Techniques and Thresholds of Significance for Using WSR-88D Velocity Data to Anticipate Significant Tornadoes

Abstract

Significant tornadoes (EF2+) make up a very small percentage of the total United States tornado events, but produce the overwhelming majority of tornado fatalities. Identifying significant tornado events in a shortfused warning environment has been a particular focus of the United States National Weather Service’s severe weather program in recent years, with the goal of reducing the loss of life from significant events to the greatest extent possible. This study aims to further this effort by identifying and quantifying the skill of key signals present in Weather Service Radar-1998 Doppler velocity data in the minutes prior to the onset of significant tornado damage. When separated by storm mode, several radar velocity signals are identified that show operationally useful skill in differentiating between significant and weak/nontornadic events in supercells—with lead time. The highest skill scores are achieved by combining maximum volumetric rotational speed and depth of the storm’s mesocyclone, as well as the overall change in rotational speed in the final minutes prior to the onset of significant damage. Very little, if any, predictive skill was found when only the lowest elevation scan was considered, including more frequent supplementary scans in between full volumetric scans. The same signals that showed noteworthy skill for supercells failed to discriminate between significant and weak/nontornadic events in quasi-linear convective Systems and bow echo/mesoscale convective vortex events.