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About WPC's Probabilistic Winter Precipitation Forecast (PWPF) ProductsThe operational WPC Winter Weather Desk (WWD) creates 24h forecasts of snowfall and freezing rain accumulations for each of three consecutive 24h periods (days) extending 72 hours into the future. These products are shared with the NWS Weather Forecast Offices (WFO) in a collaborative process resulting in refinement of the accumulation forecasts. After the 24h snowfall and freezing rain accumulation forecasts are finalized, the WWD issues its public products: a limited suite of probabilistic winter weather forecasts. These probabilistic forecasts are computed based on the deterministic accumulation forecasts combined with ensemble information (see below). Prior to the 201314 season, the probabilistic forecasts were manually edited by the WWD forecaster. For the 201314 season and onward, the limited suite of probabilistic forecasts is usually not edited. The probabilistic forecasts found here on the WPC PWPF page are also based on the deterministic WWD accumulation forecasts and are generated automatically using an ensemble of model forecasts along with the WWD forecasts. The automatic nature of this product generation allows an extensive set of displays of probabilities for snowfall or freezing rain exceeding a number of thresholds and accumulations of snowfall or freezing rain for various percentile levels. The percentile amounts and probabilities for 24hour intervals are generated at sixhour increments through 72 hours. The sixhour increments are made possible by disaggregation of the 24h human deterministic forecast based on sixhour accumulations from a blend of model guidance selected by the WWD forecaster. The automatic processing also allows the generation of probabilistic winter precipitation forecasts for 48h intervals based on 48h accumulations obtained by adding two 24h accumulations together. The same method used to compute the 24h probabilistic products is applied to the 48h intervals ending at 48 through 72 hours after the initial time. As with the 24hour forecasts, the 48h forecasts are produced at sixhour intervals. Finally, a single set of probabilistic forecasts are created for the entire 72hour period. A multimodel ensemble is utilized to create a distribution of values around the WPC accumulation at each grid point. The typical constituency of this ensemble is as follows:
Ensemble data ProcessingTo prepare the raw ensemble data for use by PWPF the following steps are taken: PRISMbased downscaling is applied to the quantitative precipitation forecasts (QPF) from the GEFS and GFS members. Otherwise, the modelbased QPF is used directly without adjustment. At each grid point, the precipitation type determination for the NCEP models is the dominant type algorithm (Manikin 2005). Precipitation type for nonNCEP models is determined by applying a simple decision tree algorithm using surface temperature, and temperatures on the 925hPa, 850hPa, and 700hPa mandatory isobaric levels. A unique snow level is also determined for each member based on the height of the 0.5 C Wet Bulb surface. At each grid point, a top down search is conducted to find where the wet bulb temperature first exceeds 0.5 C. It is then found where the snow level intersects the 5km topography from the Realtime Mesoscale Analysis (RTMA). Grid points above the snow level are flagged as snow in the final precipiation type mask for each member. Each member's snowfall is based on the QPF, the precipiation type, and a snowtoliquid ratio (SLR). The 6h SLR at each grid point is an average of the value obtained using the Roebber et al (2007) neural network algorithm (Rnna) applied to the NAM forecast, the value from the Rnna applied to the GFS forecast, a seasonal climatological value, and 11. For each member, the freezing rain accumulation is based on the QPF, the precipiation type, and an icetoliquid ratio (ILR). The ILR is based on a simplified version of the Freezing Rain Accumulation Model (FRAM; Sanders and Barjenbruch 2016) which uses precipiation rate as the sole predictor. A binormal (Toth and Szentimrey 1990) probability distribution or density function (PDF), which allows skewness, is utilized for the PWPF. The fitting of the binormal distribution is a method of moments approach. The WPC forecast is the mode of the distribution. The placement of the WPC forecast in the ensemble order statistics determines the skewness of the distribution. The variance of the distribution is matched to the variance of the ensemble. The WPC deterministic forecast is included as an additional member of the ensemble for the computation of the variance. This fit is done at each grid point; so, the probability density function (PDF) varies from grid point to grid point. Forecast FormatsThe PWPF forecasts provide information in the following formats: Probabilities of exceeding a threshold show filled contour levels of probability that the 24hour, 48hour, or 72hour accumulation of winter precipitation will equal or exceed the given threshold. As an example, consider the 6inch threshold for snowfall. If a point of interest falls within the 40% contour on the probability map, then the chance of snowfall exceeding 6 inches is 40% or greater. As the threshold values increase, the probabilities of exceeding them decrease. Percentile accumulations for 24, 48, or 72hour intervals show filled contours of snowfall or freezing rain amounts for which the probability of observing that amount or less is given by the percentile level. For example, if the 75th percentile map shows six inches of snow at a location, then the probability of getting up to six inches of snow is 75% at that point. Conversely, there is only a 25% probability of snowfall exceeding six inches at the location in this example. Percentile accumulations increase as the percentile level increases. To illustrate this point, take the previous example, but instead of the 75th precentile map consider the 10th percentile map showing two inches of snow at the location. In this case, the probability of getting up to but no more than two inches of snow is just 10%. The probability of getting more than two inches is 90%; so, a significant accumulation of snow is likely. For more information on creation of the PWPFs and how to navigate the web page, please see this informational video. ReferencesManikin, G. S., 2005: An overview of precipitation type forecasting using NAM and SREF data. Preprints, 21st Conf. on Wea. Analysis & Forecasting / 17th Conf. on Numerical Weather Prediction, Washington, DC, Amer. Meteor. Soc., 8A.6. Roebber, P. J., M. R. Butt, S. J. Reinke, T. J. Grafenauer, 2007: Realtime forecasting of snowfall using a neural network. Wea. Forecasting, 22, 676684. Sanders, K.J. and B.L. Barjenbruch, 2016: Analysis of IcetoLiquid Ratios during Freezing Rain and the Development of an Ice Accumulation Model. Wea. Forecasting, 31, 10411060. Toth, Z., and T. Szentimrey, 1990: The binormal distribution: A distribution for representing asymmetrical but normallike weather elements. J. Climate, 3, 128136.
