The Probabilistic Storm Total Snowfall Forecast Experiment: 2015-16 Results and Future Plans

During the winter months of 2014-2015, four WFOs in the Eastern Region of the NWS conducted an experiment in producing probabilistic storm total snowfall information as a means to communicate uncertainty in the forecast. During the winter of 2015-16, this experiment was expanded to 13 WFOs in the Eastern Region, 4 WFOs in the Central Region, and 1 WFO in each of the Southern and Western regions.  The forecast technique utilizes NCEP's Weather Prediction Center's (WPC) probabilistic snowfall percentile accumulation values to create a cumulative probability distribution function utilizing the official NWS WFO storm total snowfall forecast as the mode of the distribution. Tenth and 90th percentile graphics, exceedance probabilities, and categorical range probabilities for a variety of thresholds are output and made available to end users for decision making. The 10th and 90th percentile values are communicated as the minimum (expect at least this much) and maximum (potential for this much) storm total snowfall, respectively, while the official NWS WFO storm total snowfall is communicated as the most likely snowfall amount.

Point forecast verification was conducted across all of the 13 Eastern Region WFO county warning areas for numerous events during the experiment, yielding a total of over 800 forecast-observed pairs. The probabilistic forecasts were verified against storm total snowfall reports from WFO Public Information Statements and/or computed from daily climate reports. Several statistical metrics were used to assess the forecasts from both a deterministic and probabilistic perspective.  Feedback from emergency managers as well as the general public indicates that the probabilistic snowfall information was generally well received and appeared to be understood.  Lessons learned from the 2015-16 results and feedback yielded some adjustments to the forecast methodology, and the graphical depictions of the probabilistic information that will be implemented for 2016-17.  In addition to expanding the experiment to additional offices nationwide with an eventual goal of national implementation, a social science based study of the design, user interpretation and use of the probabilistic snowfall information is planned for 2016-17.