The National Weather Service (NWS) is adopting a new approach towards defining the area extent of its short-term warnings. This new methodology defines the area that is warned by using polygons rather than county outlines. This allows meteorologists to focus the warning on areas that are most vulnerable, without regards to geopolitical boundaries such as counties and states. The agency's customers have enthusiastically welcomed the new approach as a step towards providing a better service.

The Agency has long used a system of warning verification that was based on counties. If an event occurred anywhere within a county for which a warning was issued then the warning was considered verified. If no event occurred then the warning was a false alarm. The metrics were computed using a database-matching program. Now, however, with warnings defined as polygons, a different approach is required. This approach is the use of Geographic Information System (GIS) software. Using GIS, forecasters can compute verification statistics by using a combination of built-in tools along with custom software development.

This project uses ArcGIS along with customized code using Visual Basic for Applications (VBA) object programming. VBA provides an interface to the geospatial themes and allows specialized analysis such as the comparison of polygon area values to county area values and also computation of complicated performance measurements that would require many more hours to compute manually. This paper will highlight some of the methods used to analyze over 90,000 historical NWS warnings and events and produce performance metrics that will be used to evaluate NWS warning performance trends.