Examining potential improvements to severe weather warnings from a geospatial verification perspective

NWS severe thunderstorm and tornado warnings are issued as recently-adopted “storm-based” polygons that cover a two-dimensional area of expected threat of severe weather within a specific time period up to 60 minutes. The entire spatial and temporal extent of a warning polygon is verified when at least one ground truth report of severe weather occurs within the valid time period of the warning. The time difference between the time of the first severe weather report within the valid time and area of the warning and the issuance time of the warning is calculated as the lead time. Any warning polygon with (without) a matching report of severe weather is considered a hit (false alarm), and any severe weather report with (without) a warning is deemed a hit (missed) event. A single point in space and time can be used to verify an areal polygon warning. Consequently, there is no penalty for issuing warnings which cover a larger area and a longer duration, even though one of the main intents of the NWS storm-based warnings is to reduce the area and time covered by warnings in comparison to the original practice of issuing warnings for whole counties.

We continue our exploration of alternative methods for assessing the goodness of NWS severe thunderstorm and tornado warnings. At a previous conference, we reported on our work to verify warnings geospatially on a high-resolution grid using data collected during a springtime warning experiment at the NOAA Hazardous Weather Testbed (HWT). For each grid point, we compute location-specific probability of detection, false alarm ratio, and various other skill statistics (e.g., critical success index), as well as lead time, departure time, and valid warning time for each event affecting the grid location. For the period and spatial domain of an entire storm event, we compute the averages of the above measures, as well as aggregate false alarm areas and times.

We will compare the effects of improved warning precision (area and duration) on the current and experimental verification schemes. In addition, we will explore the advantages, from a service improvement standpoint, of two potential upgrades to severe weather warning information that were tested in the HWT: 1) “Threats-In-Motion” where warning polygons move continuously with the storm threats, and 2) probabilistic information for locations where the threat levels are currently below warning criteria or for locations still downstream of current warnings.