighted the challenges of communicating snowfall uncertainty.
The U.S. weather enterprise communicates snowfall uncertainty in a variety of ways, including the probability of occurrence and probability of exceeding thresholds. Morss et al. (2008) found that the general U.S. public prefers ranges of information relative to deterministic information, and, indeed, the most common approach to communicating snowfall is a range (e.g., 2-4 in.). However, it is often unclear whether this range represents the variation of snowfall over an area, or the uncertainty in values at a point. In many cases, it is also unclear whether...