A Gridded Snowfall Verification Method Using ArcGIS

National Weather Service Weather Forecast Offices verify snowfall for Winter Storm Warnings by computing the mean snowfall within each NWS forecast zone. To determine the mean snowfall within each zone, snowfall reports are added together and divided by the number of reports. This legacy method does not factor in distance between points and is heavily weighted towards areas of greater population where typically more snowfall observations exist. This method can be unrepresentative of outlying and higher terrain areas where snowfall can vary significantly. A gridded snowfall verification method using Geographic Information Systems has been developed at NWS Albany, New York. The verification is still performed by calculating the zoneaveraged snowfall, but utilizes a gridded methodology for a more innovative and representative approach. A contoured snowfall analysis map is created using an interpolation scheme to obtain a graphical representation of observed snowfall across a given area. Zonal statistics are then calculated using the observed snowfall analysis map, resulting in a more representative mean snowfall for each forecast zone. The end result is a more precise snowfall verification. A method for creating error plots for gridded snowfall forecasts has also been developed and is introduced. The error plots are calculated by taking the difference between NWS gridded snowfall forecasts and observed snowfall analysis maps after an event. Examples of observed snowfall maps and forecast error plots from the 2013-2014 winter season are presented and discussed.